Charging NiMH using a bench supply?

[cs.dk]

I've charged a lot of LA/SLA and Li batteries with my bench PSU, however i can't find the optimal charge for NiMH AA cells.. Googling around shows alot of different numbers. People tend more to agree on charge voltage/current of LA and Li.

So, to charge 6 AA cells in series, what would I adjust my PSU at/What V/c and what current is optimal?
It is Panasonic AA 2400 mAh batteries.

Thanks in advance.

[Kleinstein]

NiMH charging is not that simple, as there is no simple voltage limit to stop charging. When full the cells get hot / warm and the voltage drops slightly.

One option would be charging at C/10 for about 12 hours, if one starts from empty.
 

[Andreas]

So, to charge 6 AA cells in series, what would I adjust my PSU at/What V/c and what current is optimal?

Hello,

that depends mainly on temperature.
At normal room temperature (20-25 deg C) you can use 1.45V/cell with a current limit of 0.5-0.75 A.
I am using a diode (P600D) in series with the lab supply which adds additional 0.7V.
On hot summer days you should slightly reduce the voltage per cell.

so for 6 cells I would use 6*1.45V = 8.7V + the 0.7V for the diode (= 9.4V)
The "end voltage" should be adjusted so that after several hours the current goes down below C/30 .. C/50.

with best regards

Andreas

[Gyro]

For basic charging, then charging with constant current C/10 (240mA for 2400mAh cells) for 12-14 hours will ensure full charge, You can charge several batteries in series (up to the voltage limit of your PSU) Never charge in parallel. Constant current charging will tend to equalize the charge in the cells once they are full. It's a good idea to include a series diode too in case your PSU gets turned off.

C/10 charging can be maintained for a significant period of time (say, a day or two) without degrading the batteries. I remember some research that indicated that NiMH cells can be left on charge at up to C/30 (80mA in your case) indefinitely without ill-effects.

Voltage slope monitoring, and temperature sensing are only required by so called 'fast chargers' which charge at anything from C/5 to C/1 (?) for much shorter periods.

[John Coloccia]

C/10 is standard "overnight" charge rate for NiCd and NiMH, and has been forever.

[cs.dk]

Thanks alot,
I tried with 1,45 V/c with 240 mAh currentlimit, it seems to work just fine.

However, while charging them, it did come to my mind that I got an old dusty Trust brand (remember them?) charger laying around. I let it top them up. http://www.trust.com/en/product/13955-quick-battery-charger-430bq

[Gyro]

You mean you had a battery charger all along and only remembered after you'd charged your batteries? Classic 

P.S. I wouldn't put a hard limit on the cell charging voltage, it will settle automatically as it achieves full charge on CC.

[cs.dk]

You mean you had a battery charger all along and only remembered after you'd charged your batteries? Classic 

Yeah
I usually don't use rechargeable AA's, so the charger was put away for long time ago, it took me some time to find the damn thing.
Anyway, now i know how to charge AA's with a labsupply

[KL27x]

I've charged a lot of LA/SLA and Li batteries with my bench PSU, however i can't find the optimal charge for NiMH AA cells.. Googling around shows alot of different numbers. People tend more to agree on charge voltage/current of LA and Li.

The more research you do on NiMh batteries, the more weird it gets. I came across a NASA doc, detailing how they prep NiMh for spacey-stuff before launch, and half of it sounds like superstition.

One of the main problems seems to stem from this issue. In lead acid or Li ion, SoC is a function of voltage, and voltage is a function of SoC. IOW, if you are to graph SoC on the horizontal axis and voltage on the vertical axis, there will never be two different points on the curve that have the same voltage. This does not appear to be the case for NiMh. This why you should avoid charging NiMh cells in parallel, if possible, and why you need a thermal feedback to actually know when the battery is fully charged.

[amyk]

One of the main problems seems to stem from this issue. In lead acid or Li ion, SoC is a function of voltage, and voltage is a function of SoC. IOW, if you are to graph SoC on the horizontal axis and voltage on the vertical axis, there will never be two different points on the curve that have the same voltage.

Voltage of lithium ion also varies depending on load, and is very flat for most of the discharge/charge cycle. The difference is that the endpoints are well known.

[KL27x]

Voltage of lithium ion also varies depending on load, and is very flat for most of the discharge/charge cycle. The difference is that the endpoints are well known.

I cannot verify one way or the other, personally. But my previous statement has completely different implications re: li ion vs NiMh. And I'm talking strictly float voltage. Never mentioned load.

Two li ion cells that both measure xV float voltage are in a similar SoC. Even though the curve may be flat in this area, you could tell they are both within a certain, contiguous range of SoC. And given enough decimal places and a high enough test probe impedance, you could get a pretty good rough estimate.

Apparently, two NiMh batteries at xV may be in two completely different, isolated points where that SoC curve goes up/down through xV. So at xV volts, SoC might be (for sake of argument) between 20%-30%... OR it might be between 80%-90%. And you wouldn't be able to tell by the voltage. Again, I can't personally attest, but what I understand is that there is at least one "hump" in the voltage-over-SoC graph for NiMh, or maybe it's not even completely consistent between individual cells and/or actually even changes through charge cycles.

Imagine what occurs in parallel charging. In lead acid or li ion, the batteries will end up charged at the same time. The one that may start to lag behind will suck relatively more current, in a self-balancing manner, and they will finish at the same time. In NiMh, once the cells reach one of those negative slopes on the curve, one of them will break through first, and the one with the higher SoC will start to suck more of the current in a positive feedback loop, possibly even sucking some current from the lower SoC cell during this time.

Been many years since I had any interest in NiMh, but this was my understanding of why NiMh do not like to be charged in parallel. Yeah, I can imagine the thermal feedback might be necessary even if the curve were simply particularly flat at that point, I suppose.