[Note: Various answers to other questions address this, but I cannot find another question focused on just this. This is my attempt to create a definitive place for answers to this.
[If at all possible, please include references in your answer. If you're answering based on experience and your references are not handy, also giving the approximate era will be helpful since it seems that the advisibility of indefinite charging and acceptable rates may have changed over time as NiMH batteries have developed.]
If it makes a difference, this question is about commonly available modern consumer AA cells, particularly low self-discharge (LSD) cells, since all consumer cells now seem to be LSD.
NiMH batteries are generally charged with a constant current charger that stops charging at some point based on one or more of various criteria (time, voltage, -ΔV, temperature, ΔT/Δt, etc.). Here I am asking about continuous charging with no definite endpoint (e.g., applying charging current for days or weeks at a time).
Note also that I am asking only about constant current charging: this question discusses constant voltage ("float voltage") charging of NiMH batteries.
Various sources suggest that NiMH batteries may be "trickle charged" to maintain their charge state. For example, Linden and Reddy [lin02] state:
Trickle Charge. A number of applications require the use of batteries which are maintained in a fully charged condition. This is done by trickle charging at a rate that will replace the capacity loss due to self-discharge. A trickle charge at a current of between the 0.03 and 0.05C rates is recommended.... Trickle charge may be used following any of the previously discussed charging methods. (§29.5.2 p.29.27 PDF p.889)
The "previously discussed" methods are low-rate 0.1C, quick 0.3C and fast 0.5–1C rates. They go on to give a three-step charge procedure involving a 1C rate charge terminated with ΔT/Δt or -ΔV, a 0.1C topping charge terminated by timer after 0.5–1h, and:
- The third step is a maintenance charge of indefinite duration at a current of between the 0.05 and 0.02C rates. The battery should also be protected with a thermal cutoff device to terminate the charge so that the temperature does not exceed 60°C.
However, this source is from 2002, before modern LSD NiMH cells became available, and also capacities of consumer cells (such as AA) appear to have increased since the book was published.
A 2018 Energizer datasheet [ene18] says in its "Recommended Charging Rates" section, after discussing smart fast chargers and slow timer-based chargers, says:
Finally a maintenance (or trickle) charge rate of less than 0.025 C (C/40) is recommended. The use of very small trickle charges is preferred to reduce the negative effects of overcharging. (p.11)
Note that they don't indicate here whether by "trickle" charge they mean a continuous charge or just a low-rate charge. No definition is given in this document, but from their definition in in [ene08] it seems it could be either:
A method of recharging in which a secondary battery is either continuously or intermittently [emphasis mine] connected to a constant current supply that maintains the battery in a fully or near full charged condition. Typical trickle charges are between 0.03C and 0.05C. (p.4)
Further on, they do mention overcharge, but seem to say that only "full" charge currents (presumably C/10 or higher?) are a problem:
Establishing the appropriate degree of overcharge for a battery-powered application is dependent on the usage scenario. Some overcharge of the battery is vital to ensure that all batteries are fully charged and balanced, but maintenance of full charge currents for extended periods once the battery has reached full charge can reduce life. (p.12)
So from all this it's not clear to me if they're saying that this overcharging will have a noticeable negative effect on the battery, but it's worthwhile if your application needs to maintain fully charged batteries, or if at C/40 current the effect is essentially negligible.
I've also looked for an Eneloop datasheet but have been unable to find one that discusses charging in any but the most cursory detail.
A final component of this question is, if some particular low current rate (C/20, C/40, whatever) is acceptable for indefinite charging, is that rate also sufficient to charge a discharged battery given enough time? For example, if an NiMH battery (a particular model or in general) can handle an indefinite C/50 charging current, will that still charge the battery to full capacity if the battery becomes discharged and the current is then applied for 60-75 hours after that? Or will such low charge rates merely overcome the self-discharge and hold the battery "steady" at its currently charged capacity?
- [lin02] Linden, Reddy, Handbook of Batteries, 3rd ed., McGraw-Hill, 2002. Ch. 29: Portable Sealed Nickel-Metal Hydride Batteries.
- [ene18] Energizer, Nickel Metal Hydride (NiMNH) Handbook and Application Manual, 2018. (The quoted text is the same in the 2010 edition, "Version: NiMH02.01.")
- [ene08] Energizer, NiMH Battery Chargers Handbook and Application Manual, 2008.
For reference, here are some answers to other questions that also discuss continuous charging of NiMH cells. Others should feel free to add to this list if the come across ones that I've missed.
- Several answers to "How fast may I trickle charge a full LSD NiMH battery?"
- Russell McMahon answering "How can I trickle charge NiMH batteries as part of an IOT controller?"
- Russell McMahon answering, "Are there any dangers to consider when replacing a permanent AA NiMH battery?"
- Russell McMahon answering a question about charging NiMH cells in parallel.
- Jasen answering "Can you charge a NIMH battery with a trickle charge NiCad battery charger"