Estimate charge time
Time = capacity ÷ current × inefficiency factor.
About 2 hours 50 minutes.
Is this rate healthy?
Charge rate as a fraction of capacity (C rate).
Charger current per slot is often lower than the headline figure once every slot is full - check your charger's manual.
The formula
NiMH uses roughly 1.4, because NiMH charging is only about 70% efficient - the rest leaves as heat, most of it right at the end of the charge. Li-ion is far more efficient at around 95%, but it spends the last stretch in a constant-voltage taper where current falls away, so the last 10% takes disproportionately long. The 1.15 factor is a rough allowance for that taper.
How a NiMH charger knows when to stop
This is the single most important thing a charger does, and it is invisible on the box. As a NiMH cell reaches full charge its voltage rises, peaks, then dips very slightly. A smart charger watches for that dip - negative delta V, or -dV - and terminates. Good chargers also watch cell temperature and run a backup timer, because -dV is a small and occasionally unreliable signal.
A dumb charger does none of this. It charges for a fixed time, or until you unplug it, and cooks the cell. That is why every charger in our charger finder uses independent, individually terminated slots.
The rate window
There is a usable band, and it has walls on both sides.
- Below ~0.3C: the -dV signal gets faint and a charger can miss it entirely. Trickle charging needs a charger designed for it, not just a fast charger turned down.
- 0.3C to 1C: the sweet spot. Fast enough to be practical, gentle enough that heat stays manageable.
- Above 1C: works, and manufacturers rate quality cells for it, but heat climbs and heat is what ages cells. Ultra-fast chargers that ignore the rated current are the classic cell-killer.
Common questions
How long does it take to charge a NiMH battery?
Divide the capacity in mAh by the charger current in mA, then multiply by roughly 1.4 to allow for charging inefficiency. A 2000mAh AA on a 1000mA charger takes about 2.8 hours. Charging NiMH is only around 70% efficient, so you always put in more charge than you get back out.
Why do I multiply by 1.4?
NiMH charging wastes energy as heat, particularly near the end of the charge, so you must push in roughly 140% of the cell capacity to fill it. That inefficiency is where the 1.4 comes from. Li-ion is much more efficient at about 95%, so it uses a factor closer to 1.1 - but Li-ion also tapers off in its constant-voltage phase, which adds time back.
Is charging batteries faster bad for them?
Within the manufacturer standard rates, fast charging is fine and modern smart chargers handle it well. The problem is heat, and heat is what ages a cell. Panasonic rates Eneloop for fast charging, but ultra-fast chargers that run well above the rated current will shorten cell life. Around 0.5C is a good balance of speed and longevity for NiMH.
Can charging too slowly be a problem?
Yes, and this surprises people. NiMH chargers detect a full cell by watching for a tiny voltage drop (-dV) at the end of charge. Below roughly 0.3C that signal gets too faint to detect reliably, so a charger can miss it and keep charging. Good chargers add a backup timer and temperature cut-off. Very low currents are best left to chargers designed for trickle charging.
Next: find a charger that terminates properly, or check how long the charge will last.