Proper termination voltage for "Full" NiMh

[was.lost.but.now.found]

I made the switch from the 15 minute Energizer charger to the BC-900. I wanted something that would be a little softer on my batteries. I seem to understand there are two prevailing theories - (1) charge as low a rate as possible - (2) but not so low that the cutoff cannot be detected. I've also heard that the BC-900 may not be the best for determining this at the slowest charging rate (200 mA). Is this correct so far?

As soon as I got the charger Tuesday night I went right to running a "Test" cycle on the supplied free batteries. Immediately I could tell that two of the four AAs were junk as they discharged at a much higher rate and finished at an accumulated capacity of 1251 mAh, and 1420 mAh (out of stated 2600 mAh). The other two are still going so they should be much stronger using a rough estimate of the total time for charge/discharge/charge of the bad two compared to the other two.

I did run this test at the default 200 mA charge; I watched closely every few minutes toward the end of the charge to ensure they would terminate and not endlessly charge. The final voltage when they switched to "Full" was 1.45 volts, and after a short rest into the trickle charge they went to 1.44. I was wondering if this sounds correct or if it is too "hot" and I should up the charge rate to 500 mA so the charger can better detect what I believe is called delta-v?

As another note, my next test will be to run a full refresh cycle and see if the capacity improves. I would prefer to use the lowest charge rate which will be the most gentle on the batteries, but not if it's going to damage them. Am I worrying too much? Is there not much difference between 200 and 500 charge rates?

Thanks in advance for your answers!

 

[metlarules]

1.42 to 1.45 is what my regular aa's come off of the charger. My eneloop lsd batteries come off as high as 1.51v.

 

[AlexLED]

Hello now.found !

For low charging currents (up to about 200mA or 0.1C), 1.45V sounds reasonable and to some extent does show the state of full charge. Some people and chargers even recommend this as evidence.

But more into detail, there is not really such a thing as a proper termination voltage, since the voltage during the charge is mainly determined by the current setting and the internal resistance of the cell. (To be complete, also by the voltage of the actual chemical reaction.) When charging NiHM at 0.5C or more, the voltage commonly shows 1.6V and more, but this does not really pose any problems.

Your fear about higher charging currents is not really necessary. Even with 0.5C to 1C charging currents, healthy cells don't really get hot. At least not if you use a decent charger (like your BC-900), which detects a small increase of temperature soon enough.
On the other hand, overcharging NiMH with low currents (resulting in no increase of the temperature and hence no chance of detecting full-charge) is as bad as high temperatures during charging.
So, to be gentle on your cells, you need to find a compromise between actually generating a temperature increase in order to determined the state of full charge and keeping the cells cool. This compromise for healthy cells and decent chargers is at about 0.5C.

Does this answer your questions ?

 

[Ragiska]

run another test on them and i bet you'll get a much higher capacity. those lacrosse cells need broken in, in my experience

btw, i think the test cycle gives discharge capacity, not accumulated capacity

 

[was.lost.but.now.found]

run another test on them and i bet you'll get a much higher capacity. those lacrosse cells need broken in, in my experience

btw, i think the test cycle gives discharge capacity, not accumulated capacity

I think you are dead on for both accounts.

I got home last night and went ahead and started a discharge refresh cycle for all four cells, and based on Alex's advice this time I bumped it up to 250 discharge / 500 charge. This is still more like .2C compared to .5C but I'm taking it slow. The two cells that came out of the first "test" cycle at such a low capacity finished their first round of refresh at better than 2500 mAh, already well within the stated +/-10% of 2600 mAh. This is fantastic; maybe they're not junk after all. You were right also about the discharge capacity, it does not measure accumulated capacity.

When I'm all done with these I plan to do some refresh and test cycles on my Tenergy LSD AAs that have a stated capacity of 2300 mAh. I believe everyone here when they say eneloop are the best (I don't yet own any), but I will then be able to make my own cost/benefit analysis to see how well these are doing, or whether I'd like to make the switch to eneloops.

Thanks to all for the helpful info.

 

[AlexLED]

I agree with the above, you should be able to get a higher capacity after some cycles.

You might even be able to get a higher capacity from the non-eneloops, but the self-discharge is a lot higher than with eneloops, so even after some days part of the charge will be gone.

 

[was.lost.but.now.found]

Man you guys are good.

After quite a few tests across a variety of brands and charge levels over the last couple of weeks, I've noticed with consistency a termination of 1.45/1.46 for most batteries at low charge level, and 1.48/1.49 at higher levels (.33C to .5C, I haven't gone over that yet). It's cool to watch the charger; the second number is the highest voltage reading, the first is what it drops to when it reaches delta v - moments before it reads FULL.

It seems to be terminating correctly (and consistently) even at low charge rates.

 

[mdocod]

ALl your numbers look perfectly acceptable...

Funny though, that you WERE charging at 10 amps (with your energizer battery deep frier), and now are debating if 0.5A is *too* high... hehe....

IMO. for AA cells. Anything in the 200mA to 2000mA is a perfectly acceptable charge rate. The higher rates will terminate more accurately, but at 200mA, it's basically a trickle rate anyways (0.1C) so missing termination wouldn't be the end of the world.

Eric

 

[was.lost.but.now.found]

Funny though, that you WERE charging at 10 amps (with your energizer battery deep frier), and now are debating if 0.5A is *too* high... hehe....

Very true, though with the Energizer charger stood up to allow maximum fan cooling, the batteries got about the same temp as .5C charge level on the BC-900. I need to get a little fan to aim at it.

 

[qwertyydude]

I've measured my energizer battery fryer and it "only" does 7.5 amps. I got tired of it rejecting half my good cells so now I only charge at 2 amps on my hobby charger. Fresh off the charge is about 1.45 volts which settles down to 1.38 or so after an hour and then holds steady at 1.35 or so for as long as I keep them stored.