Exhibit 10.4

                                PLEDGE AGREEMENT


         PLEDGE AGREEMENT, dated as of September 17, 2001, executed and
delivered by William E. Stanton ("Pledgor"), to Beacon Power Corporation
("Pledgee") under the below-described Note.

                              W I T N E S S E T H :
                               - - - - - - - - - -

         WHEREAS, Pledgor is the record and beneficial owner of the shares of
Common Stock and options to purchase Common Stock described in Exhibit A hereto
(the "Pledged Securities") and issued by the Pledgee; and

         WHEREAS, Pledgor has executed a Note of even date herewith (as amended,
modified, supplemented and restated from time to time, the "Note"), pursuant to
which the Pledgee has agreed to make certain loans to the Pledgor (the
"Obligations"); and

         WHEREAS, in accordance with the terms of the Note, and as security for
all of the amounts outstanding thereunder, the Pledgee is requiring that Pledgor
execute and deliver this Pledge Agreement and grant the security interest
contemplated hereby.

         NOW, THEREFORE, in consideration of the premises and the covenants
hereinafter contained, and to induce the Pledgee to make the loans under the
Note, it is agreed as follows:

         1. Definitions. Unless otherwise defined herein, terms defined in the
Note are used herein as therein defined, and the following shall have (unless
otherwise provided elsewhere in this Pledge Agreement) the following respective
meanings (such meanings being equally applicable to both the singular and plural
form of the terms defined):

         "Agreement" shall mean this Pledge Agreement, including all amendments,
modifications and supplements and any exhibits or schedules to any of the
foregoing, and shall refer to the Agreement as the same may be in effect at the
time such reference becomes operative.

         "Bankruptcy Code" shall mean Title 11, United States Code, as amended
from time to time, and any successor statute thereto.

         "Event of Default" shall mean any of the following events:

                  (a) there shall occur any "Event of Default" under the Note,
         as such term is defined therein, or demand shall be made for payment of
         all principal and interest thereunder and such payment shall remain
         unpaid for 10 or more days;

                  (b) any of the Pledged Collateral shall be attached or levied
         upon or seized in any legal proceedings, or held by virtue of any lien
         or distress;

                  (c) Pledgor shall default in the observance or performance of
         any covenant or agreement set forth in this Agreement or the Note and
         such default shall continue for thirty (30) days or more after written
         notice from Pledgee; or

                  (d) Pledgor makes any representations or warranties in this
         Agreement or in any certificate or statement furnished at any time
         hereunder or thereunder or in connection herewith or therewith which
         proves to have been untrue or misleading in any material respect when
         made or furnished and which continues to be untrue or misleading in any
         material respect.

         "Pledged Collateral" shall have the meaning assigned to such term in
Section 2 hereof.

         2. Pledge.  Pledgor hereby pledges, conveys, hypothecates, mortgages,
assigns, sets over, delivers and grants to the Pledgee a security interest in
all of the following (collectively, the "Pledged Collateral"):

                  2.1 the Pledged Securities, all Investment Property (as such
term is defined in the Uniform Commercial Code) in connection with the Pledged
Securities, all general intangibles related to the Pledged Securities, the
certificates and agreements, if any, representing the Pledged Securities, and
all dividends, distributions, cash, instruments and other property or proceeds
from time to time received, receivable or otherwise distributed in respect of or
in exchange for any or all of the Pledged Securities; and

                  2.2 all additional shares or options to purchase shares or
other Investment Property of Pledgee from time to time acquired by Pledgor in
any manner (which interests shall be deemed to be part of the Pledged
Securities), and the certificates and agreements, if any, representing such
additional interests, and all dividends, distributions, cash, instruments and
other property or proceeds from time to time received, receivable or otherwise
distributed in respect of or in exchange for any or all of such interests; and

                  2.3 all proceeds of any of the foregoing.

         3. Security for Obligations.  This Agreement secures, and the Pledged
Collateral is security for, the payment and performance of all of the
Obligations.

         4. Delivery of Pledged Collateral. All certificates and agreements
representing or evidencing the Pledged Securities shall be delivered to and held
by or on behalf of the Pledgee pursuant hereto and shall be accompanied by duly
executed instruments of transfer or assignment in blank, all in form and
substance satisfactory to the Pledgee. The Pledgor agrees to deliver to the
Pledgee all stock certificates representing the Shares immediately following
release of such certificates from SSB. The Pledgee shall have the right, in its
discretion and without notice to Pledgor at any time after the occurrence and
during the continuance of an Event of Default, to transfer to or to register in
the name of the Pledgee, or any of its nominees, subject to the terms of this
Agreement, any or all of the Pledged Securities. In addition, the Pledgee shall
have the right at any time following an Event of Default to exchange
certificates or instruments representing or evidencing Pledged Securities for
certificates or instruments of smaller or larger denominations.

         5. Representations and Warranties.  Pledgor represents and warrants to
the Pledgee that:

                  5.1 Pledgor is, and at the time of delivery of the Pledged
Securities to the Pledgee pursuant to Section 4 hereof will be, the sole holder
of record and the sole beneficial owner of the Pledged Collateral free and clear
of any Lien thereon or affecting the title thereto except for the Lien created
by this Agreement (and prior to repayment of the SSB Loan, a lien in favor of
SSB).

                  5.2 The Pledged Securities are fully paid and non-assessable.

                  5.3 None of the Pledged Securities has been issued or
transferred in violation of the securities registration, securities disclosure
or similar laws of any jurisdiction to which such issuance or transfer may be
subject. Pledgor's execution and delivery of this Agreement and the pledge of
the Pledged Collateral hereunder do not, directly or indirectly, violate or
result in a violation of any such laws.

                  5.4 No consent, approval, authorization or other order of any
Person and no consent, authorization, approval, or other action by, and no
notice to or filing with, any governmental departments, commissions, boards,
bureaus, agencies or other instrumentalities, domestic or foreign, is required
to be made or obtained by Pledgor either (a) for the pledge of the Pledged
Collateral pursuant to this Agreement or for the execution, delivery or
performance of this Agreement by Pledgor or (b) for the exercise by the Pledgee
of the remedies in respect of the Pledged Collateral pursuant to this Agreement,
except as may be required in connection with such disposition by laws affecting
the offering and sale of securities generally and any restrictions imposed on
the pledged Options under any applicable plan or agreement.

                  5.5 Upon delivery of the Pledged Collateral to Pledgee upon
release from SSB, the pledge, assignment and delivery of the Pledged Collateral
pursuant to this Agreement will create a valid first priority Lien on and a
first priority perfected security interest in the Pledged Collateral pledged by
Pledgor, and the proceeds thereof, securing the payment of the Obligations,
subject to no other Lien.

                  5.6 This Agreement has been duly executed and delivered by
Pledgor and constitutes a legal, valid and binding obligation of Pledgor
enforceable in accordance with its terms, except as enforceability may be
limited by bankruptcy, insolvency, or other similar laws affecting the rights of
creditors generally or by the application of general equity principles.

         The representations and warranties set forth in this Section 5 shall
survive the execution and delivery of this Agreement.

         6. Covenants. Pledgor covenants and agrees that until the  indefeasible
payment in full of the Obligations:

                  6.1 Except as provided herein, without the prior written
consent of the Pledgee, Pledgor will not sell, assign, transfer, pledge, or
otherwise encumber any of its rights in or to the Pledged Collateral or any
unpaid dividends or other distributions or payments with respect thereto or
grant a Lien therein.

                  6.2 Pledgor will, at its expense, promptly execute,
acknowledge and deliver all such instruments and take all such action as the
Pledgee from time to time may reasonably request in order to ensure to the
Pledgee the benefits of the Liens in and to the Pledged Collateral intended to
be created by this Agreement, including the filing of any necessary or desirable
Uniform Commercial Code financing statements, which may be filed by the Pledgee
with or without the signature of Pledgor, and will cooperate with the Pledgee,
at Pledgor's expense, in obtaining all necessary approvals and making all
necessary filings under federal or state law in connection with such Liens or
any sale or transfer of the Pledged Collateral.

                  6.3 Pledgor has and will defend the title to the Pledged
Collateral and the Liens of the Pledgee thereon against the claim of any Person
and will maintain and preserve such Liens.

                  6.4 Pledgor will, upon obtaining any additional shares of
stock or options to purchase stock of the Pledgee which are not already Pledged
Collateral, promptly (and in any event within three (3) Business Days) deliver
to the Pledgee such additional Pledged Securities pursuant to this Agreement.
Pledgor hereby agrees that all shares of stock or options to purchase stock so
delivered to the Pledgee shall for all purposes hereunder be considered Pledged
Securities or Pledged Collateral, as applicable.

                  6.5 Pledgor will pay all taxes, assessments and charges
levied, assessed or imposed upon the Pledged Collateral owned by it before the
same become delinquent or become Liens upon any of the Pledged Collateral except
where such taxes, assessments and charges may be contested in good faith by
appropriate proceedings.

                  6.6 Pledgor will not create, grant or suffer to exist any Lien
on any of the Pledged Collateral except those in favor of the Pledgee.

         7. Distributions; Etc.

                  7.1 Delivery by Pledgor of Distributions. Until the
Obligations are paid in full, the Pledgee shall have the sole and exclusive
right to receive and retain any distributions on the Pledged Collateral which
Pledgor would otherwise be authorized to receive and retain. In such event,
Pledgor shall pay over to the Pledgee any distributions received by Pledgor with
respect to the Pledged Collateral and any and all money and other property paid
over to or received by the Pledgee pursuant to the provisions of this Section
7.1 shall be applied to the repayment of the Obligations in accordance with the
provisions hereof. Any and all stock or liquidating distributions, other
distributions in property, return of capital or other distributions made on or
in respect of Pledged Collateral, whether resulting from a subdivision,
combination or reclassification of the outstanding capital stock of Pledgee or
received in exchange for Pledged Collateral or any part thereof or as a result
of any merger, consolidation, acquisition or other exchange of assets to which
Pledgee may be a party or otherwise, shall be and become part of the Pledged
Collateral pledged hereunder and, if received by Pledgor, shall be received in
trust for benefit of the Pledgee, be segregated from the other property and
funds of Pledgor, and shall forthwith be delivered to the Pledgee to be held
subject to the terms of this Agreement.

                  7.2 Holding Pledged Collateral; Exchanges. The Pledgee may
hold any of the Pledged Collateral, endorsed or assigned in blank, and following
an Event of Default, may make denominational exchanges or registrations or
transfers or for such other reasonable purpose in furtherance of this Agreement
as the Pledgee may deem desirable. The Pledgee shall have the right, if
necessary to perfect its security interest, to transfer to or register in the
name of the Pledgee or any of its nominees, any or all of the Pledged
Collateral; provided that notwithstanding the foregoing, until any transfer of
beneficial ownership with respect to the Pledged Collateral pursuant to any
exercise of remedies under Section 8 hereof, Pledgor shall continue to be the
beneficial owner of the Pledged Collateral. In addition, the Pledgee shall have
the right at any time following the occurrence and continuance of an Event of
Default to exchange certificates or instruments representing or evidencing
Pledged Collateral for certificates or instruments of smaller or larger
denominations.

         8. Remedies.  Upon and after an Event of Default, the Pledgee shall
have the following rights and remedies:

                  8.1 Secured Creditor. All of the rights and remedies of a
secured party under the Uniform Commercial Code of the State where such rights
and remedies are asserted, or under other applicable law, all of which rights
and remedies shall be cumulative, and none of which shall be exclusive, to the
extent permitted by law, in addition to any other rights and remedies contained
in this Agreement.

                  8.2 Retirement of Pledged Securities. The Pledgee may, without
demand and without advertisement, notice or legal process of any kind (except as
may be required by law), all of which Pledgor waives, at any time or times (a)
apply any cash distributions received by the Pledgee pursuant to Section 7.1
hereof to the Obligations, if not previously applied, and (b) if following such
application there remains outstanding any Obligations, foreclose on the Pledged
Securities and retire all or any part of the Pledged Securities as treasury
stock. Upon foreclosure and retirement of all or any part of the Pledged
Securities, the Pledgee shall calculate the value of the Pledged Securities at
the average of the closing sales prices of the Pledgee's common stock on NASDAQ
for the 10 trading days preceding the date of foreclosure. The value of the
Pledged Securities shall be applied by the Pledgee, without any marshaling of
assets, in the following order:

                  (a) first, to the payment of all of the reasonable costs and
         expenses of such foreclosure and retirement, including, without
         limitation, reasonable legal fees, and all other expenses, liabilities
         and advances made or incurred by the Pledgee in connection therewith
         (including, without limitation, costs and expenses incurred in
         connection with any bankruptcy, reorganization or insolvency
         proceeding);

                  (b) second, to the payment of the Obligations as set forth in
the Note, until payment in full thereof; and

                  (c) third, any remaining Pledged Securities shall be returned
to the Pledgor.

                  8.3 Notice. In addition thereto, Pledgor further agrees that
in the event that notice is necessary under applicable law, written notice
mailed to Pledgor in the manner specified in Section 16 hereof ten (10) days
prior to the date of the foreclosure and retirement, shall constitute
commercially reasonable and fair notice.

                  8.4 Waiver of Certain Rights. Pledgor agrees that following
the occurrence and during the continuance of an Event of Default it will not at
any time plead, claim or take the benefit of any appraisal, valuation, stay,
extension, moratorium or redemption law now or hereafter in force in order to
prevent or delay the enforcement of this Agreement, or the retirement by Pledgee
of the Pledged Securities, and Pledgor waives the benefit of all such laws to
the extent it lawfully may do so. Pledgor agrees that it will not interfere with
any right, power or remedy of the Pledgee provided for in this Agreement or now
or hereafter existing at law or in equity or by statute or otherwise, or the
exercise or beginning of the exercise by the Pledgee of any one or more of such
rights, powers or remedies. No failure or delay on the part of the Pledgee to
exercise any such right, power or remedy and no notice or demand which may be
given to or made upon Pledgor by the Pledgee with respect to any such remedies
shall operate as a waiver thereof, or limit or impair the Pledgee's right to
take any action or to exercise any power or remedy hereunder, without notice or
demand, or prejudice its rights as against Pledgor in any respect.

                  8.5 Specific Performance. Pledgor further agrees that a breach
of any of the covenants contained in this Section 8 will cause irreparable
injury to the Pledgee, that the Pledgee has no adequate remedy at law in respect
of such breach and, as a consequence, agrees that each and every covenant
contained in this Section 8 shall be specifically enforceable against Pledgor,
and Pledgor hereby waives and agrees not to assert any defenses against an
action for specific performance of such covenants except for a defense that the
Obligations are not then due and payable in accordance with the agreements and
instruments governing and evidencing such obligations.

         9. Power of Attorney; Proxy.

                  9.1 Upon and after an Event of Default and during the
continuance of such Event of Default, Pledgor irrevocably designates, makes,
constitutes and appoints the Pledgee (and all Persons designated by the Pledgee)
as its true and lawful attorney (and agent-in-fact) and the Pledgee, or the
Pledgee's agent, may, without notice to Pledgor, and at such time or times
thereafter as the Pledgee or said agent, in its discretion, may determine, in
the name of Pledgor or the Pledgee: (a) transfer the Pledged Collateral on the
books of the issuer thereof, with full power of substitution in the premises;
(b) endorse the name of Pledgor upon any checks, notes, acceptance, money
orders, certificates, drafts or other forms of payment of security that come
into the Pledgee's possession to the extent they constitute Pledged Collateral;
and (c) do all acts and things necessary, in the Pledgee's discretion, to
fulfill the obligations of Pledgor under this Agreement.

                  9.2 Upon the occurrence, and during the continuance, of any
Event of Default hereunder, the Pledgee, or its nominee, without notice or
demand of any kind to Pledgor, shall have the sole and exclusive right to
exercise all voting powers pertaining to any and all of the Pledged Collateral
(and to give written consents in lieu of voting thereon) and may exercise such
power in such manner as the Pledgee, in its sole discretion, shall determine.
THIS PROXY IS COUPLED WITH AN INTEREST AND IS IRREVOCABLE. The exercise by the
Pledgee of any of its rights and remedies under this Section shall not be deemed
a disposition of Pledged Collateral under Article 9 of the Uniform Commercial
Code nor an acceptance by the Pledgee of any of the Pledged Collateral in
satisfaction of any of the Obligations.

         10. Waiver. No delay on the Pledgee's part in exercising any right
hereunder, and no notice or demand which may be given to or made upon Pledgor by
the Pledgee with respect to any right hereunder, shall constitute a waiver
thereof, or limit or impair the Pledgee's right to take any action or to
exercise any right hereunder, without notice or demand, or prejudice the
Pledgee's rights as against Pledgor in any respect.

         11. Termination. This Agreement shall terminate and be of no further
force or effect at such time as the Obligations shall be paid in full. Upon such
termination of this Agreement, the Pledgee shall deliver to Pledgor the Pledged
Collateral at the time subject to this Agreement and then in the Pledgee's
possession or control and all instruments of assignment executed in connection
therewith, free and clear of the Liens hereof and, except as otherwise provided
herein, all of Pledgor's obligations hereunder shall at such time terminate.

         12. Reinstatement. This Agreement shall remain in full force and effect
and continue to be effective should any petition be filed by or against Pledgor
for liquidation or reorganization, should Pledgor become insolvent or make an
assignment for the benefit of creditors or should a receiver or trustee be
appointed for all or any significant part of Pledgor's assets, and shall
continue to be effective or be reinstated, as the case may be, if at any time
payment and performance of the Obligations, or any part thereof, is, pursuant to
applicable law, rescinded or reduced in amount, or must otherwise be restored or
returned by any obligee of the Obligations, whether as a "voidable preference",
"fraudulent conveyance", or otherwise, all as though such payment or performance
had not been made. In the event that any payment, or any part thereof, is
rescinded, reduced, restored or returned, the Obligations shall be reinstated
and deemed reduced only by such amount paid and not so rescinded, reduced,
restored or returned.

         13. Miscellaneous. This Agreement shall be binding upon Pledgor and its
successors and assigns, and shall inure to the benefit of, and be enforceable
by, the Pledgee and its successors and assigns, and shall be governed by, and
construed and enforced as a sealed instrument in accordance with, the internal
laws in effect in the Commonwealth of Massachusetts, and none of the terms or
provisions of this Agreement may be waived, altered, modified or amended except
in writing duly signed for and on behalf of the Pledgee and Pledgor.

         14. Severability.  If for any reason any provision or provisions hereof
are determined to be invalid and contrary to any existing or future law, such
invalidity shall not impair the operation of or effect those portions of this
Agreement which are valid.

         15. Notices. Except as otherwise provided herein, whenever it is
provided herein that any notice, demand, request, consent, approval, declaration
or other communication shall or may be given to or served upon any of the
parties by any other party, or whenever any of the parties desires to give or
serve upon any other a communication with respect to this Agreement, each such
notice, demand, request, consent, approval, declaration or other communication
shall be in writing and shall be delivered in accordance with the terms of the
Note.

         16. Section Titles.  The Section titles contained in this Agreement are
and shall be without substantive meaning or content of any kind whatsoever and
are not a part of the agreement between the parties hereto.

         17. Counterparts.  This Agreement may be executed in any number of
counterparts, which shall, collectively and separately, constitute one
agreement.

    IN WITNESS WHEREOF, this Agreement has been duly executed as of the date
first written above.


                                  ----------------------------
                                  William E. Stanton

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                                    EXHIBIT A

     Attached to and forming a part of that certain Pledge Agreement dated as of
September 17, 2001 executed and delivered by William E. Stanton,  as Pledgor, to
Beacon Power Corporation, as Pledgee.



Outstanding Common Stock                        Outstanding Stock Options

     306,668                                             963,332



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