Document:

THIS  WARRANT  AND THE  SHARES  PURCHASABLE  HEREUNDER  HAVE BEEN  ACQUIRED  FOR
INVESTMENT  AND HAVE NOT  BEEN  REGISTERED  UNDER  THE  SECURITIES  ACT OF 1933.
NEITHER THIS WARRANT NOR SUCH SHARES MAY BE SOLD OR  TRANSFERRED  IN THE ABSENCE
OF SUCH  REGISTRATION  IN EFFECT  UNDER SUCH ACT UNLESS THE COMPANY  RECEIVES AN
OPINION OF COUNSEL OR OTHER EVIDENCE  REASONABLY  ACCEPTABLE TO IT DEMONSTRATING
THAT SUCH  SALE OR  TRANSFER  IS EXEMPT  FROM THE  REGISTRATION  AND  PROSPECTUS
DELIVERY  REQUIREMENTS  OF SAID ACT OR UNLESS SOLD  PURSUANT TO RULE 144 OF SUCH
ACT.

                          Common Stock Purchase Warrant

                      To Purchase Shares of Common Stock of

                            Beacon Power Corporation

                                  May 24, 2005

FOR VALUE  RECEIVED,  Beacon  Power  Corporation,  a Delaware  corporation  (the
"Company"),  hereby grants to Perseus 2000 Expansion Fund, L.L.C. (together with
its permitted  successors and assigns,  the "Registered  Holder"),  the right to
purchase at any time up to and including May 24, 2010 (the "Termination  Date"),
the  number  of  shares  of fully  paid and  nonassessable  Common  Stock of the
Company,  $0.01 par value per share (the  "Common  Stock",  and those  shares of
Common Stock purchasable under this Warrant being the "Warrant  Shares"),  equal
to the  product of (i)  806,400  divided by (ii) the  Warrant  Price (as defined
below) then in effect, for a purchase price of 120% of the Per Share Price as of
the date hereof (as defined in the  Investment  Agreement,  dated as of the date
hereof,  by and among the Company,  the Registered  Holder and Perseus  Capital,
L.L.C.  ("Investment  Agreement")),  which Per Share Price may be adjusted  from
time to time for  purposes of this  Warrant and the  calculation  of the Warrant
Price only during the six-month  period  following  the date hereof  pursuant to
Section 2.4 of the Investment Agreement (the "Warrant Price"). For the avoidance
of  doubt,  any  adjustments  to the Per  Share  Price  under  the  terms of the
Investment  Agreement  that may occur after the six-month  period  following the
date hereof shall be disregarded  for purposes of calculating  the Warrant Price
hereunder.

         1.       EXERCISE.

     (a) This Warrant may be exercised by the Registered Holder on or before the
Termination  Date, in whole or in part, by surrendering  this Warrant,  with the
purchase  form  appended  hereto as Exhibit I duly  executed  by the  Registered
Holder at the  principal  executive  offices  of the  Company,  or at such other
office or agency as the Company may  designate,  accompanied by payment in full,
in lawful money of the United States, of the Warrant Price payable in respect of
the number of Warrant Shares purchased upon such exercise.

     (b) The Registered  Holder may, at its option,  elect to pay some or all of
the Warrant  Price payable upon an exercise of this Warrant by surrender of this
Warrant  at the  principal  executive  office of the  Company,  or at such other
office or agency as the Company may  designate,  together with the purchase form
attached hereto as Exhibit I to exercise by means of a net issuance exercise, in
which event the  Company  shall  issue to the  Registered  Holder that number of
Warrant  Shares equal to the total number of Warrant  Shares,  minus the product
obtained by  multiplying  (x) the total number of Warrant  Shares (or, if only a
portion of the Warrant is being  exercised,  the  portion of the  Warrant  being
cancelled,  at the date of such calculation) by (y) a fraction, the numerator of
which shall be the Warrant Price per share and the denominator of which shall be
the Fair Market  Value per share of Common Stock as of the  Exercise  Date.  The
"Fair Market Value" per share of Common Stock shall be determined as follows:

          (i) As long as the  Common  Stock is listed on a  national  securities
     exchange,  the  NASDAQ  Small Cap  Market,  the NASDAQ  National  Market or
     another  nationally  recognized trading system as of the Exercise Date, the
     Fair  Market  Value  per share of  Common  Stock  shall be deemed to be the
     average of the high and low reported  sale prices per share of Common Stock
     thereon  on  the  trading  day  immediately  preceding  the  Exercise  Date
     (provided  that if no such price is reported  on such day,  the Fair Market
     Value per share of Common  Stock  shall be  determined  pursuant  to clause
     (ii)).

          (ii) In all other  cases,  the Fair  Market  Value per share of Common
     Stock  shall be deemed to be the amount  most  recently  determined  by the
     Board of  Directors  to  represent  the fair market  value per share of the
     Common Stock.  Notwithstanding the foregoing, if the Board of Directors has
     not made such a determination  within the  three-month  period prior to the
     Exercise Date,  then (A) the Board of Directors  shall make a determination
     of the Fair Market  Value per share of the Common Stock within 20 days of a
     request by the  Registered  Holder  that it do so, and (B) the  exercise of
     this Warrant  pursuant to this  subsection 1(b) shall be delayed until such
     determination is made.

     (c) Each  exercise of this  Warrant  shall be deemed to have been  effected
immediately  prior to the close of  business  on the day on which  this  Warrant
shall have been  surrendered to the Company as provided in subsection 1(a) above
(the  "Exercise  Date").  At such  time,  the person or persons in whose name or
names any  certificates  for Warrant Shares shall be issuable upon such exercise
as provided in  subsection  1(d) below shall be deemed to have become the holder
or holders of record of the Warrant Shares represented by such certificates.

     (d) As soon as practicable after the exercise of this Warrant in full or in
part, and in any event within twenty (20) days thereafter,  the Company,  at its
expense,  will  cause to be  issued  in the  name  of,  and  delivered  to,  the
Registered  Holder,  or as such  Holder  (upon  payment  by such  Holder  of any
applicable transfer taxes) may direct:

          (i) a certificate or certificates  for the number of Warrant Shares to
     which the Registered Holder shall be entitled upon such exercise; and

          (ii) in case such  exercise is in part only, a new warrant or warrants
     (dated the date hereof) of like tenor, calling in the aggregate on the face
     or faces  thereof for the number of Warrant  Shares equal  (without  giving
     effect to any  adjustment  therein) to the number of such shares called for
     on the face of this Warrant minus the sum of: (a) the number of such shares
     purchased by the Registered Holder upon such exercise and (b) the number of
     Warrant Shares (if any) covered by the portion of this Warrant cancelled in
     payment of the total Warrant  Price payable upon such exercise  pursuant to
     subsection 1(b) above.

         2.       ADJUSTMENTS.

     (a) Adjustment for Stock Splits and  Combinations.  If the Company shall at
any time or from time to time  after the date on which  this  Warrant  was first
issued (the  "Original  Issue Date")  effect a  subdivision  of the  outstanding
Common Stock,  the number of Warrant  Shares then in effect  immediately  before
that subdivision shall be proportionately  increased and the Warrant Price shall
be proportionately  decreased.  If the Company shall at any time or from time to
time after the  Original  Issue Date  combine the  outstanding  shares of Common
Stock, the number of Warrant Shares shall be  proportionately  decreased and the
Warrant Price shall be  proportionately  increased.  Any  adjustment  under this
paragraph  shall  become  effective  at the  close of  business  on the date the
subdivision or combination becomes effective.

     (b)  Adjustments for Other  Dividends and  Distributions.  In the event the
Company  at any time or from time to time  after the  Original  Issue Date shall
make or issue, or fix a record date for the  determination  of holders of Common
Stock  entitled  to  receive,  a  dividend  or  other  distribution  payable  in
securities  of the Company or in cash or other  property,  then and in each such
event the Registered  Holder shall receive upon exercise hereof,  in addition to
the number of shares of Common Stock issuable hereunder,  the kind and amount of
securities of the Company  and/or cash and other  property  which the Registered
Holder would have been  entitled to receive had this  Warrant been  exercised on
the date of such  event and had the  Registered  Holder  thereafter,  during the
period from the date of such event to and including the Exercise Date,  retained
any such securities receivable, giving application to all adjustments called for
during  such  period  under  this  Section 2 with  respect  to the rights of the
Registered Holder.

     (c)  Adjustment for Mergers or  Reorganizations,  etc. If there shall occur
any reclassification,  reorganization, recapitalization,  consolidation, sale of
all or  substantially  all assets or merger  involving  the Company in which the
Common  Stock is  converted  into or  exchanged  for  securities,  cash or other
property  (other than a  transaction  covered by subsection  2(a) above),  then,
following   any   such   reclassification,   reorganization,   recapitalization,
consolidation,  sale of all or substantially  all assets or merger,  and without
payment of any additional  consideration  thereof,  the Registered  Holder shall
receive upon exercise  hereof the kind and amount of  securities,  cash or other
property  which the  Registered  Holder would have been  entitled to receive if,
immediately prior to such  reclassification,  reorganization,  recapitalization,
consolidation or merger, sale of all or substantially all assets, the Registered
Holder had held the number of shares of Common  Stock  subject to this  Warrant,
giving  application to all adjustments  called for during such period under this
Section 2 with respect to the rights of the Registered Holder.

     The  foregoing  provisions  of this Section 2(c) shall  similarly  apply to
successive reorganizations,  consolidations, mergers, sales and transfers and to
the stock or securities of any other corporation that are at the time receivable
upon the exercise of this Warrant. If the per share consideration payable to the
holder hereof for Warrant Shares in connection with any such transaction is in a
form  other  than  cash  or  marketable  securities,  then  the  value  of  such
consideration  shall  be  determined  in good  faith by the  Company's  Board of
Directors. In all events, appropriate adjustment (as determined in good faith by
the  Company's  Board  of  Directors)  shall be made in the  application  of the
provisions  of this  Warrant  with  respect to the rights and  interests  of the
Holder after the  transaction,  to the end that the  provisions  of this Warrant
shall be applicable  after that event, as near as reasonably may be, in relation
to any shares or other  property  deliverable  after that event upon exercise of
this Warrant.  (d)  Certificate as to  Adjustments.  Upon the occurrence of each
adjustment  or  readjustment  pursuant  to this  Section  2, the  Company at its
expense shall promptly  compute such  adjustment or  readjustment  in accordance
with the terms hereof and furnish to the Registered Holder a certificate setting
forth  such  adjustment  or  readjustment  (including  the  kind and  amount  of
securities,  cash or other property for which this Warrant shall be exercisable,
the Warrant Price and the method of  calculation  of each) and showing in detail
the facts upon which such adjustment or readjustment is based.

         3. FRACTIONAL SHARES.

     The Company  shall not be  required  upon the  exercise of this  Warrant to
issue any fractional  shares,  and shall round any fractional shares to the next
highest whole number of shares.  If a fractional  share interest arises upon any
exercise  or  conversion  of the  Warrant,  the  Company  shall  eliminate  such
fractional share interest by paying the Registered  Holder an amount computed by
multiplying the fractional  interest by the Fair Market Value of a full share of
common Stock.

         4. REPRESENTATIONS AND WARRANTIES BY THE REGISTERED HOLDER.

     The Registered Holder represents and warrants to the Company as follows:

     (a) Upon each exercise hereof,  the Registered  Holder must reaffirm to the
Company that the Registered  Holder is an "accredited  investor" as that term is
defined in Rule 501 under the  Securities  Act of 1933,  as amended (the "Act").
The Registered  Holder is purchasing the Warrant and the Warrant Shares for such
Registered Holder's own account for investment purposes only and not with a view
to,  or for the  resale in  connection  with,  any  "distribution"  thereof  for
purposes of the Act. The Registered Holder  understands that the Warrant and the
Warrant  Shares  have not  been  registered  under  the Act by  reason  of their
issuance in a transaction  exempt from the registration and prospectus  delivery
requirements  of the Act  pursuant  to Section  4(2)  thereof,  which  exemption
depends upon,  among other things,  the bona fide nature of Registered  Holder's
investment intent as expressed herein. In this connection, the Registered Holder
understands  that, in the view of the U.S.  Securities  and Exchange  Commission
(the "SEC"),  the statutory  basis for such  exemption may be unavailable if the
Registered  Holder's   representation  was  predicated  solely  upon  a  present
intention  to hold the  Warrant or the Warrant  Shares for the  minimum  capital
gains period specified under tax statutes,  for a deferred sale, for or until an
increase or decrease in the market  price of the Warrant or the Warrant  Shares,
or for a period of one year or any other fixed period in the future.

     (b) The  Registered  Holder  understands  that the  Warrant and the Warrant
Shares  must  be  held by the  Registered  Holder  indefinitely,  and  that  the
Registered  Holder must  therefore  bear the  economic  risk of such  investment
indefinitely,  unless a subsequent  disposition  thereof is registered under the
Act or is  exempted  from  such  registration.  The  Registered  Holder  further
understands  that the  Warrant and the  Warrant  Shares have not been  qualified
under any state's  blue sky laws by reason of their  issuance  in a  transaction
exempt from the  qualification  requirements of applicable blue sky laws,  which
exemptions  depend  upon,  among  other  things,  the bona  fide  nature  of the
Registered  Holder's  investment  intent  expressed  above.  In  addition,   the
Registered  Holder  understands that any certificate  evidencing the Warrant and
the Warrant  Shares will be imprinted  with a legend that prohibits the transfer
of the  Warrant  and the  Warrant  Shares  unless  they are  registered  or such
registration is not required in the opinion of counsel for the Company.

     (c) The  Registered  Holder is familiar  with the  provisions  of Rule 144,
promulgated under the Act, which, in substance, permits limited public resale of
"restricted  securities"  acquired,  directly  or  indirectly,  from the  issuer
thereof,  in a  non-public  offering  subject  to the  satisfaction  of  certain
conditions.

     (d) The Registered Holder further  understands that in the event all of the
applicable  requirements of Rule 144 are not satisfied,  registration  under the
Act, compliance with Regulation A, or some other registration  exemption will be
required; and that, notwithstanding the fact that Rule 144 is not exclusive, the
staff of the SEC has  expressed  its  opinion  that  persons  proposing  to sell
private placement  securities other than in a registered  offering and otherwise
than  pursuant  to  Rule  144  will  have  a  substantial  burden  of  proof  in
establishing that an exemption from registration is available for such offers or
sales,  and that such persons and their  respective  brokers who  participate in
such transactions do so at their own risk.

         5.       REQUIREMENTS FOR TRANSFER.

     (a) This Warrant and the Warrant  Shares  shall not be sold or  transferred
unless either (i) they first shall have been registered  under the Act and under
applicable  state  securities  or blue sky laws, or (ii) the Company first shall
have been furnished with an opinion of legal counsel, reasonably satisfactory to
the  Company,  to the  effect  that such  sale or  transfer  is exempt  from the
registration requirements of the Act.

     (b)  Each  certificate  representing  Warrant  Shares  shall  bear a legend
substantially in the following form:

         "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR
         INVESTMENT AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
         1933. SUCH SHARES MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF SUCH
         REGISTRATION IN EFFECT UNDER SUCH ACT UNLESS THE COMPANY RECEIVES AN
         OPINION OF COUNSEL OR OTHER EVIDENCE REASONABLY ACCEPTABLE TO IT
         DEMONSTRATING THAT SUCH SALE OR TRANSFER IS EXEMPT FROM THE
         REGISTRATION AND PROSPECTUS DELIVERY REQUIREMENTS OF SAID ACT OR UNLESS
         SOLD PURSUANT TO RULE 144 OF SUCH ACT.

     The foregoing  legend shall be removed from the  certificates  representing
any Warrant Shares,  at the request of the holder  thereof,  at such time as (i)
they become eligible for resale pursuant to an effective  registration statement
or Rule 144(k) under the Act or (ii) the Company shall have  received  either an
opinion of counsel or a  "no-action"  letter from the SEC to the effect that any
transfer of the Warrant Shares represented by such certificates will not violate
the Act and applicable state securities laws.

         6. NO IMPAIRMENT.

     The Company will not, by amendment of its charter  through  reorganization,
transfer  of assets,  consolidation,  merger,  dissolution,  issuance or sale of
securities or any other voluntary action,  avoid or seek to avoid the observance
or  performance  of any of the terms of this  Warrant,  but will at all times in
good faith assist in the carrying out of all such terms and in the taking of all
such action as may be necessary or appropriate in order to protect the rights of
the holder of this Warrant against impairment.

         7. NOTICES OF RECORD DATE, ETC. In the event:

     (a) that the Company shall take a record of the holders of its Common Stock
(or other stock or securities at the time  deliverable upon the exercise of this
Warrant) for the purpose of  entitling or enabling  them to receive any dividend
or other distribution,  or to receive any right to subscribe for or purchase any
shares of stock of any class or any other  securities,  or to receive  any other
right; or

     (b) of any capital  reorganization of the Company,  any reclassification of
the Common Stock of the Company, any consolidation or merger of the Company with
or into another  corporation  (other than a consolidation or merger in which the
Company is the surviving  entity and its Common Stock is not  converted  into or
exchanged  for any other  securities  or  property),  or any  transfer of all or
substantially all of the assets of the Company; or

     (c) of the voluntary or involuntary dissolution,  liquidation or winding-up
of the Company,  then,  and in each such case, the Company will mail or cause to
be mailed to the Registered Holder a notice specifying,  as the case may be, (i)
the record date for such  dividend,  distribution  or right,  and the amount and
character of such dividend, distribution or right, or (ii) the effective date on
which such reorganization,  reclassification,  consolidation,  merger, transfer,
dissolution, liquidation or winding-up is to take place, and the time, if any is
to be fixed,  as of which the  holders of record of Common  Stock (or such other
stock or securities at the time  deliverable  upon the exercise of this Warrant)
shall be entitled to exchange  their shares of Common Stock (or such other stock
or  securities)  for  securities  or  other  property   deliverable   upon  such
reorganization, reclassification,  consolidation, merger, transfer, dissolution,
liquidation or  winding-up.  Such notice shall be mailed at least ten days prior
to the record date or effective date for the event specified in such notice.

         8. STOCK FULLY PAID, RESERVATION OF STOCK.

     All  of the  Warrant  Shares  issuable  upon  the  exercise  of the  rights
represented  by this  Warrant  will,  upon  issuance and receipt of the purchase
price therefor,  be validly issued, fully paid and nonassessable,  and free from
all taxes,  liens and charges except for  restrictions on transfer  provided for
herein or under  applicable  federal and state securities laws. The Company will
at all times reserve and keep  available,  solely for issuance and delivery upon
the  exercise of this  Warrant,  such number of shares of Common Stock and other
securities,  cash and/or  property,  as from time to time shall be issuable upon
the exercise of this Warrant.

         9. EXCHANGE OF WARRANTS.

     Upon the  surrender  of this  Warrant by the  Registered  Holder,  properly
endorsed,  to the Company at the principal executive offices of the Company, the
Company will,  subject to the  provisions of Sections 4 and 5 hereof,  issue and
deliver to or upon the order of such Holder,  at the  Company's  expense,  a new
Warrant or Warrants of like tenor,  in the name of the  Registered  Holder or as
the Registered  Holder (upon payment by the Registered  Holder of any applicable
transfer  taxes)  may  direct,  calling  in the  aggregate  on the face or faces
thereof  for the  number of shares of Common  Stock (or other  securities,  cash
and/or property) then issuable upon exercise of this Warrant.

         10. REPLACEMENT OF WARRANTS.

     Upon  receipt of  evidence  reasonably  satisfactory  to the Company of the
loss, theft, destruction or mutilation of this Warrant and (in the case of loss,
theft or  destruction)  upon  delivery of an  indemnity  agreement  (without any
obligation  for a surety or other  security  therefor)  in an amount  reasonably
satisfactory to the Company,  or (in the case of mutilation)  upon surrender and
cancellation  of this Warrant,  the Company will issue,  in lieu thereof,  a new
Warrant of like tenor.

         11. TRANSFERS, ETC.

     (a) The Company will maintain a register containing the name and address of
the Registered  Holder of this Warrant.  The Registered Holder may change its or
his  address as shown on the warrant  register by written  notice to the Company
requesting such change.

     (b) This Warrant and all rights hereunder are transferable,  in whole or in
part,  upon surrender of this Warrant with a properly  executed  assignment in a
form reasonably  acceptable to the Company at the principal executive offices of
the Company.

     (c) Until any transfer of this Warrant is made in the Warrant register, the
Company may treat the  Registered  Holder as the  absolute  owner hereof for all
purposes;  provided, however, that if and when this Warrant is properly assigned
in blank,  the  Company  may (but  shall not be  obligated  to) treat the bearer
hereof as the absolute owner hereof for all purposes, notwithstanding any notice
to the contrary.

         12. MAILING OF NOTICES, ETC.

     All notices  and other  communications  from the Company to the  Registered
Holder shall be mailed by  first-class  certified or  registered  mail,  postage
prepaid,  to the  address  last  furnished  to the  Company  in  writing  by the
Registered  Holder.  All notices and other  communications  from the  Registered
Holder or in connection  herewith to the Company shall be mailed by  first-class
certified or registered mail,  postage prepaid,  to the Company at its principal
executive  office.  If the Company should at any time change the location of its
principal  executive  office to a place other than as set forth below,  it shall
give  prompt  written  notice  to  the  Registered  Holder  and  thereafter  all
references in this Warrant to the location of its principal  executive office at
the particular time shall be as so specified in such notice.

         13. NO RIGHTS AS STOCKHOLDER.

     Until the exercise of this Warrant, the Registered Holder shall not have or
exercise any rights by virtue hereof as a stockholder of the Company.

         15. CHANGE OR WAIVER.

     Any term of this Warrant may be amended or waived upon the written  consent
of the Company and the Registered Holder.

         16. SECTION HEADINGS.

     The Section headings in this Warrant are for the convenience of the parties
and  in  no  way  alter,  modify,  amend,  limit  or  restrict  the  contractual
obligations of the parties.

         17. GOVERNING LAW.

     This  Warrant  will be governed by and  construed  in  accordance  with the
internal laws of the State of Delaware  without giving effect to the conflict or
choice of law  provision  thereof  that would give rise the  application  of the
domestic substantive law of any other jurisdiction.

<PAGE>
         EXECUTED as of the Date of Issuance indicated above.

                                       BEACON POWER CORPORATION

                                       By:/s/James M. Spiezio
                                          ------------------------------------
                                          Name: James M. Spiezio
                                          Title: Chief Financial Officer

<PAGE>

                                    EXHIBIT I

                                  PURCHASE FORM

To:  Beacon Power Corporation                            Dated:  ____________

The undersigned, pursuant to the provisions set forth in the attached Warrant,
hereby irrevocably elects to purchase (check applicable box):

         |_|      _____ shares of the Common Stock covered by such Warrant; or

         |_|      the maximum number of shares of Common Stock covered by such
                  Warrant pursuant to the cashless exercise procedure set forth
                  in Section 1(b).

The undersigned herewith makes payment of the full Warrant Price for such shares
at the  price per  share  provided  for in such  Warrant,  which is $____.  Such
payment takes the form of (check applicable box or boxes):

         |_|      $______ in lawful money of the United States; and/or

         |_|      the cancellation of such number of Warrant Shares as is
                  necessary, in accordance with the formula set forth in Section
                  1(b), to exercise this Warrant with respect to the maximum
                  number of Warrant Shares purchasable pursuant to the cashless
                  exercise procedure set forth in Section 1(b).

Please issue a certificate representing said shares of Common Stock in the name
of the undersigned or in such other name as is specified below:

The undersigned is an "accredited investor" as defined in Regulation D
promulgated under the Securities Act of 1933, as amended.

                           Name of Entity:
                                          -------------------------------------

                           Signature of
                           Authorized Person:
                                             ----------------------------------

                           Address:
                                   --------------------------------------------

                           Date:
                                --------------------------------------------THE SECURITIES  EVIDENCED BY THIS WARRANT  CERTIFICATE  HAVE NOT BEEN REGISTERED
UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"), AND MAY NOT
BE SOLD,  TRANSFERRED,  OR ASSIGNED OR OFFERED FOR SALE OR OTHERWISE DISTRIBUTED
UNLESS THERE IS AN EFFECTIVE  REGISTRATION  STATEMENT  UNDER THE  SECURITIES ACT
COVERING SUCH SECURITIES, THE SALE IS MADE IN COMPLIANCE WITH RULE 144 UNDER THE
SECURITIES ACT, OR THE COMPANY  RECEIVES AN OPINION OF COUNSEL FOR THE HOLDER OF
SUCH SECURITIES  REASONABLY  SATISFACTORY TO THE COMPANY STATING THAT SUCH SALE,
TRANSFER OR ASSIGNMENT IS EXEMPT FROM THE REGISTRATION  AND PROSPECTUS  DELIVERY
REQUIREMENTS OF THE SECURITIES ACT.

THE SALE OR  TRANSFER  AND  CERTAIN  OTHER  RIGHTS  RELATING  TO THE  SECURITIES
REPRESENTED  BY THIS  DOCUMENT  ARE  SUBJECT  TO THE TERMS AND  CONDITIONS  OF A
CERTAIN  INVESTORS  RIGHTS  AGREEMENT  BY AND  AMONG  THE  COMPANY  AND  CERTAIN
STOCKHOLDERS  OF THE  COMPANY.  A COPY  OF  SUCH  AGREEMENT  IS ON  FILE  AT THE
PRINCIPAL OFFICE OF THE COMPANY.

No.: F-10

                              AMENDED AND RESTATED
                                     WARRANT
                            TO PURCHASE COMMON STOCK
                                       OF
                            BEACON POWER CORPORATION

                            (void after May 23, 2007)

          THIS AMENDED AND RESTATED WARRANT TO PURCHASE COMMON STOCK OF BEACON
POWER CORPORATION (the "Warrant") is entered into as of May 24, 2005 by and
between Beacon Power Corporation, a Delaware corporation (the "Company"), and
the Holder.

          WHEREAS, in connection with the execution of the Securities Purchase
Agreement (as defined below), the Company and the Holder entered into that
certain Warrant to Purchase Common Stock of the Company dated as of May 23, 2000
(the "Original Warrant") pursuant to which the Holder, or its assigns, is
entitled to purchase from the Company 1,333,333 shares of Common Stock (as
defined below) at any time before 5:00 p.m. New York City time on May 23, 2005
(the "Original Termination Date"); and

          WHEREAS, the Company and Beacon Power both desire to amend and restate
the Original Warrant to extend the Original Termination Date.

          NOW THEREFORE, the Company and Beacon Power agree, pursuant to Section
11 of the Original Warrant, that the Original Warrant is hereby amended and
restated in its entirety as follows:

         1. Issuance of Warrant. FOR VALUE RECEIVED, from and after the
Commencement Time, and subject to the terms and conditions herein set forth, the
Holder (as defined below) is entitled to purchase from the Company, at any time
before 5:00 p.m. New York City time on May 23, 2007 (the "Termination Date"),
the Warrant Stock (as defined below) at a price per share equal to the Warrant
Price (as defined below) upon exercise of this Warrant pursuant to Section 6
hereof.

         2. Definitions. As used in this Warrant, the following terms have the
definitions ascribed to them below:

(a) "Business Day" means any day other than a Saturday, Sunday or other day on
which the national or state banks located in the Commonwealth of Massachusetts,
the State of New York or the District of Columbia are authorized to be closed.

(b) "Commencement Time" means immediately prior to the consummation of a
Liquidity Event.

(c) "Common Stock" means the Company's Common Stock, $.01 par value per share.

(d) "Holder" means Perseus Capital, L.L.C., or its assigns.

(e) "Liquidity Event" means (i) the consummation of the sale or other
disposition of all or substantially all of the assets of the Company to another
Person or the acquisition of at least a majority of the outstanding voting stock
of the Company by another Person by means of a merger, consolidation or
otherwise, in either case on financial terms that reflect a total equity value
of at least $200 million for the Company immediately prior to the consummation
of such transaction (a "Sale Transaction") or (ii) the consummation of an
underwritten public offering of Common Stock pursuant to an effective
registration statement under the Securities Act of 1933, as amended, that (a)
results in the Company receiving at least $30 million in gross proceeds and (b)
is made at an initial offering price per share (the "Initial Price") that
reflects a market capitalization value of at least $200 million for the Company
(a "Qualified Public Offering").

(f) "Person" means any individual, corporation, partnership, limited liability
company, or other entity.

(g) "Securities Purchase Agreement" means the Securities Purchase Agreement
dated as of May 23, 2000 by and among the Company, Perseus Capital, L.L.C., DQE
Enterprises, Inc., Micro Generation Technology Fund, L.L.C., GE Capital Equity
Investments, Inc., Mechanical Technologies Incorporated, Penske Corporation, and
The Beacon Group, LLC (collectively, the "Purchasers").

(h) "Warrant Price" means 75% of the lower of (i) the effective price per share
of Common Stock paid by the acquirer in a Sale Transaction as determined by a
nationally recognized investment banking firm chosen by the Company and approved
by Purchasers that hold a majority of the aggregate Preferred Shares and
Conversion Shares (as such terms are defined in the Securities Purchase
Agreement) as of the date of consummation of such Sale Transaction and (ii) the
Initial Price in first Qualified Offering, in each case subject to adjustment
under Section 3.

(i) "Warrant Stock" means a number of shares of Common Stock equal to the
quotient obtained by dividing (i) 50% of the aggregate stated value of the
Preferred Shares acquired by the initial Holder of this Warrant pursuant to the
Securities Purchase Agreement by (ii) the Warrant Price, subject to adjustment
under Section 3.

(j) "Warrant Stock Class" means the class of capital stock or other securities
that includes the Warrant Stock.

         3. Adjustments and Notices. The Warrant Price and/or the Warrant Stock
shall be subject to adjustment from time to time in accordance with this Section
3. The Warrant Price and/or the Warrant Stock shall be adjusted to reflect all
of the following events that occur on or after the Commencement Time.

                  (a) Subdivision, Stock Dividends or Combinations. In case the
Company shall at any time subdivide the outstanding shares of the Warrant Stock
Class or shall issue a stock dividend with respect to the Warrant Stock Class,
the Warrant Price in effect immediately prior to such subdivision or the
issuance of such dividend shall be proportionately decreased, and in case the
Company shall at any time combine the outstanding shares of the Warrant Stock
Class, the Warrant Price in effect immediately prior to such combination shall
be proportionately increased, in each case effective at the close of business on
the date of such subdivision, dividend or combination, as the case may be.

                  (b) Reclassification, Exchange, Substitution, In-Kind
Distribution. Upon any reclassifications, exchange, substitution or other event
that results in a change of the number and/or class of the securities issuable
upon exercise or conversion of this Warrant or upon the payment of a dividend in
securities or property other than shares of the Warrant Stock Class, the Holder
shall be entitled to receive, upon exercise or conversion of this Warrant, the
number and kind of securities and property that Holder would have received if
this Warrant had been exercised or converted immediately before the record date
for such reclassification, exchange, substitution, or other event or immediately
prior to the record date for such dividend. The Company or its successor shall
promptly issue to Holder a new warrant for such new securities or other
property. The new warrant shall provide for adjustments which shall be as nearly
equivalent as may be practicable to the adjustments provided for in this Section
3 including, without limitation, adjustments to the Warrant Price and to the
number of securities or property issuable upon exercise or conversion of the new
warrant. The provisions of this Section 3(b) shall similarly apply to successive
reclassifications, exchanges, substitutions, or other events and successive
dividends.

                  (c) Reorganization, Merger etc. In case of any (i) merger or
consolidation of the Company into or with another corporation where the Company
is not the surviving corporation, (ii) sale, transfer or lease (but not
including a transfer or lease by pledge or mortgage to a bona fide lender) of
all or substantially all of the assets of the Company or (iii) sale by the
Company's shareholders of 50% or more of the Company's outstanding securities in
one or more related transactions, the Company, or such successor or purchasing
corporation, as the case may be, shall, as a condition to closing any such
reorganization, merger or sale, duly execute and deliver to the Holder hereof a
new warrant so that the Holder shall have the right to receive, at a total
purchase price not to exceed that payable upon the exercise or conversion of the
unexercised or unconverted portion of this Warrant, and in lieu of the shares of
Warrant Stock theretofore issuable upon exercise or conversion of this Warrant,
the kind and amount of shares of stock, other securities, money and property
receivable upon such reorganization, merger or sale by the Holder of the number
of shares of Warrant Stock then purchasable under this Warrant. Such new warrant
shall provide for adjustments that shall be as nearly equivalent as may be
practicable to the adjustments provided for in this Section 3. The provisions of
this subparagraph (c) shall similarly apply to successive transactions of the
type described in this subparagraph (c).

                  (d) Certificate of Adjustment. In each case of an adjustment
or readjustment of the Warrant Price, the Corporation, at its own expense, shall
cause its Chief Financial Officer to compute such adjustment or readjustment in
accordance with the provisions hereof and prepare a certificate showing such
adjustment or readjustment, and shall mail such certificate, by first class
mail, postage prepaid, to the Holder. The certificate shall set forth such
adjustment or readjustment, showing in detail the facts upon which such
adjustment or readjustment is based. No adjustment of the Warrant Price shall be
required to be made unless it would result in an increase or decrease of at
least one cent, but any adjustments not made because of this sentence shall be
carried forward and taken into account in any subsequent adjustment otherwise
required hereunder.

                  (e) No Impairment. The Company shall not, by amendment of its
Certificate of Incorporation or through a reorganization, transfer of assets,
consolidation, merger, dissolution, issue, or sale of securities or any other
voluntary action, avoid or seek to avoid the observance or performance of any of
the terms to be observed or performed under this Warrant by the Company, but
shall at all times in good faith assist in carrying out all of the provisions of
this Section 3 and in taking all such action as may be necessary or appropriate
to protect the Holder's rights under this Section 3 against impairment. If the
Company takes any action affecting the Warrant Stock Class other than as
described above that adversely affects the Holder's rights under this Warrant,
the Warrant Price shall be adjusted downward.

                  (f) Fractional Shares. No fractional shares shall be issuable
upon exercise or conversion of the Warrant and the number of shares to be issued
shall be rounded down to the nearest whole share. If a fractional share interest
arises upon any exercise or conversion of the Warrant, the Company shall
eliminate such fractional share interest by paying the Holder an amount computed
by multiplying the fractional interest by the fair market value of a full share.

         4. No Shareholder Rights. This Warrant, by itself, as distinguished
from any shares purchased hereunder, shall not entitle its Holder to any of the
rights of a shareholder of the Company.

         5. Reservation of Stock. The Company will reserve from its authorized
and unissued stock a sufficient number of shares to provide for the issuance of
Warrant Stock upon the exercise or conversion of this Warrant. Issuance of this
Warrant shall constitute full authority to the Company's officers who are
charged with the duty of executing stock certificates to execute and issue the
necessary certificates for shares of Warrant Stock issuable upon the exercise or
conversion of this Warrant.

         6. Exercise of Warrant. This Warrant may be exercised as a whole or in
part by the Holder, at any time from and after the Commencement Time and prior
to the termination of this Warrant, by the surrender of this Warrant, together
with the Notice of Exercise and Investment Representation Statement in the forms
attached hereto as Attachments 1 and 2, respectively, duly completed and
executed at the principal office of the Company, specifying the portion of the
Warrant to be exercised and accompanied by payment in full of the Warrant Price
in cash or by check with respect to the shares of Warrant Stock being purchased.
This Warrant shall be deemed to have been exercised immediately prior to the
close of business on the date of its surrender for exercise as provided above,
and the person entitled to receive the shares of Warrant Stock issuable upon
such exercise shall be treated for all purposes as the holder of such shares of
record as of the close of business on such date. As promptly as practicable
after such date, the Company shall issue and deliver to the person or persons
entitled to receive the same a certificate or certificates for the number of
full shares of Warrant Stock issuable upon such exercise. If this Warrant shall
be exercised for less than the total number of shares of Warrant Stock then
issuable upon exercise, promptly after surrender of this Warrant upon such
exercise, the Company will execute and deliver a new warrant, dated the date
hereof, evidencing the right of the Holder to the balance of the Warrant Stock
purchasable hereunder upon the same terms and conditions set forth herein.

         7. Conversion. In lieu of exercising this Warrant or any portion
hereof, at any time from and after the Commencement Time and prior to the
termination of this Warrant, the Holder hereof shall have the right to convert
this Warrant or any portion hereof into Warrant Stock by executing and
delivering to the Company at its principal office the written Notice of
Conversion and Investment Representation Statement in the forms attached hereto
as Attachments 2 and 3, specifying the portion of the Warrant to be converted,
and accompanied by this Warrant. The number of shares of Warrant Stock to be
issued to Holder upon such conversion shall be computed using the following
formula:

X=(P)(Y)(A-B)/A

where

X  = the number of shares of Warrant  Stock to be issued to the Holder for the
     portion of the Warrant being converted.

P  = the  portion  of the  Warrant  being  converted  expressed  as a  decimal
     fraction.

Y  = the total number of shares of Warrant Stock issuable upon exercise of the
     Warrant in full.

A  = the fair market value of one share of the Warrant Stock Class which means
     (i) the fair market value of one share of the Warrant Stock Class as of the
     last Business Day immediately prior to the date the notice of conversion is
     received  by the  Company,  as reported  in the  principal  market for such
     securities or, if no such market exists, as determined in good faith by the
     Company's Board of Directors, or (ii) if this Warrant is being converted in
     conjunction  with a Liquidity  Event,  133.33% of the Warrant Price then in
     effect.

B  = the Warrant Price on the date of conversion.

Any portion of this Warrant that is converted shall be immediately canceled.
This Warrant or any portion hereof shall be deemed to have been converted
immediately prior to the close of business on the date of its surrender for
conversion as provided above, and the person entitled to receive the shares of
Warrant Stock issuable upon such conversion shall be treated for all purposes as
the holder of such shares of record as of the close of business on such date. As
promptly as practicable after such date, the Company shall issue and deliver to
the person or persons entitled to receive the same a certificate or certificates
for the number of full shares of Warrant Stock issuable upon such conversion. If
the Warrant shall be converted for less than the total number of shares of
Warrant Stock then issuable upon conversion, promptly after surrender of the
Warrant upon such conversion, the Company will execute and deliver a new
warrant, dated the date hereof, evidencing the right of the Holder to the
balance of the Warrant Stock purchasable hereunder upon the same terms and
conditions set forth herein. If this Warrant is converted, as a whole or in
part, after the occurrence of a transaction as to which Section 3(c) is
applicable, the Holder shall receive the consideration contemplated by Section
3(c) in lieu of Warrant Stock of the Company.

         8. Notice of Liquidity Event.The Company shall provide the Holder with
at least 10 Business Days advance written notice prior to the consummation of
any Liquidity Event describing in reasonable detail the terms and conditions of
such Liquidity Event and an estimate of the Warrant Price derivable from such
Liquidity Event.

         9. Transfer of Warrant. This Warrant may be transferred or assigned by
the Holder hereof as a whole or in part, provided that the transferor provides,
at the Company's request, an opinion of counsel satisfactory to the Company that
such transfer does not require registration under the Securities Act and the
securities law applicable with respect to any other applicable jurisdiction.

         10. Termination. This Warrant shall terminate at 5:00 p.m. New York
City time on the Termination Date.

         11. Miscellaneous. This Warrant shall be governed by the laws of the
State of Delaware, as such laws are applied to contracts to be entered into and
performed entirely in Delaware by Delaware residents. The headings in this
Warrant are for purposes of convenience and reference only, and shall not be
deemed to constitute a part hereof. Neither this Warrant nor any term hereof may
be changed or waived orally, but only by an instrument in writing signed by the
Company and the Holder of this Warrant. All notices and other communications
from the Company to the Holder of this Warrant shall be delivered personally or
by facsimile transmission or mailed by first class mail, postage prepaid, to the
address or facsimile number furnished to the Company in writing by the last
Holder of this Warrant who shall have furnished an address or facsimile number
to the Company in writing, and if mailed shall be deemed given three days after
deposit in the United States mail.

<PAGE>

         EXECUTED as of the Date of Issuance indicated above.

                                       BEACON POWER CORPORATION

                                       By:/s/James M. Spiezio
                                       Name:James M. Spiezio
                                       Title:Chief Financial Officer

<PAGE>

                                                        Attachment 1

NOTICE OF EXERCISE

TO:      BEACON POWER CORPORATION

1.   The undersigned hereby elects to purchase _______________ shares of Warrant
     Stock of Beacon Power Corporation pursuant to the terms of the attached
     Warrant, and tenders herewith payment of the purchase price in full,
     together with all applicable transfer taxes, if any.

2.   Please issue a certificate or certificates representing said shares of
     Warrant Stock in the name of the undersigned or in such other name as is
     specified below:

                                          ------------------------------
                                     (Name)

                                          -------------------------------
                                    (Address)

---------------------------------------     -----------------------------------
(Date) (Name of Warrant Holder)

                                       By:________________________________

                                       Title:_______________________________

<PAGE>

                                                               Attachment 2

                       INVESTMENT REPRESENTATION STATEMENT

         In connection with the purchase of the shares of Warrant Stock upon
exercise of the enclosed Warrant, the undersigned hereby represents to Beacon
Power Corporation (the "Company") as follows:

(a) The securities to be received upon the exercise of the Warrant (the
"Securities") will be acquired for investment for its own account, not as a
nominee or agent, and not with a view to the sale or distribution of any part
thereof, and the undersigned has no present intention of selling, granting
participation in or otherwise distributing the same, but subject, nevertheless,
to any requirement of law that the disposition of its property shall at all
times be within its control. By executing this statement, the undersigned
further represents that it does not have any contract, undertaking, agreement or
arrangement with any person to sell, transfer, or grant participations to such
person or to any third person, with respect to any Securities issuable upon
exercise of the Warrant.

(b) The undersigned understands that the Securities issuable upon exercise of
the Warrant at the time of issuance may not be registered under the Securities
Act of 1933, as amended (the "Securities Act"), and applicable state securities
laws, on the ground that the issuance of such securities is exempt pursuant to
Section 4(2) of the Securities Act and state law exemptions relating to offers
and sales not by means of a public offering, and that the Company's reliance on
such exemptions is predicated on the undersigned's representations set forth
herein.

(c) The undersigned agrees that in no event will it make a disposition of any
Securities acquired upon the exercise of the Warrant unless and until (i) it
shall have notified the Company of the proposed disposition and shall have
furnished the Company with a statement of the circumstances surrounding the
proposed disposition, and (ii) it shall have furnished the Company with an
opinion of counsel satisfactory to the Company and Company's counsel to the
effect that (A) appropriate action necessary for compliance with the Securities
Act and any applicable state securities laws has been taken or an exemption from
the registration requirements of the Securities Act and such laws is available,
and (B) the proposed transfer will not violate any of said laws.

(d) The undersigned acknowledges that an investment in the Company is highly
speculative and represents that it is able to fend for itself in the
transactions contemplated by this statement, has such knowledge and experience
in financial and business matters as to be capable of evaluating the merits and
risks of its investments, and has the ability to bear the economic risks
(including the risk of a total loss) of its investment. The undersigned
represents that it has had the opportunity to ask questions of the Company
concerning the Company's business and assets and to obtain any additional
information which it considered necessary to verify the accuracy of or to
amplify the Company's disclosures, and has had all questions which have been
asked by it satisfactorily answered by the Company

(e) The undersigned acknowledges that the Securities issuable upon exercise or
conversion of the Warrant must be held indefinitely unless subsequently
registered under the Securities Act or an exemption from such registration is
available. The undersigned is aware of the provisions of Rule 144 promulgated
under the Securities Act which permit limited resale of shares purchased in a
private placement subject to the satisfaction of certain conditions, including,
among other things, the existence of a public market for the shares, the
availability of certain current public information about the Company, the resale
occurring not less than one year after a party has purchased and paid for the
security to be sold from the Company or any affiliate of the Company, the sale
being through a "broker's transaction" or in transactions directly with a
"market maker" (as provided by Rule 144(f)) and the number of shares being sold
during any three month period not exceeding specified limitations.

         Dated:________________________

         ---------------------------------------
         (Typed or Printed Name)

         By:____________________________________
              (Signature)

         ---------------------------------------
         (Name)
         ---------------------------------------
         (Title)

<PAGE>

                                                              Attachment 3

NOTICE OF CONVERSION

TO:      BEACON POWER CORPORATION

1. The undersigned hereby elects to acquire _______________ shares of the
Warrant Stock of Beacon Power Corporation pursuant to the terms of the attached
Warrant, by conversion of _________ percent (_____%) of the Warrant.

2. Please issue a certificate or certificates representing said shares of
Warrant Stock in the name of the undersigned or in such other name as is
specified below:

                                          ------------------------------
                                     (Name)

                                          -------------------------------
                                    (Address)

---------------------------------------     -----------------------------------
(Date) (Name of Warrant Holder)

                                       By:________________________________

                                       Title:_______________________________
                                    (Title and signature of authorized person)

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