(I'm cross-posting this to the electric forum. Way too many forums here, and I might get good advice there, too.)

I have the Hobbico Accu-Charger Elite. This is a nice charger, and I like that it gives me a lot of info about the batteries. (How much energy it took to charge, the amount of capacity on discharge, and charging rates.)

But I'm having a problem: When I try to charge higher-capacity NiMh packs, it seems to never complete the charge. For example, the battery in my glow stick is a 3400mAh sub-c cell, and when I charge it at a modest rate of around 350mA, the charger never peaks - even after pumping 5000mAh in to the cell! My 4 cell 1600mAh NiMh flight pack does the same thing.

I'm pretty sure the charger is working right; it has no problems with the 900mAh batteries I use in my Easy Star, and the 600mAh NiCd packs that came with my transmitter and receiver seem to work fine. It's just these high-capacity NiMh packs that won't peak.

Do I need to change how they're charging? More current? Adjust the peak detection? (it will go from 2mv up to 10mv, I think. The default is 8mv) I want these batteries to peak and not overcharge, and I prefer keeping the charging rates low - but if they never peak out, I'm thinking that I'm doing something wrong.

Any ideas?

You are charging the battery at too low of a rate.

You are safe up to 1C w/o problems, so your 3400mAh pack would accept a 3.4 amp charge current.

Cut that in half and you are still at 1.7 amps, so charge at 1.5 to 2 amps if you cannot hit 1.7.

With the 900mAh batteries at 350mA you are closer to their .5C current... which is why they work for you.

I remember seeing some battery charging curves, plotting battery voltage vs. time with different constant charge current settings. The higher currents would peak out the voltage and then it would start coming down (which is what "negative delta-v" peak chargers look for). At lower charge currents, the voltage would level off at a maximum and stay there (i.e. not decrease), which would likely fool most peak chargers. I thought you had to get down below 0.1C before the peaking went away, but I guess it depends on the cells your charging.

That may be but some of this may be tied to the condition and usage of his packs.

I've seen the problem he is having with a couple of relatively new packs.

After a full charge/discharge/charge cycle, I could go back to the lower rate.

Even with NiMH's some manufacturers recommend a higher & longer initial "forming" charge.

That's kind of what I thought. I was intentionally setting the rate so that it would take 8-10 hours to charge, but maybe I need to look for 3-4 hours.

I was charging the 3400 cell at somewhere around 350ma, so it's probably just not enough to peak the cell. I'll try running it at something more like 850 next time and see if that works better.

Thanks.

also if i remember correctly that charger is only capable of 2A charge rates, and may not even be able to supply that on the transformer that it comes with. I think that it is able to supply 2A on both sides if you run it off a car battery due to higher input capacity to the charger than the AC transformer is able to supply.

He would have no problems charging those batteries at a 2A rate.

They should not even get a bit beyond barely warm at that rate.

This is correct. It will only supply 2.00 amps total on the DC power supply. So if you are charging two batteries it is only capable of charging each a 1.00 amp.

Austin

Yeah, the transformer is rated to 2.5A. There's a chart in the manual; basically, I really only want to use it for diagnostics, to make sure a battery is up to snuff. To do that, I'll cycle it and check the total mAh at the end of the cycle; but since I haven't gotten the new batteries to peak, I don't have a baseline yet.

I didn't want to cycle my cells needlessly, but I might do one cycle on the receiver pack tonight just to see if it works. I just got a new radio set with a new receiver pack, so it's not like I don't have spares (3 rx packs, 3 tx packs.) The only thing that's a disappointment is that the pack from my 4 or 6ch transmitters won't fit in the 7ch unit.