Continuously charging NiMH - best approach?

I've built a simple project based on an idea from a video,




The long charge time (in days/weeks) is compensated for by there being a lot more batteries than needed for any one device.

The project was based on the idea that slow trickle-charge is safe for NiCD/NiMH due to self-catalysis, where any generated gas is recombined back to electrolyte, as long as the current is low enough.

And many legit-looking articles, blogs, and anwers on the webs suggest that this is a good idea.

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However, turns out the world is not simple.

suggests that modern high-capacity cells should never be trickle-charged. Not even at very low currents corresponding to C/100 or C/300:

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When cell capacities exceeded about mAh AA packaged cells this mechanism was removed to allow more room for active material.
As a consequence modern NimH cells must not be trickle charged even at very very low rates. It is posisble that modern low capacity cells still have this mechanism inlcuded, but this is not certain and should not be relied on.
Most manufacturers indicate cells should not be trickle charged at all. A very few say a very low level of trickle charge (maybe C/100) may be applied for an extremely short period.

The poster suggests that this depends on the specific manufacturer chemistry, and

I tried to find what the specific change in chemistry is, how can one tell if a battery is safe to trickle-charge or not, but the only mention is "LSD", or low-self-discharge, and several sources (blog posts, forum posts) seem to agree that LSD is either always

Since I'm using IKEA LADDA batteries that some sources mention to be LSD, I looked around and found out that IKEA's own charger seems to

...

However,

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36.  Does trickle charge damage regular eneloops? lite and pro?

Trickle charge doesn&#;t by itself, but continuous charge does.

This could explain why the charger is doing PWM when trickle-charging.

Looking into some datasheets for ICs,

...

Even here on the forum, people seem to generally mention that trickle-charging NiMH is not good, though it may be less damaging if done with a lower-than-maximum voltage and with a voltage-limiting by the charger. But there is no universal consensus from what I could find.

There seem to be some special batteries designed for trickle-charging (as used by cordless phones), though it's unclear where these have genuinely different chemistry, or are simply de-rated regular high capacity cells.

...

So, is there an off-the-shelf charger, an IC, a DIY project, or even just an algorithm I could implement in an MCU that would keep a set of NiMH batteries always ready-to-use, without degrading their lifespan significantly?

Is really the best approach to do -dV/dt (or dT/dt) at a higher current (so one can detect the voltage drop or change in temperature), and then switch to a once-every-hour pulsed "trickle" (maintenance) charging? Even if I don't need the batteries charged quickly?

Thanks!

Hello,I've built a simple project based on an idea from a video, https://www.youtube.com/watch?v=wVnAH17f4jg, essentially using simple resistors to limit charging current to ~20mA (C/100) or even lower, to continuously trickle-charge many NiMH batteries 24/7, so there are always fully-charged batteries available when you need them for some device.The long charge time (in days/weeks) is compensated for by there being a lot more batteries than needed for any one device.The project was based on the idea that slow trickle-charge is safe for NiCD/NiMH due to self-catalysis, where any generated gas is recombined back to electrolyte, as long as the current is low enough.And many legit-looking articles, blogs, and anwers on the webs suggest that this is a good idea....However, turns out the world is not simple. A post on electronics.stackexchange.com suggests that modern high-capacity cells should never be trickle-charged. Not even at very low currents corresponding to C/100 or C/300:The poster suggests that this depends on the specific manufacturer chemistry, and another, older, post suggests this varies even across Eneloop battery ranges.I tried to find what the specific change in chemistry is, how can one tell if a battery is safe to trickle-charge or not, but the only mention is "LSD", or low-self-discharge, and several sources (blog posts, forum posts) seem to agree that LSD is either always damaged by trickle-charging , or at least they LSD batteries have significantly lower recombination capabilities.Since I'm using IKEA LADDA batteries that some sources mention to be LSD, I looked around and found out that IKEA's own charger seems to use PWM for trickle-charging , with a frequency of around 0.5 Hz and 10-20% duty cycle....However, an interview with an Eneloop engineer suggests, amongst other interesting things (over-discharge being far worse than over-charge), that it's not the current that's the problem. It's the continuous 24/7 charging part.This could explain why the charger is doing PWM when trickle-charging.Looking into some datasheets for ICs, UB suggest a similar thing with its charging logic (~20s periods for top-off charge pulses and ~20 minute periods for trickle charging), though some chargers/ICs seem to completely turn the charging off after a few hours of trickle-charging....Even here on the forum, people seem to generally mention that trickle-charging NiMH is not good, though it may be less damaging if done with a lower-than-maximum voltage and with a voltage-limiting by the charger. But there is no universal consensus from what I could find.There seem to be some special batteries designed for trickle-charging (as used by cordless phones), though it's unclear where these have genuinely different chemistry, or are simply de-rated regular high capacity cells....So, is there an off-the-shelf charger, an IC, a DIY project, or even just an algorithm I could implement in an MCU that would keep a set of NiMH batteries always ready-to-use, without degrading their lifespan significantly?Is really the best approach to do -dV/dt (or dT/dt) at a higher current (so one can detect the voltage drop or change in temperature), and then switch to a once-every-hour pulsed "trickle" (maintenance) charging? Even if I don't need the batteries charged quickly?Thanks!

1. Sensitivity to Overcharging: NiMH batteries do not handle overcharging as well as other types. When NiMH batteries are overcharged, they can become overheated, which may cause them to degrade faster. Continuous trickle charging at a higher rate can lead to significant heat generation, potential venting, and even leakage or rupture.
2. Appropriate Charging Rate: If you need to trickle charge NiMH batteries, it should be done at a very low rate. A common recommendation is to use a trickle charge rate of about 0.05C to 0.1C, where "C" represents the battery&#;s capacity in amp-hours (Ah). For example, for a battery with a capacity of mAh (2 Ah), the trickle charge rate should be about 100 mA to 200 mA.
3. Using Smart Chargers: Many modern chargers designed for NiMH batteries have smart features that monitor the battery&#;s state and adjust the charging current accordingly. These chargers typically transition to a maintenance or "top-off" mode once the battery is fully charged, applying a very low current that&#;s just enough to keep the battery fully charged without causing damage. These chargers are preferable for maintaining NiMH batteries over long periods.
4. Duration of Trickle Charging: Even with low rates, it's advisable not to leave NiMH batteries on a trickle charger indefinitely. Long-term trickle charging, even at low currents, might still lead to reduced battery performance and lifespan. Ideally, once fully charged, the batteries should be removed from the charger, or the charger should automatically switch off or shift to a safe maintenance mode.

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