"overcharging" with LaCrosse BC-700

I went to test 4 Eneloop AAAs that I'd charged a couple of days earlier. When I put them in, the charger said FULL. When I clicked to Test, it charged at an indicated 500ma for 90 minutes! At first I was alarmed, but the voltage was only 1.42 and an infrared thermometer showed that they were at 90F, the same as cells with plenty of capacity yet to be charged.

This seeming overcharging has happened with cells that weren't Eneloops, both on Charge and Test. They stay at a normal temperature and register full when the voltage finally reaches 1.52 or so.

I don't believe they're really being fed the 500ma indicated. I wonder if under some circumstances (for example to top off for a test or when a cell that isn't charged gets too warm) the charger reduces current by narrowing the pulses, but the readout reads as if the pulses were still full-width.

I think I see why sometimes a cell may take far too long on a 500ma charge. At 1.5V, 500ma would be 0.75W. If charging is 90% efficient, 10%, or 0.075W goes into warming the battery. If on a given day, efficiency is only 70%, then .225W would go to warm the battery. As it warmed above 86F, efficiency would be less and less, warming the battery more. So it could receive 500ma for an unusually long time without overcharging.

My experience putting charged cells on Test was something else. If 4 charged 800mah cells stayed on charge another 90 minutes without warming above 90F, it seems clear that they weren't receiving the 500ma indicated.

I've had several problems with charging at 500mA (with AA). I stick to 700mA every time.

I've had a few missed terminations with older batteries and even newer batteries on my BC-700, but mostly in the beginning, when I was fiddling with things and trying to revive older, more worn out batteries.

It's not a perfect charger, but I'm glad I have it and I still pull it out here and there and charge stuff up with it.

If I could only keep one, I'd keep the Maha and I use my C-9000 most often.

Chris

How did you know they were overcharging?

Duracell recommends a 1C rate and termination when the temperature change is 1C (1.8F) per minute. That sounds more foolproof than looking for a slight reduction on voltage.

At 1C, Duracell says you're 85% charged when the cell is 35C (15C above ambient). Then the temperature climbs faster. Duracell says you're fully charged at a temperature of 42C (22C above ambient). I took that from a chart, so it's what they found measuring a certain cell under certain conditions. They recommend a cutoff at 60C, a lot hotter.

At 32C, I'm sure my 4 AAA Eneloops weren't overcharging even if the charger said it was pumping in 500ma for 90 minutes after they read full. The old AAA that rose to 52C must have been overcharging. It hadn't been charging long enough to have received the charge I'd just drawn from it.

I wonder if the problem was bubbles. I read about them in the late 1990s. If the gases didn't recombine fast enough, bubbles could reduce the effective surface area of the plates (if plates is the right term in a AAA cell). Maybe the gases recombine better in more recent batteries.