Batteries: the terms the rules use, and the hazard regulators describe.

The terms in the rule books

No standards-body text defining the hobby's "S" cell count, its "C" rating, Kv, or the difference between LiPo and LiFe was read for this site, so these terms are used only as the racing rules use them.

Lithium based, LiPo and LiFe

EFRA's 2026 Appendix 4 says its approved cells are lithium based, either LiPo or LiFe, and EFRA has approved only lithium-based batteries since 2009 (3.1). IFMAR's March 2023 batteries and motors document likewise approves only lithium-based batteries.

Cell count (1S, 2S, 4S)

The labels the rules use for pack sizes, each with its own voltage limits. EFRA describes a 1S pack as having cells only in parallel (Appendix 4, 1.4).

Nominal voltage

Both EFRA (Appendix 4, 1.4) and IFMAR (2.4 and 2.5) rate cells at no more than 3.8 V nominal for LiPo and 3.3 V for LiFe. So a 2S pack is at most 7.6 V (LiPo) or 6.6 V (LiFe) under both; EFRA also sets a 4S pack at 15.2 V or 13.2 V, which IFMAR's March 2023 document does not cover. EFRA's handbook dates the 3.8 V rating two ways: Appendix 4 (1.4) says 3.8 V cells have been allowed since 1 April 2017, while Appendix 3 (A 3.1) gives 1 March 2017 as the effective date for 3.8 V 1/12 batteries. Earlier 3.7 V approvals still stand.

Maximum charged voltage

A separate, higher figure: the most a pack may hold at an event. EFRA (Appendix 4, 2.7) and IFMAR (2.5 and 3.12 to 3.14) both set 4.20 V per cell for LiPo, so 8.40 V for 2S. ROAR's 2011 rules allowed 4.25 V; see battery rules by body.

Hard case

Both bodies require a hard protective case of ABS or similar that completely encloses the cells, factory sealed so that opening it destroys it, cuboid in shape, with openings only for wires or connectors (EFRA 1.1; IFMAR 2.1 to 2.3). A BRCA Electric Board listing from 2017 describes the difference between hard-case packs (foil cells inside a hard plastic box) and soft-case packs (no plastic case protecting the cells).

Saddle pack

A pack shape both bodies allow for 2S, as long as it fits within 139.0 mm end to end (EFRA 1.2; IFMAR 2.1 to 2.3). IFMAR does not allow 1S saddle packs.

Label: Wh and C rating

EFRA (1.6) requires the original label to state the part number, nominal voltage, chemistry, energy in Wh and the "C" rating, with a readable brand. IFMAR (2.6 to 2.8) asks for the maker or importer, part number, nominal voltage, chemistry, capacity in Wh and C rating.

Wh (watt-hours)

The measure air-transport rules use. IATA's guidance of 31 March 2026 defines it as rated capacity in ampere-hours times nominal voltage. See travelling with batteries.

UN 38.3

A set of safety tests (T1 to T8) in the UN Manual of Tests and Criteria, Part III, subsection 38.3. EFRA (Appendix 4, 3.3(a)) and IFMAR (section 1) both require certification to it before a battery can be approved.

Which pack each class uses

Whether a car may also carry a separate receiver pack is a point on which IFMAR and EFRA disagree for 1/10 off-road; the detail is on battery rules by body.

The hazard, as regulators and safety bodies describe it

No study of RC car battery fires was found. What follows is how regulators and safety bodies describe lithium battery risk in general, each in its own words, summarised:

• OSHA (Lithium-ion Battery Safety page, undated) describes thermal runaway as a chain reaction in which heat from one failing cell damages its neighbours, which can lead to fire, explosion and release of chemical by-products. It lists internal short circuits from manufacturing defects, mechanical damage, excessive heat or cold, and improper charging among the causes, and names hydrogen fluoride among the hazards such fires can produce.
• A 2023 UL fact sheet hosted by the New Jersey Division of Fire Safety calls thermal runaway the primary driver of lithium-ion battery risk and says it generates more than 1,000 degrees F. It says a person may have less than a minute to escape from the first sign of a problem, and names swelling, hissing or popping, heat, a strange odour and white or grey wispy smoke as warning signs. A search summary of NTSB recommendations hosted by the FAA gives 1,100 degrees F or higher; that figure is unconfirmed and differs from UL's.
• PHMSA and the FAA, in safety advisory notice 09-05 (2009), said defective, misused or overcharged lithium batteries can overheat and ignite, and that once ignited such fires can be especially difficult to extinguish.
• The US CPSC, in consumer alert 08-189 (12 February 2008), warned that rechargeable lithium batteries in remote-controlled helicopters and airplanes can overheat, catch fire or explode if overcharged, and recorded 26 reports of helicopters igniting while being charged. The alert concerns RC aircraft, not RC cars, and does not name cars.
• Essex County Fire and Rescue Service (UK) reported a single incident on 27 October 2021 in which a remote-control car battery overheated and exploded while on charge in a house kitchen, causing a fire; no people or animals were hurt. One incident is not a rate.

More on battery fire risk, and on regulators' battery and charger recalls at recalls.

LiPo sacks

EFRA and IFMAR rules require drive batteries to be in a "LiPo sack" at certain times at their events, and both define the sack as a receptacle built to contain a LiPo or LiFe fire. That is the rules' definition, not evidence that a sack prevents or contains a fire, and this site does not present it as protection. When each rule requires one is set out on battery rules by body.