NiMH preventing self discharge.

I`m considering keeping a whole bunch of NiMH batteries on standby, I know they don`t mind being series charged, so I thought if I had a 6x D cell holder filled with identical fully charged D`s, and gave that pack 9 volts through a 900 Ohm resistor, that should be a 10ma "trickle charge", my goal isn`t to charge them exactly but rather stop them self dischaging over time.

Would this work?

Katherine Alicia said:

I`m considering keeping a whole bunch of NiMH batteries on standby, I know they don`t mind being series charged, so I thought if I had a 6x D cell holder filled with identical fully charged D`s, and gave that pack 9 volts through a 900 Ohm resistor, that should be a 10ma "trickle charge", my goal isn`t to charge them exactly but rather stop them self dischaging over time.

Would this work?

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i just picked up some 2800 cells but i should of went with the 10 year eneloops

raggie33 said:

i just picked up some 2800 cells but i should of went with the 10 year eneloops

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it`s exactly because I`m upgrading that I`m going to end up with a load of D cells as well (36 of them! LOL). but you do the best you can at the time with the knowledge (and resouces) that you have, and at that time NiMH D cells were the best I could come up with, like your 2800`s

Katherine Alicia said:

I`m considering keeping a whole bunch of NiMH batteries on standby, I know they don`t mind being series charged, so I thought if I had a 6x D cell holder filled with identical fully charged D`s, and gave that pack 9 volts through a 900 Ohm resistor, that should be a 10ma "trickle charge", my goal isn`t to charge them exactly but rather stop them self dischaging over time.

Would this work?

Click to expand...

It would work but is not a good idea at all.
1) Nimh do not like being trickle charged unlike Nicd trickle charging slowly degrades them over time
2) Nimh can be series charged but if not matched up well cells could complete charging before other cells and then be subjected to trickle charging at a high current rate which would weaken them.
Get Eneloops (LSD) and they should retain about 80-85% of charge for a year and the discharge rate should drop even more after that.

Lynx_Arc said:

It would work but is not a good idea at all.
1) Nimh do not like being trickle charged unlike Nicd trickle charging slowly degrades them over time
2) Nimh can be series charged but if not matched up well cells could complete charging before other cells and then be subjected to trickle charging at a high current rate which would weaken them.
Get Eneloops (LSD) and they should retain about 80-85% of charge for a year and the discharge rate should drop even more after that.

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yeah, I had a rethink of the math, and 10ma would be like 240ma per day which is Waaaay more than it loses! I`m probably better of with a quarter of that, a 3.6k resistor giving them all 2.5ma per hour sounds much better.
I would use eneloops, and indeed do! (about 30 of them) but these batts have had maybe 3 charge cycles since I bought them this year, so I`m not going to just scrap them, but at the same time I refuse to be charging 36 cells every month or so "just in case".
There must be a way to acheive stasis with them, where they`re neither charging nor discharging, but held at a balance.

If they're high self discharge, maybe you could set up a trickle charge on a timer to run an hour or two per day to keep them topped off without over charging/over heating them. It's probably best to under charge them a little and do a full charge on the individual cells once or twice a year.


If they're low self discharge there's no need – best to just charge them in individually 1-2 times per year or as needed after using them.

Katherine Alicia said:

yeah, I had a rethink of the math, and 10ma would be like 240ma per day which is Waaaay more than it loses! I`m probably better of with a quarter of that, a 3.6k resistor giving them all 2.5ma per hour sounds much better.
I would use eneloops, and indeed do! (about 30 of them) but these batts have had maybe 3 charge cycles since I bought them this year, so I`m not going to just scrap them, but at the same time I refuse to be charging 36 cells every month or so "just in case".
There must be a way to acheive stasis with them, where they`re neither charging nor discharging, but held at a balance.

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What you should do is measure the resting voltage of the batteries and when they drop to a certain level then charge them. I had some decent regular nimh and they do ok for 2-3 months but at 6 months they seemed to fall under half capacity. I quit using them because they discharged too fast. I now rely more on 18650s which self discharge way slower than LSD nimh.

Lynx_Arc said:

What you should do is measure the resting voltage of the batteries and when they drop to a certain level then charge them. I had some decent regular nimh and they do ok for 2-3 months but at 6 months they seemed to fall under half capacity. I quit using them because they discharged too fast. I now rely more on 18650s which self discharge way slower than LSD nimh.

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That is one thing i learned from li-ions that they really hold their charge really well, a bit too well i'd say.

But unlike nimh, i read that it is not a good idea to keep li-ions topped off correct? You are right about them being more reliable than nimh.

when you mentioned measuring the voltage you gave me an idea, if I take a reading of the total voltage then again at 24 hours, that should give me the initial discharge rate, so if I can match that loss with a holding charge of the same amount then it should work (I hope).

XXO: I think you`re right about not keeping them charged to the max and holding them there, maybe 1.4v per cell would be better.

MarioJP said:

That is one thing i learned from li-ions that they really hold their charge really well, a bit too well i'd say.

But unlike nimh, i read that it is not a good idea to keep li-ions topped off correct? You are right about them being more reliable than nimh.

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It really depends on how you use them. It doesn't hurt them at all to top them off only counts as a partial charge when you do so and you then know how much runtime you have. As for storing them unused for long periods of time some people discharge them to a certain level that can extend the lifetime some but IMO if you can buy more easily enough at decent prices I see little need to overstock up on them such that you have a bunch stored for years without using them. At the price of them vs alkaleaks even if you lose a dozen recharge cycles and some capacity you still come out way ahead of using primaries.

Katherine Alicia said:

when you mentioned measuring the voltage you gave me an idea, if I take a reading of the total voltage then again at 24 hours, that should give me the initial discharge rate, so if I can match that loss with a holding charge of the same amount then it should work (I hope).

XXO: I think you`re right about not keeping them charged to the max and holding them there, maybe 1.4v per cell would be better.

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I would measure voltage 2-3 days after charging them and subtract about 10% from that and set that as the voltage to consider recharging them. Typical nimh drop to about 1.35 or so after a few days after charging it varies by brand and chemistry (LSD, non LSD, low/high capacity etc) Keeping them at 1.4v would probably be hard on them. Personally unless I absolutely needed them on standby all the time I wouldn't bother recharging them but every 6-12 months or so and invest in some Energizer lithium primaries enough to hold you till you can recharge a set of them for use. For all but high drain devices LSD should have 80% capacity after a year so a 2000mah cell should be about 1500-1600mah which is plenty if you have a lot of them to suffice and gives you time to top off or totally recharge the depleted ones used.

Charge your Eneloops before storage, and stick them in storage for as long as you want.

I'm running some long-term testing on Eneloop self-discharge characteristics. I'm far from done yet, but I've noticed these trends:

4 year old Eneloops, new out of the package: 72% charged (measured using available energy left in the cells in watt hours). Keep in mind they don't fully charge the cells before packaging them.

1 week of storage, after a full charge: 94% charged. This is also about the same after 1 month. So, most of the initial self-discharge happens very soon after charging.

By 4 months of storage, it's down to about 85% charged. Note that this is energy left, not milliamp hours, and I suspect the mAh would be higher since the voltage is highest on the first part of discharge.

By 6 months, it's still 85%. And it sticks around that for a long time.

So, to conclude, I think you can reasonably expect Eneloops to hold at about 80% - 85% charged for many years, with most of the self-discharge happening within the first few months of storage. When they reach that level, self-discharge basically stops.

That level I measure at about 1.32v to 1.33v. So, if you're designing a system to top up an Eneloop, I'd do it at 1.32v, and it will probably never be reached except when you use the battery. That will keep the battery at about 80% charged.

some helpful figures there, Thanks!

I have about a dozen duraloops (white top 2000mah Japan) that I last charged about 5 years ago at 1.285v now I figure they probably have more than half the power they do charged fully. Since I have had them for over 10 years now and never used these batteries in any emergency I'm going to leave them alone till I do need them. I have plenty of lights with batteries in them about half are 18650 now and that probably accounts for about 2/3 of my "runtime" if needed.

These are fascinating studies. I never could understand why do cells self-discharge in the first place and also when it comes to percentages, why does it drop from 100% quickly and then tapers off and stabilizes? This has been a phenomenon in batteries in general.