Document:

<PAGE>

                                                                   Exhibit 10.41

                                PROMISSORY NOTE
                                ---------------

                                                                 October 6, 1999
$10,000.00                               Cambridge, Massachusetts

FOR VALUE RECEIVED, Michael C. Turmelle (the "Maker"), promises to pay to SatCon
Technology Corporation (the "Corporation"), or order, at the offices of the
Corporation, located at 161 First Street, Cambridge, Massachusetts 02142, or at
such other place as the holder of this Note may designate, the principal sum of
TEN THOUSAND DOLLARS ($10,000) together with interest on the unpaid principal
balance of this Note from time to time outstanding at the rate of 5.74% per year
until paid in full.  Principal and interest shall be due and payable in full on
December 15, 2000.

Interest on this Note shall be computed on the basis of a year of 365 days for
the actual number of days elapsed.  All payments by the Maker under this Note
shall be in immediately available funds.

This Note shall become immediately due and payable without notice or demand upon
the occurrence at any time of any of the following events of default
(individually, "an Event of Default" and collectively, "Events of Default"):

     (1)  default in the payment or performance of this or any other liability
          or obligation of the Maker to the holder, including the payment when
          due of any principal, premium or interest under this Note;

     (2)  the liquidation, termination of existence, dissolution, insolvency or
          business failure of the Maker, or the appointment of a receiver or
          custodian for the Maker or any part of its property if such
          appointment is not terminated or dismissed within thirty (30) days;

     (3)  the institution against the Maker or any indorser or guarantor of this
          Note of any proceedings under the United States Bankruptcy Code or any
          other federal or state bankruptcy, reorganization, receivership,
          insolvency or other similar law affecting the rights of creditors
          generally, which proceeding is not dismissed within thirty (30) days
          of filing;

     (4)  the institution by the Maker or any indorser or guarantor of this Note
          of any proceedings under the United States Bankruptcy Code or any
          other federal or state bankruptcy, reorganization, receivership,
          insolvency or other similar law affecting the rights of creditors
          generally or the making by the Maker or any indorser or guarantor of
          this Note of a composition or an assignment or trust mortgage for the
          benefit of creditors; or

     (5)  determination by the holder that it is insecure with respect to the
          payment of any obligation of the Maker to the holder.
<PAGE>

Upon the occurrence of an Event of Default, the holder shall have then, or at
any time thereafter, all of the rights and remedies afforded by the Uniform
Commercial Code as from time to time in effect in the Commonwealth of
Massachusetts or afforded by other applicable law.

Every amount overdue under this Note shall bear interest from and after the date
on which such amount first became overdue at an annual rate which is two (2)
percentage points above the rate per year specified in the first paragraph of
this Note.  Such interest on overdue amounts under this Note shall be payable on
demand and shall accrue and be compounded monthly until the obligation of the
Maker with respect to the payment of such interest has been discharged (whether
before or after judgment).

In no event shall any interest charged, collected or reserved under this Note
exceed the maximum rate then permitted by applicable law and if any such payment
is paid by the Maker, then such excess sum shall be credited by the holder as a
payment of principal.

All payments by the Maker under this Note shall be made without set-off or
counterclaim and be free and clear and without any deduction or withholding for
any taxes or fees of any nature whatever, unless the obligation to make such
deduction or withholding is imposed by law.  The Maker shall pay and save the
holder harmless from all liabilities with respect to or resulting from any delay
or omission to make any such deduction or withholding required by law.

Whenever any amount is paid under this Note, all or part of the amount paid may
be applied to principal, premium or interest in such order and manner as shall
be determined by the holder in its discretion.

No reference in this Note to any guaranty or other document shall impair the
obligation of the Maker, which is absolute and unconditional, to pay all amounts
under this Note strictly in accordance with the terms of this Note.

The Maker agrees to pay on demand all costs of collection, including reasonable
attorneys' fees, incurred by the holder in enforcing the obligations of the
Maker under this Note.

No delay or omission on the part of the holder in exercising any right under
this Note shall operate as a waiver of such right or of any other right of such
holder, nor shall any delay, omission or waiver on any one occasion be deemed a
bar to or waiver of the same or any other right on any future occasion.  The
Maker and every indorser or guarantor of this Note regardless of the time, order
or place of signing waives presentment, demand, protest and notices of every
kind and assents to any extension or postponement of the time of payment or any
other indulgence, to any substitution, exchange or release of collateral, and to
the addition or release of any other party or person primarily or secondarily
liable.

This Note may be prepaid in whole or in part at any time or from time to time.
Any such prepayment shall be without premium or penalty.

None of the terms or provisions of this Note may be excluded, modified or
amended except by a written instrument duly executed on behalf of the holder
expressly referring to this Note and setting forth the provision so excluded,
modified or amended.

                                       2
<PAGE>

All rights and obligations hereunder shall be governed by the laws of the
Commonwealth of Massachusetts and this Note is executed as an instrument under
seal.

                                         /s/ Michael C. Turmelle
                                         -------------------------
                                         Michael C. Turmelle

                                       3
<PAGE>

                                PROMISSORY NOTE
                                ---------------

                                                                December 6, 1999
$75,000.00                               Cambridge, Massachusetts

FOR VALUE RECEIVED, Michael C. Turmelle (the "Maker"), promises to pay to SatCon
Technology Corporation (the "Corporation"), or order, at the offices of the
Corporation, located at 161 First Street, Cambridge, Massachusetts 02142, or at
such other place as the holder of this Note may designate, the principal sum of
SEVENTY-FIVE THOUSAND DOLLARS ($75,000) together with interest on the unpaid
principal balance of this Note from time to time outstanding at the rate of
5.74% per year until paid in full.  Principal and interest shall be due and
payable in full on December 15, 2000.

Interest on this Note shall be computed on the basis of a year of 365 days for
the actual number of days elapsed.  All payments by the Maker under this Note
shall be in immediately available funds.

This Note shall become immediately due and payable without notice or demand upon
the occurrence at any time of any of the following events of default
(individually, "an Event of Default" and collectively, "Events of Default"):

     (1)  default in the payment or performance of this or any other liability
          or obligation of the Maker to the holder, including the payment when
          due of any principal, premium or interest under this Note;

     (2)  the liquidation, termination of existence, dissolution, insolvency or
          business failure of the Maker, or the appointment of a receiver or
          custodian for the Maker or any part of its property if such
          appointment is not terminated or dismissed within thirty (30) days;

     (3)  the institution against the Maker or any indorser or guarantor of this
          Note of any proceedings under the United States Bankruptcy Code or any
          other federal or state bankruptcy, reorganization, receivership,
          insolvency or other similar law affecting the rights of creditors
          generally, which proceeding is not dismissed within thirty (30) days
          of filing;

     (4)  the institution by the Maker or any indorser or guarantor of this Note
          of any proceedings under the United States Bankruptcy Code or any
          other federal or state bankruptcy, reorganization, receivership,
          insolvency or other similar law affecting the rights of creditors
          generally or the making by the Maker or any indorser or guarantor of
          this Note of a composition or an assignment or trust mortgage for the
          benefit of creditors; or

     (5)  determination by the holder that it is insecure with respect to the
          payment of any obligation of the Maker to the holder.
<PAGE>

Upon the occurrence of an Event of Default, the holder shall have then, or at
any time thereafter, all of the rights and remedies afforded by the Uniform
Commercial Code as from time to time in effect in the Commonwealth of
Massachusetts or afforded by other applicable law.

Every amount overdue under this Note shall bear interest from and after the date
on which such amount first became overdue at an annual rate which is two (2)
percentage points above the rate per year specified in the first paragraph of
this Note.  Such interest on overdue amounts under this Note shall be payable on
demand and shall accrue and be compounded monthly until the obligation of the
Maker with respect to the payment of such interest has been discharged (whether
before or after judgment).

In no event shall any interest charged, collected or reserved under this Note
exceed the maximum rate then permitted by applicable law and if any such payment
is paid by the Maker, then such excess sum shall be credited by the holder as a
payment of principal.

All payments by the Maker under this Note shall be made without set-off or
counterclaim and be free and clear and without any deduction or withholding for
any taxes or fees of any nature whatever, unless the obligation to make such
deduction or withholding is imposed by law.  The Maker shall pay and save the
holder harmless from all liabilities with respect to or resulting from any delay
or omission to make any such deduction or withholding required by law.

Whenever any amount is paid under this Note, all or part of the amount paid may
be applied to principal, premium or interest in such order and manner as shall
be determined by the holder in its discretion.

No reference in this Note to any guaranty or other document shall impair the
obligation of the Maker, which is absolute and unconditional, to pay all amounts
under this Note strictly in accordance with the terms of this Note.

The Maker agrees to pay on demand all costs of collection, including reasonable
attorneys' fees, incurred by the holder in enforcing the obligations of the
Maker under this Note.

No delay or omission on the part of the holder in exercising any right under
this Note shall operate as a waiver of such right or of any other right of such
holder, nor shall any delay, omission or waiver on any one occasion be deemed a
bar to or waiver of the same or any other right on any future occasion.  The
Maker and every indorser or guarantor of this Note regardless of the time, order
or place of signing waives presentment, demand, protest and notices of every
kind and assents to any extension or postponement of the time of payment or any
other indulgence, to any substitution, exchange or release of collateral, and to
the addition or release of any other party or person primarily or secondarily
liable.

This Note may be prepaid in whole or in part at any time or from time to time.
Any such prepayment shall be without premium or penalty.

None of the terms or provisions of this Note may be excluded, modified or
amended except by a written instrument duly executed on behalf of the holder
expressly referring to this Note and setting forth the provision so excluded,
modified or amended.

                                       2
<PAGE>

All rights and obligations hereunder shall be governed by the laws of the
Commonwealth of Massachusetts and this Note is executed as an instrument under
seal.

                                         /s/ Michael C. Turmelle
                                         -------------------------
                                         Michael C. Turmelle

                                       3<PAGE>

                                                                   Exhibit 10.42
                                                                   -------------

                   SENIOR SECURED CONVERTIBLE PROMISSORY NOTE

THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED
(THE "SECURITIES ACT") OR ANY APPLICABLE STATE SECURITIES LAWS. IT MAY NOT BE
SOLD OR OFFERED FOR SALE IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT
UNDER THE SECURITIES ACT AND COMPLIANCE WITH SUCH STATE SECURITIES LAWS OR AN
OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION AND/OR
COMPLIANCE IS NOT REQUIRED.

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 SHAREHOLDER AGREEMENT BY AND AMONG THE COMPANY AND CERTAIN SHAREHOLDERS
OF THE COMPANY.  A COPY OF SUCH AGREEMENT IS ON FILE AT THE PRINCIPAL OFFICE OF
THE COMPANY.

$200,000                                                         January 7, 2000

                                                                 WOBURN, MA

  FOR VALUE RECEIVED, Beacon Power Corporation, a Delaware corporation (the
"Company"), promises to pay to the order of SatCon Technology Corporation, or
its registered assigns (the "Holder"), the principal sum of TWO HUNDRED THOUSAND
DOLLARS ($200,000) or such lesser amount as shall then equal the outstanding
principal amount hereof, together with interest computed in the following
fashion:

  Interest accruing from the date of issuance of this Note on the unpaid
principal balance hereof at a rate equal to twelve and one-half percent (12
1/2%) per annum, computed on the basis of the actual number of days elapsed and
a year of 365 days; provided that if the Funding Date does not occur within four
(4) months from the date hereof, such interest rate shall increase effective as
of the close of business on such day to occur four (4) months from the date
hereof to fifteen percent (15%) per annum.  Interest on the unpaid principal
balance hereof may be paid in cash, or, if a Funding Date occurs and the Note is
converted, in the form of Class E Preferred Stock at the price per share at
which the Class E Preferred Stock is issued on the Funding Date.

  All unpaid principal, together with any accrued but unpaid interest and other
amounts payable hereunder, shall be due and payable on the earlier of (i) the
date of conversion (or, absent a conversion, demand by the holder) as referenced
below in Section 6 (the "Maturity Date") or (ii) upon or after the occurrence of
an Event of Default (as defined below), when such amounts are declared due and
payable by the Holder or made automatically due and payable.

  This Note is issued pursuant to the Note and Warrant Purchase Agreement (the
"Purchase Agreement") dated as of January 7, 2000 by and among the Company,
Perseus Capital, L.L.C., Duquesne Enterprises, Inc., Micro Generation Technology
Fund, L.L.C. and SatCon Technology Corporation.

  The following is a statement of the rights of the Holder and the conditions to
which this Note is subject, and to which the Holder hereof, by the acceptance of
this Note, agrees:

         1.      Definitions.  As used in this Note, the following capitalized
         --      ------------
terms have the following meanings:

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

          (b) "Funding Date" shall refer to date on which the purchase of at
least $5 million of the Company's Class E Preferred Stock occurs.

          (c) "Obligations" means the principal, interest and other amounts
payable under this Note.

          (d) "Transaction Documents" shall mean this Note, the Purchase
Agreement, and any other promissory note issued pursuant to the Purchase
Agreement.

     2.   Events of Default.  The occurrence of any of the following
          -----------------
shall constitute an "Event of Default" under this Note:

          (a)  Failure to Pay.  The Company shall fail to pay (i) when due
any principal payment on this Note or (ii) any interest or other payment
required under the terms of this Note or any other Transaction Document within
five Business Days of its due date; or

          (b) Breaches of Other Covenants. The Company shall materially fail to
observe or to perform any other covenant, obligation, condition or agreement
contained in this Note or the other Transaction Documents, other than those
specified in Section 2(a) hereof, and such failure shall continue for 10 days
after written notice thereof to the Company; or

          (c) Voluntary Bankruptcy or Insolvency Proceedings. The Company shall
(i) apply for or consent to the appointment of a receiver, trustee, liquidator
or custodian of itself or of all or a substantial part of its property, (ii) be
unable, or admit in writing its inability, to pay its debts generally as they
mature, (iii) make a general assignment for the benefit of its or any of its
creditors, (iv) be dissolved or liquidated in full or in part, (v) become
insolvent (as such term may be defined or interpreted under any applicable
statute), (vi) commence a voluntary case or other proceeding seeking
liquidation, reorganization or other relief with respect to itself or its debts
under any bankruptcy, insolvency or other similar law now or hereafter in effect
or consent to any such relief or to the appointment of or taking possession of
its property by any official in an involuntary case or other proceeding
commenced against it or (vii) take any action for the purpose of effecting any
of the foregoing; or

          (d) Involuntary Bankruptcy or Insolvency Proceedings. Proceedings for
             ------------------------------------------------
the appointment of a receiver, trustee, liquidator or custodian of the Company
or of all or a substantial part of the property thereof, or an involuntary case
or other proceedings seeking liquidation, reorganization or other relief with
respect to the Company or the debts thereof under any bankruptcy, insolvency or
other similar law now or hereafter in effect shall be commenced and an order for
relief entered, or such case or proceeding shall not be dismissed or discharged
within 45 days of commencement; or

          (e) Cross-Default. The Company or any of its subsidiaries shall
              -------------
default under any bond, debenture, note or other evidence of indebtedness for
money borrowed (excluding any capital lease), under any guarantee or under any
mortgage, or indenture pursuant to which there shall be issued or by which there
shall be secured or evidenced any indebtedness for money borrowed by the Company
or any of its subsidiaries, whether such indebtedness now exists or shall
hereafter be created, which default shall have resulted in indebtedness of at
least $10,000 being due and payable prior to the date on which it would
otherwise become due and payable and shall not have been issued by the Company
or waived by the lender; or

          (f) Undischarged Judgment. One or more judgments for the payment of
             ---------------------
money in an amount in excess of $10,000 in the aggregate shall be rendered
against the Company or any of its subsidiaries (or any combination thereof) and
shall remain undischarged for a period of ten consecutive days during which
execution shall not be effectively stayed, or any action is legally taken by a
judgment creditor to levy upon any such judgment.

     3.   Rights of Holder Upon Default.  Upon the occurrence or existence of
          -----------------------------
any Event of Default (other than an Event of Default referred to in Sections
2(c) and 2(d) hereof) and at any time thereafter during the continuance of such
Event of Default, the Holder may declare all outstanding Obligations payable by
the Company hereunder to be immediately due and payable without presentment,
demand, protest or any other notice of any

                                      -2-
<PAGE>

kind, all of which are hereby expressly waived, anything contained herein or in
the other Transaction Documents to the contrary notwithstanding.  Upon the
occurrence or existence of any Event of Default described in Sections 2(c) and
2(d) hereof, immediately and without notice, all outstanding Obligations payable
by the Company hereunder shall automatically become immediately due and payable,
without presentment, demand, protest or any other notice of any kind, all of
which are hereby expressly waived, anything contained herein or in the other
Transaction Documents to the contrary notwithstanding.  In addition to the
foregoing remedies, upon the occurrence or existence of any Event of Default,
the Holder may exercise any other right, power or remedy granted to it by the
Transaction Documents or otherwise permitted to it by law, either by suit in
equity or by action at law, or both.

     4.   Collateral. To secure the Company's payment and performance of the
          ----------
Obligations and to secure the Company's prompt, full and faithful performance
and observance of all of the provisions under this Note and the other
Transaction Documents, the Company hereby grants the Holder a security interest,
ranking pari passu with to the Senior Secured Convertible Promissory Notes
issued by the Company pursuant to the Note and Warrant Purchase Agreement dated
August 2, 1999, as amended by and among the Company and certain investors in the
Company and the other Notes being issued pursuant to the Purchase Agreement, and
senior to all other security interests except for any now or hereafter existing
commercial bank or institutional lender financings and equipment leases, in all
of the Company's right, title and interest in and to the following, whether now
owned or hereafter acquired or existing and wherever located:

  (a) All inventory and equipment, and all parts thereof, attachments,
accessories and accessions thereto, products thereof and documents therefor;

  (b) All accounts, contract rights, chattel paper, instruments, deposit
accounts, general intangibles and other obligations of any kind, and all rights
now or hereafter existing in and to all mortgages, security agreements, leases
or other contracts securing or otherwise relating to any of the same;

  (c) All intellectual property and trade secrets, including, without
limitation,

          (i) all patents, patent applications and patentable inventions and (i)
the inventions and improvements described and claimed therein; (ii) any
continuation, division, renewal, extension, substitute or reissue thereof or any
legal equivalent in a foreign country for the full term thereof or the terms for
which the same may be granted; (iii) all rights to income, royalties, profits,
awards, damages and other rights relating to said patents, applications and
inventions, including the right to sue for past, present and future infringement
and (iv) any other rights and benefits relating to said patents, applications
and inventions including any rights as a licensor or licensee of said patents,
applications and inventions (the "Patents");

          (ii) all trademarks, trademark registrations, trademark applications,
service marks, service mark registrations and service mark applications, trade
names, fictitious business name, tradestyles, and the goodwill underlying those
trademarks and service marks and (i) any similar marks or amendments,
modifications and renewals thereof and the goodwill represented by those and any
legal equivalent in a foreign country for the full term or terms for which the
same may be granted; (ii) all rights to income, royalties, profits, damages and
other rights relating to said trademarks and service marks including the right
to sue for past, present or future infringement and (iii) any other rights and
benefits relating to said trademarks and service marks including any rights as a
licensor or licensee of said trademark and service mark (the "Trademarks");

          (iii) all copyrights, copyright registrations and copyright
applications, including without limitation those copyrights for computer
programs, computer databases, flow diagrams, maskworks, maskwork applications,
source codes and object codes, computer software, technical knowledge and
processes, trade secrets, know-how, customer lists, franchises, systems,
inventions, designs, blueprints, formal or informal licensing arrangements, and
all property embodying or incorporating such copyrights and (i) any similar
rights or amendments, modifications and renewals thereof and any legal
equivalent in a foreign country for the full term or terms for which the same
may be granted; (ii) all rights to income, past, present and future infringement
and (iii) any other rights and benefits relating to said copyrights (the
"Copyrights");

                                      -3-
<PAGE>

          (d) all substitutions and replacements for, and all rights to exploit,
all of the foregoing;

          (e) all books and records pertaining to any of the foregoing; and

          (f) all proceeds of all of the foregoing and, to the extent not
otherwise included, all payments under insurance or any indemnity, warranty or
guaranty, payable by reason of loss or damage to or otherwise with respect to
any of the foregoing.

All of the above assets are hereinafter collectively referred to as
"Collateral."

The Company covenants and agrees with Holder that:  (x) the security interest
granted under this Note is in addition to any other security interest from time
to time held by the Holder; (y) the Holder may realize upon all or part of any
Collateral in any order it desires and any realization by any means upon any
Collateral will not bar realization upon any other Collateral; and (z) the
security interest hereby created is a continuing security interest and will
cover and secure the payment of all Obligations both present and future of the
Company to Holder pursuant to this Note and the other Transaction Documents.
The Company further covenants and agrees to take all actions requested by the
Holder to establish or perfect the security interest granted under this Note.

  5.  Prepayment.  This Note may be prepaid as a whole or in part at any time
      ----------
prior to the Maturity Date upon at least ten Business Days prior written notice
to the Holder, provided that nothing in this Section 5 shall limit the Holder's
conversion rights set forth in Section 6 herein during such notice period.  Any
such prepayment shall be applied first to the payment of expenses due under this
Note, second to interest accrued on this Note and third, if the amount of
prepayment exceeds the amount of all such expenses and accrued interest, to the
payment of principal of this Note.

  6.  Conversion.
      ----------

      (a) If there is a Funding Date by February 2, 2000, the Holder shall have
the choice of (a) declaring in writing that such Note has become a demand
obligation, and thereafter may make demand for payment of the Note at any time
thereafter or (b) convert the Note into shares of Class E Preferred Stock on
such date at a price equal to the price at which the Class E Preferred Stock is
issued on the Funding Date.  In the event that the Holder does not so notify the
Company as provided above, the Holder shall be issued a Warrant as provided for
in the Purchase Agreement.

      (b) If the Funding Date has not occurred on or before February 2, 2000,
then (a) all outstanding principal and accrued interest under this Note will
convert to the Company's common or preferred stock at a price and on terms to be
negotiated at that time between the Holder and the Company, or (b) if no
agreement can be reached between the Holder and the Company within 60 days
following February 2, 2000, at the Holder's option, exercisable by the Holder at
any time after expiration of the 60 day period, the Holder may declare in
writing that such Note has become a demand obligation, and thereafter may make
demand for payment of the Note at any time thereafter.  Any such demand shall be
in writing, and require payment by the Company no earlier than 10 days after
such demand.

  7.  Successors and Assigns.  Subject to the restrictions on transfer
      ----------------------
described in Sections 9 and 10 hereof, the rights and obligations of the Company
and the Holder of this Note shall be binding upon and benefit the successors,
assigns, heirs, administrators and transferees of the parties.

  8.  Waiver and Amendment.  Any provision of this Note may be amended, waived
      --------------------
or modified only as to the Holder of this Note upon the written consent of the
Company and the Holder.

  9.  Transfer of this Note or Securities Issuable on Conversion Hereof.  This
      -----------------------------------------------------------------
Note may not be transferred in violation of any restrictive legend set forth
hereon.  Each new Note issued upon transfer of this Note shall bear a legend as
to the applicable restrictions on transferability in order to ensure compliance
with the Securities Act, unless in the opinion of counsel for the Company such
legend is not required in order to ensure compliance with the Securities Act.
The Company may issue stop transfer instructions to its transfer agent in

                                      -4-
<PAGE>

connection with such restrictions.  Subject to the foregoing, transfers of this
Note shall be registered upon registration books maintained for such purpose by
or on behalf of the Company.  Prior to presentation of this Note for
registration of transfer, the Company shall treat the registered holder hereof
as the owner and holder of this Note for the purpose of receiving all payments
of principal and interest hereon and for all other purposes whatsoever, whether
or not this Note shall be overdue and the Company shall not be affected by
notice to the contrary.

  10.  Assignment by the Company.  Neither this Note nor any of the rights,
       -------------------------
interests or obligations hereunder may be assigned, by operation of law or
otherwise, as a whole or in part, by the Company, without the prior written
consent of the Holder.

  11.  Treatment of Note.  To the extent permitted by generally accepted
       -----------------
accounting principles, the Company will treat, account and report the Note as
debt and not equity for accounting purposes and with respect to any returns
filed with federal, state or local tax authorities.

  12.  Notices.  Any notice, request or other communication required or
       -------
permitted hereunder shall be in writing and shall be deemed to have been duly
given if personally delivered or mailed by registered or certified mail, postage
prepaid, or by recognized overnight courier, personal delivery or facsimile
transmission at the respective addresses or facsimile number of the parties as
set forth in the Purchase Agreement or on the register maintained by the
Company.  Any party hereto may by notice so given change its address or
facsimile number for future notice hereunder.  Notice shall conclusively be
deemed to have been given when received.

  13.  Expenses; Waivers.  If action is instituted to collect this Note, the
       -----------------
Company promises to pay all costs and expenses, including, without limitation,
reasonable attorneys' fees and costs, incurred in connection with such action.
The Company hereby waives notice of default, presentment or demand for payment,
protest or notice of nonpayment or dishonor and all other notices or demands
relative to this instrument.

  14.  Governing Law.  This Note and all actions arising out of or in connection
       -------------
with this Note shall be governed by and construed in accordance with the laws of
the Commonwealth of Massachusetts, without regard to conflict of laws provisions
of the Commonwealth of Massachusetts or of any other state. In the event of any
dispute among or between any of the parties to this Note arising out of the
terms of this Note, the parties hereby consent to the exclusive jurisdiction of
the federal and state courts located in the Commonwealth of Massachusetts for
resolution of such dispute, and agree not to contest such exclusive jurisdiction
or seek to transfer any action relating to such dispute to any other
jurisdiction.

     IN WITNESS WHEREOF, the Company has caused this Note to be issued as of the
date first written above.

                                         BEACON POWER CORPORATION

                                         By:    /s/ William E. Stanton
                                                ----------------------

                                         Name:  William E. Stanton
                                                ------------------

                                         Title: President & CEO
                                                ---------------

                                      -5-

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