Exhibit 5.2

Waltham, Massachusetts 02451

Ladies and Gentlemen:

We have acted as special counsel to A123 Systems, Inc., a Delaware corporation (the “Company”), in connection with the issuance of up to $143,750,000 aggregate principal amount of the Company’s 3.75% Convertible Subordinated Notes due 2016 (the “Notes”), convertible into common stock, par value $0.001 per share, of the Company (the “Common Stock”).  The Notes have been issued under an indenture, dated as of April 6, 2011 (the “Base Indenture”), and a supplemental indenture, dated as of April 6, 2011 (the “Supplemental Indenture,” and together with the Base Indenture, the “Indenture”), between the Company and U.S. Bank National Association, as trustee, and included in a registration statement on Form S-3 under the Securities Act of 1933, as amended (the “Act”), filed with the Securities and Exchange Commission (the “Commission”) on March 28, 2011 (Registration No. 333—173122) (as amended, the “Registration Statement”) and an underwriting agreement by and among Deutsche Bank Securities, Inc., Goldman, Sachs & Co. and the several underwriters named in Schedule II thereto (the “Underwriters”) and the Company, dated March 31, 2011 (the “Underwriting Agreement”).  This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or related prospectus, other than as expressly stated herein with respect to the issue of the Notes and the Common Stock.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter.  With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters.  We are opining herein as to the internal laws of the State of New York and the General Corporation Law of the State of

Delaware (the “DGCL”), and we express no opinion with respect to the applicability thereto, or the effect thereon, of the laws of any other jurisdiction or, in the case of Delaware, any other laws, or as to any matters of municipal law or the laws of any local agencies within any state.

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof:

(1)           The Notes have been duly authorized by all necessary corporate action of the Company, and when the Notes have been duly executed, issued and authenticated in accordance with the terms of the Indenture and delivered against payment therefor in the circumstances contemplated by the Underwriting Agreement, the Notes will be legally valid and binding obligations of the Company enforceable against the Company in accordance with their terms.

(2)           The Common Stock initially reserved for issuance upon conversion of the Notes has been duly authorized by all necessary corporate action of the Company, and when the Common Stock has been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the holders of the Notes, and has been delivered in accordance with the terms of the authorization thereof and the Indenture upon conversion of Notes in a principal amount not less than the par value of the Common Stock to be issued, such Common Stock will be validly issued, fully paid and nonassessable.  In rendering the foregoing opinion, we have assumed that the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the DGCL.

Our opinions are subject to: (i) the effect of bankruptcy, insolvency, reorganization, fraudulent transfer, moratorium or other similar laws relating to or affecting the rights and remedies of creditors; (ii) the effect of general principles of equity, whether considered in a proceeding in equity or at law (including the possible unavailability of specific performance or injunctive relief), concepts of materiality, reasonableness, good faith and fair dealing, and the discretion of the court before which a proceeding therefor may be brought; (iii) the invalidity under certain circumstances under law or court decisions of provisions providing for the indemnification of or contribution to a party with respect to a liability where such indemnification or contribution is contrary to public policy; and (iv) we express no opinion with respect to (a) provisions for liquidated damages, default interest, late charges, monetary penalties, make-whole premiums or other economic remedies to the extent such provisions are deemed to constitute a penalty, (b) consents to, or restrictions upon, governing law, jurisdiction, venue, arbitration, remedies or judicial relief, (c) any provision requiring the payment of attorneys’ fees, where such payment is contrary to law or public policy, (d) any provision permitting, upon acceleration of the Notes, collection of that portion of the stated principal amount thereof which might be determined to constitute unearned interest thereon, (e) advance waivers of claims, defenses, rights granted by law, or notice, opportunity for hearing, evidentiary requirements, statutes of limitation, trial by jury or at law, or other procedural rights, (f) waivers of broadly or vaguely stated rights, (g) provisions for exclusivity, election or cumulation of rights or remedies, (h) provisions authorizing or validating conclusive or discretionary determinations, (i) proxies, powers and trusts, (j) provisions prohibiting, restricting, or requiring consent to

2

assignment or transfer of any right or property and (k) the severability, if invalid, of provisions to the foregoing effect.

With your consent, we have assumed (a) that the Indenture and the Notes (collectively, the “Documents”) have been duly authorized, executed and delivered by the parties thereto other than the Company, (b) that the Documents constitute legally valid and binding obligations of the parties thereto other than the Company, enforceable against each of them in accordance with their respective terms, and (c) that the status of the Documents as legally valid and binding obligations of the parties is not affected by any (i) breaches of, or defaults under, agreements or instruments, (ii) violations of statutes, rules, regulations or court or governmental orders, or (iii) failures to obtain required consents, approvals or authorizations from, or make required registrations, declarations or filings with, governmental authorities.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act.  We consent to your filing this opinion as an exhibit to the Company’s Form 8-K dated April 6, 2011 and to the reference to our firm in the prospectus under the heading “Legal Matters.”  In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.

3