Check a combination
Pick what your charger supports, and what you want to put in it.
Safe
A multi-chemistry charger handles NiMH correctly.
Not sure what you have?
Read the cell, not the packaging.
Look at the voltage printed on the cell. It is the single most reliable tell:
- 1.2V - NiMH. A rechargeable AA, AAA, C or D.
- 1.5V - alkaline (disposable), unless it has a USB-C port, in which case it is a lithium cell with built-in regulation.
- 3.2V - LiFePO4.
- 3.6V or 3.7V - Li-ion. If it is AA-sized and says 3.7V, it is a 14500 and it is not an AA.
- 3.0V - a lithium coin cell. Not rechargeable unless it says LIR.
Still unsure? The equivalents lookup will identify a cell from the code printed on it.
The compatibility matrix
| Charger type | NiMH cell | Li-ion cell | LiFePO4 cell | Alkaline |
|---|---|---|---|---|
| NiMH only | Yes | Never | Never | Never |
| Li-ion only | Never | Yes | Only if stated | Never |
| Multi-chemistry | Yes | Yes | Only if stated | Never |
| LiFePO4 capable | Only if stated | Only if stated | Yes | Never |
USB-C rechargeable AA cells are absent deliberately: they charge over their own port and must never go in a battery charger at all.
Why chemistry is a hard rule and not a preference
The two chemistries are charged in fundamentally different ways, and each charger only knows one of them.
NiMH: constant current, stop on the dip
A NiMH charger pushes a steady current and watches the cell's voltage. As the cell fills, voltage rises, peaks, then dips very slightly - negative delta V. The charger sees the dip and stops. Good chargers also watch temperature and run a backup timer, because the dip is a small signal.
Li-ion: constant current, then constant voltage, hard ceiling at 4.2V
A Li-ion charger pushes current until the cell reaches 4.2V, then holds exactly 4.2V while current tapers away, and stops when the current falls low enough. There is no voltage dip to look for. The 4.2V ceiling is not a target, it is a limit.
What goes wrong when you cross them
Put Li-ion in a NiMH charger and the charger waits for a voltage dip that Li-ion will never produce. It keeps pushing current past 4.2V. Overcharged Li-ion vents, and can burn or explode. Put NiMH in a Li-ion-only charger and it drives toward 4.2V against a cell that tops out near 1.45V - the cell overheats, vents hydrogen and leaks.
The reason this catches people is that neither failure is certain on the first go. Someone does it once, nothing dramatic happens, and they conclude it is fine.
Common questions
Can I charge any NiMH battery in any NiMH charger?
Generally yes - NiMH is NiMH regardless of brand, so an Eneloop charges fine in an EBL charger and vice versa. The real question is whether the charger is any good. A smart charger with independent slots that terminates on -dV and temperature is safe with any NiMH cell. A dumb timer-based charger will overcharge cells whose capacity does not match its assumptions, whatever the brand.
Will an Eneloop charger charge other brands of NiMH?
Yes. Panasonic Eneloop chargers charge any NiMH AA or AAA cell. Panasonic only honours the Eneloop warranty if you used a Panasonic or Sanyo charger, which is a warranty policy rather than a technical limit. There is no chip in an Eneloop that a charger checks.
Can I charge Li-ion in a NiMH charger?
No. This is genuinely dangerous. NiMH chargers push constant current and stop by detecting a small voltage drop that Li-ion never produces, so the charger keeps going past 4.2V. Overcharged Li-ion can vent, catch fire or explode. Only ever charge Li-ion in a charger that explicitly names Li-ion support.
Can I charge NiMH in a Li-ion charger?
Not in a Li-ion-only charger. It will try to drive the cell to 4.2V, and NiMH tops out around 1.45V, so the cell is massively overcharged - it will overheat, vent hydrogen and leak. Multi-chemistry chargers that support both are a different matter and are entirely safe, because they detect or let you select the chemistry first.
What happens if I use the wrong charger?
It depends which way round. NiMH in a Li-ion charger overheats, vents and leaks. Li-ion in a NiMH charger overcharges past its safe ceiling and can vent with flame. Neither failure is guaranteed on the first attempt, which is exactly what makes it dangerous - people do it once, nothing obvious happens, and they assume it is fine.
Can I charge alkaline batteries?
No. Standard alkaline cells are not designed to be recharged. Attempting it generates hydrogen inside a sealed case, and they leak, rupture or burst. Some chargers advertise an alkaline mode; it is a bad idea with a small payoff, and it can ruin the device the cell later goes into. If you want to recharge AA or AAA cells, buy NiMH.
Need a charger that covers everything you own? Run the charger finder, or read the best chargers guide.