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.


         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




                                    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:
                                --------------------------------------------