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I can see there I’d so much information on the net about charging NiMH batteries and so many forum discussions on it that I am a reticent to start another in fear of the stock answers that are already out there. My question however comes from a slightly different angle to all the others I’ve seen (and hence why I have been unable to find an answer) - I want to build a simple constant voltage NiMH maintenance battery charger where the goal ISN’T to fully charge the batteries (nor to charge them in the minimum time).

Just before publishing this I saw this question that actively asks the same though I’m not satisfied with the detail of the responses (that ended up discussing between themselves rather than the original question) so perhaps it’s worth asking again. I’ve read that a fully charged NiMH battery open circuit if 1.4V. So if I simply write up a 1.4V (or to be safe a 1.35V) voltage regulator to a bank of AA battery holders because the applied voltage is limited to 1.35-1.4V (before the voltage peak seen with constant current charging) when the battery reaches this voltage the current will slowly peter down to a negligible value. Sure, the capacity won’t be a maximum but as long as it above 60% I don’t really care.

So my question is, firstly would this work (or is there a fundamental oversight in this idea that would result in damage to the batteries). And second (assuming the idea would work) exactly what is the maximum safe charging voltage should be used. Thirdly, a response to the previously mentioned question have a value of 30% charge capacity using this method - is that really all you could get? (That is, is the other 70% really given to the battery during the peak characteristic of constant current charging?)

toolic
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N Hughes
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  • Why do you want to do this? Do you have a specific use case? There are much better solutions today with LiIon-cells. NiMH have self discharge and still some memory effect and a low, not so good useable voltage, so that you have to switch them in a row. – MikroPower Apr 07 '23 at 18:55
  • Each battery and each battery brand is different. Do you have modern chemistry Eneloops made in Japan or antique chemistry from China? The old Ni-MH chemistry loses some of its charge is a couple of weeks and needs to be frequently re-charged. Eneloops hold a charge for 1 year. I recommend using a charger that is made for charging the chemistry of your battery fully, then it disconnects. – Audioguru Apr 07 '23 at 19:00
  • I agree with @Audioguru about Eneloop. I just found an old box of saved NiMH cells (AAA, AA, C, and D.) Many of them were Eneloop that I bought from Costco, used for a while, and then piled into a box with other brand NiMH. Upon subjecting them all to re-charging for the first time in more than a decade, all of the Eneloop came back (but one) and appear to work pretty well right now. Most of the others (not all, but most) either couldn't be charged properly or else took only a tiny charge and are, my opinion, now worthless. (I used an XTAR Dragon VR4 Plus for charging.) I know: not an answer. – periblepsis Apr 07 '23 at 20:18
  • I'm not sufficiently expert on NiMH (which is why I prefer to use a commercial charger at this time.) You need to hear from an expert who is familiar, broadly, on their chemistry and more specifically also familiar with all of its various incarnations by different manufacturers. Short of having both that sufficient and necessary breadth and depth of knowledge and experience, I'm not sure you can get a simplified "yes/no" answer you can fully trust. But I think that's the kind of expert you will need to hear from. Simple yes/no questions often require great depth and breadth to answer. – periblepsis Apr 07 '23 at 20:22
  • The use case is for charging AA & AAA batteries that are much cheaper as NiMH. I want to have a bank of perhaps 10 of each that can be on constant maintenance charge so that they can be ready for use (with at least 60% capacity) at any time. This method also will maximize the cycle life of the batteries by ensuring they won't be overcharged. It is also a simpler solution in having just one place batteries are stored (on charge) rather than having to put the batteries on charge and then take them out of the charger to put in storage somewhere to have (to charger again before use). – N Hughes Apr 07 '23 at 21:28
  • @NHughes Sounds like you already have your answer, then. I certainly wouldn't feel comfortable saying "... will maximize the cycle life ..." So you are way ahead of me. – periblepsis Apr 08 '23 at 00:37
  • @periblepsis I’m only saying that based on the established principle that NiMH batteries’ cycle life is reduced by overcharging - so presumably it is maximised by undercharging. I don’t claim to have a thorough understanding of NiMH chemistry while charging so I’m waiting hoping for a response from someone who does … – N Hughes Apr 08 '23 at 05:41

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