Lithium Batteries
Pipeline and Hazardous Materials Safety Administration · Feb 5, 2026 · Reference No. 25-0102
Interprets: 49 CFR § 173.185(c)(1)(i)
Reggie’s quick take
Someone asked whether a lithium ion battery made of two cells, each over 20 Wh but totaling under 100 Wh, can use the small-battery exception in § 173.185(c)(1)(i). The agency said yes. The 100 Wh limit applies to the assembled battery, so the 20 Wh limit on individual cells does not need to be checked for the cells inside it. The agency also said a newer UN Manual of Tests and Criteria definition of "battery" does not cancel its 2015 letter of interpretation on this point.
Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.
February 5, 2026
Ana Diaz
Regulatory Compliance Specialist
Currie Associates, Inc.
101 Ridge Street Suite I
Glens Falls, NY 12801
Reference No. 25-0102
Dear Ms. Diaz
This letter is in response to your July 22, 2025 letter requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to size limits for lithium ion batteries with respect to exceptions for smaller cells or batteries and prior clarification provided in a previous letter of interpretation (Reference No. 15-0009). You indicate that the definitions section in 38.3.2.3 of the Eighth Revised Edition of the United Nations (UN) Manual of Tests and Criteria (MTC) now defines a battery as “two or more cells or batteries which are electrically connected together and fitted with devices necessary for use...” Given this revised definition, you are seeking confirmation that: (1) a lithium ion battery consisting of two cells, each exceeding 20 watt-hours (Wh) but with a combined total of less than 100 Wh, qualifies for the exception outlined in § 173.185(c)(1)(i); and (2) the prior clarification remains valid.
Yes, your understanding is correct. Furthermore, the letter of interpretation (Ref. No. 15-0009) cited in your incoming letter remains valid. PHMSA can confirm that the 100 Wh limit applies to the assembled battery, and that the 20 Wh limit for individual cells does not need to be considered for the component cells within that battery. While it is accurate that the UN MTC has d the definition of a “battery,” this revised definition does not invalidate the 2015 letter of interpretation. If the assembled battery’s total energy density is under 100 Wh,
the exceptions specified in § 173.185(c)(1)(i) apply, even if each component cell exceeds the 20 Wh limit.
I hope this information is helpful. Please contact us if we can be of further assistance.
Sincerely,
Dirk DerKinderen
Chief, Standards Development Branch
Standards and Rulemaking Division
173.185(c)
A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.