$564,822.46 September 17, 2001
FOR GOOD AND VALUABLE CONSIDERATION, William E. Stanton (the "Borrower"), hereby
unconditionally promises to pay to the order of Beacon Power Corporation (the "Lender"), in lawful money of the
United States of America and in immediately available funds, the principal amount of Five Hundred Sixty-four
Thousand Eight Hundred Twenty-two and 46/100 Dollars ($564,822.46) (the "Loan"), together with accrued
and unpaid interest as provided herein, UPON DEMAND (within 10 days thereafter) at any time. This Demand
Note (as amended, supplemented, extended, restated, renewed, refunded, replaced, refinanced, increased in
amount or otherwise modified, in each case from time to time and whether in whole or in part, this "Note")
evidences the unpaid principal amount of the Loan together with all accrued and unpaid interest thereon.
The Lender agrees to disburse principal amounts of the Loan from time to time ("Disbursements") at such time as
the Borrower receives a margin call from Salomon Smith Barney ("SSB") in connection with the loan (the "SSB
Loan") that Borrower has outstanding with SSB incurred by the Borrower in connection with Borrower's holding
or exercise of Borrower's options ("Options") to purchase shares of the Lender's Common Stock (the "Shares").
Borrower shall provide the Lender with a written copy of the SSB margin call prior to making the Disbursement.
The Borrower hereby irrevocably authorizes the Lender to make or cause to be made on the schedule attached
to this Note, at or following the time of making any Disbursement of the Loan and of receiving any payment of
principal or interest, an appropriate notation reflecting such transaction and the then aggregate unpaid balance of
principal. Failure of the Lender to make any such notation shall not, however affect any obligation of the
Borrower hereunder. The unpaid principal amount of this Note, as recorded by the Lender from time to time on
such schedule, shall constitute presumpt