UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K /A

Amendment No. 1

For the fiscal year ended March 31, 2013 or

For the transition period from                      to                     

Commission file no.: 001-32253

ENERSYS

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (610) 208-1991

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  x    No  ¨

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x

Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or for shorter period that the registrant was required to submit and post such files.    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of Registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

Large accelerated filer  x        Accelerated filer  ¨        Non-accelerated filer  ¨        Smaller reporting company  ¨

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Act).    Yes  ¨    No  x

State the aggregate market value of the voting and non-voting common equity held by non-affiliates at September 30, 2012: $1,696,495,217 (1) (based upon its closing transaction price on the New York Stock Exchange on September 28, 2012). (1) For this purpose only, “non-affiliates” excludes directors and executive officers.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive Proxy Statement for its Annual Meeting to be held on August 1, 2013 are incorporated by reference in Part III of this Annual Report.

Explanatory Note

This Amendment No. 1 to the Annual Report on Form 10-K of EnerSys for the fiscal year ended March 31, 2013, originally filed on May 28, 2013 (the “Original Filing”), is being filed solely to correct an error in the number of shares of common stock outstanding as of May 24, 2013 shown on the cover page. The correct number of common shares outstanding as of such date is 48,121,845, as indicated on the cover page of this Amendment No. 1.

Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, as amended, this Form 10-K/A also contains new certifications pursuant to Section 302 of the Sarbanes-Oxley Act of 2002, which are attached hereto. Because no financial statements have been included in this Form 10-K/A and this Form 10-K/A does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4, and 5 of the certifications have been omitted.

Except as described above, no other changes have been made to the Original Filing, and this Form 10-K/A does not modify, amend or update in any way any of the financial or other information contained in the Original Filing. This Form 10-K/A does not reflect events that may have occurred subsequent to the filing date of the Original Filing.

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SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

June 25, 2013

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EXHIBIT INDEX

The following is a list of all exhibits filed as a part of this Annual Report on Form 10-K/A.