Continuously charging NiMH - best approach?

[tooki]

In the coming weeks I will be rebuilding a couple NiCd packs with NiMh cells and my plan was to leave them permanently trickle charging. I do not quite understand how can that be a problem. How can it be a problem to supply a trickle current equivalent to the self-discharge? Or even a bit less?

They're not optimized for that, and you're basically constantly overcharging them.

[jirij]

Now that we know what your "X" is, I think it would be a better idea to design a relatively well-engineered modern-day fast charger, with the twist that once done, it goes into very low leakage mode, so that you can be confident that leaving the cells on the charger does not discharge them even in long term; basically combining the pros of your initial proposal with the pros of the usual fast chargers.

Yeah, I might eventually do that. I looked around at some charger ICs like MC33342 or BQ24401, and some of them are SPI/i2c programmable, so I could program in my desired top-up charge period (in minutes), and "duty cycle" (in seconds). But many are old (2001 to 2010) and no longer available, and what remains is pretty expensive, given that I'd need ~15 of them. And, as it happens, lcsc.com doesn't have filtering based on battery chemistry, so I'll have to dig through the results to find something cheap (and programmable). ... I guess I could always roll my own solution with adjustable linear regulators, given that I'd already need MCU(s) anyway.

I have several NiCD/NiMH chargers, but they all seem to suck at terminating the charge. They all over-charge to >1.5V while the batteries are rather hot to touch. So I'd like to go with a SMD thermistor as close to the cell as I can get it (fractions of a mm, cutting a hole in each battery holder, maybe adding a thermal pad or a formed thermal conductive silicone plug, for perfect contact).

How can it be a problem to supply a trickle current equivalent to the self-discharge? Or even a bit less?

I don't pretend to understand all the herbs and spices in today's NiMH cells, but there can logically be a difference between applying a voltage potential to a chemical cell long-term (with little current), and for that to have a different effect than applying a higher current short-term.

Not a chemist either, but I know enough to know that the rate of some reactions needs to be controlled, or you get different resulting compounds (see ie. polymerization, or even crystalization).

So it's quite possible that a modern NiMH cell can't be modelled by a simple energy storage equation (X goes in, X minus self-loss comes out). The cell might benefit from a "top-up" charge once in a while, as opposed to constant current equal to self-discharge.

[vaq]

My two sats:

I've been keeping several AA nimh cells charged with 7.8 mA going through them for around a year (some slightly longer, some 11 months) 24x7.
I did this for use in a simple 'Power Outage LED' project which is a clear jiffy box glued ontop of a nice flat 240v USB power supply (2x AAs each).
FWIW, Wikipedia says "Duracell further suggests that a trickle charge at C/300 can be used for batteries that must be kept in a fully charged state."
https://en.wikipedia.org/wiki/Nickel%E2%80%93metal_hydride_battery#Trickle_charging
I read this thread and thought I'd better test them.
I'm using 2x "PKCELL 2800mah" for one unit and 2x Ikea Ladda 2450mah cells for the other.
I tested them using a BT-C3100 charger, which isn't the best for testing but in this case is good enough I'd say.
(Side note: for the BT-C3100, I use a 0.1F capacitor (made up of 10x caps) and a 5A 12v PSU so it can hammer away all it likes.
 I bought 10x caps so I thought I might as well use them all and fill up the jiffy box that I put them in. Uses a 555, relay and resistor for soft start.)
I feared that the Ladda (made in Japan) cells would have increased internal resistance after 11 months @ 7.8mA trickling, but both are ~90-100mR (about the same as a new one).
The PKCELL cells shows ~120mR.
The capacity tests are still good as new (the "PKCELL 2800mah" cells are only ~90% of the Ladda 2450mah cells, new and now).
Side note: keeping them at 7.8mA keeps them at ~1.43v.

[iMo]

I trickle charged Eneloops 2000mAh with 75mA aprox 2416hx7d (6pcs in series) in my handheld TRX for perhaps 10 years long, they are still working as new. 3 months back I replaced them out of my curiosity with LADDA 1900mAh and they are trickle charged the same way, still working fine.
PS: edited the hours (it charges when my wifi router and co. is on).. The 0.05*2000=100mA, my TRX has got a built in trickle charger (originally intended for the 1000mAh NiCd charging used when new 23y ago)..

[wraper]

There was a Motorola wireless barcode scanner at my work that was barely used and stayed on its docking station all the time. Original and replacement NiMH battery pack lasted for about 1.5 years until ESR became abysmally high. Told them to just leave it nearby and only put on the docking station for charging once in a few weeks. Still working fine after about 6-7 years.

[Andy Chee]

I seem to recall having read in a Panasonic NiMH document, that charging at 0.05C should be considered the maximum continuous charge.  Charging at any higher current means the cell will be unable to recombine hydrogen fast enough once the cell reaches fully charged state.

[Conrad Hoffman]

I was hoping someone else had done the work already!  There's quite a few devices that used NiCad batteries as voltage regulators of a sort.  I have two of them,  Fluke 731B voltage reference and an HP403B AC voltmeter.  I think I might just try to track down the Varta cells that tooki mentioned for the 731B and just use the zener/capacitor method for the 403B.  Then I can do a multi-year test in the device.  I think I can reduce the charge current and use a removable battery holder instead of soldering up a permanent pack.

I curse the designer of those NiCd power supplies, especially the ones that used the battery string to create a dual supply with low impedance ground from a single supply. NiCd batteries don't particularly like being trickle charged either, and those battery packs never lasted as long as one might hope. I replace the singles with a big cap and zener, which usually works fine, like the 731B. The duals, like used in the HP 204D/209 signal generators, are more of a problem and really need a new supply with a CT transformer.

I'm not aware of anything, save for lead-acid cells and high-value capacitors, that does well with long term float or trickle charging.

[Andy Chee]

I found a new version of the Panasonic NiMH document:

https://eu.industrial.panasonic.com/sites/default/pidseu/files/downloads/files/id_ni-mh_1104_e.pdf

Page 17 contains the charging specifications.