AA cell discharger

Why?

Stack up Schottky diodes until their combined forward voltages are about

Although it might be worth investing in a smart charger that can charge and discharge cells at different currents while measuring their capacity.

Theo

Are you saying that you're charging these in parallel?

I can only recommend getting a half decent charger that detects the end point, not just a timer variety of charger.

Build yourself a push-pull regulator. The linear regulator ADR510, could discharge at 10mA on its own. Amplified with a ua741, that raises the drive level to 25mA or so. This OpAmp (DIP package, 8 legs), can sink or source 80mA. The OpAmp is a "rail-to-rail" design, well not really, but close enough to that property, for this project to work. There are a few OpAmps that swing right down to zero volts (using a negative bias generator internally).

VCC=3V VOH=2.63V VOL=0.18V # Rail to Rail swing, 1V output possible VCC=5V VOH=4.4V VOL=0.30V

Eneloop 1.45V off the charger 4.7 ohms or 5.6 ohms to a 1V discharge terminal 1.27V rough median while discharging (57mA)

This is a 1.000V reference. ADR510ARTZ. SOT23 (eye sight test!)

If you want less than 80mA, make R2 have a higher resistance value. R1 isn't too critical, and you could use 10K there if you wanted.

Vdiff^2 ----- = Pdiss of Resistor R2, 1/4 watt resistor is fine for the 4.7 ohm resistor R2

The purpose of this circuit, is not just discharge. You can leave this on the battery all day long if you like, as the discharger will not allow the battery to go below 1.0V . Regular discharge circuits (that resistor that was burning your hand), they could drag the battery below the knee you were trying to hit. With a push pull regulator, that can't happen.

It still approaches the desired voltage asymptotically. If the amp had a good quality current limit behavior, you could remove R2 entirely, safe in the knowledge more than 80mA would not flow. The reason I put R2 in there, is to "avoid surprises". With R2 gone, it gets to 1.0V a lot faster. As the Vdiff gets smaller and smaller, the discharger becomes less and less effective due to R2 being there. The discharger could "slap the battery silly" ... if it was allowed to run using its internal current limit to fix the behavior.

Paul

So that I can recharge them slowly.

It does that. If necessary it conditions the cells first. There are 4 separate channels that can be adjusted individually.

How will that help to "recharge them slowly"?

It might be helpful to answer this question.

Or provide details of the charger?

Didn't he say the charger had 4 individual bays?

A re-read does say that and I now understand what he's trying to say.

It does seems that at 350mA the end point is being detected early.

Presumably the conditioning the charger is doing is to run the cell down (to a voltage like the 1.1V you mentioned) and then charging up.

AA Eneloops hold 2000 mAh and he's charging at a slow rate (350mA is approx 0.2C). Maybe the charger is detecting some trivial voltage fluctuation long before the full charge point. For this reason I prefer

However he's been charging his cells for some time before this problem. So perhaps the cells are exhausted, as Eneloops tend not to have a memory effect.

>>

>>> >>>
>>> <snip>
>>>
>>>> For a while I was charging them on 150mA but it takes a long time for 12 +
>>>> other cells (done in batches), so started using the default of 350mA. >>>
>>> Are you saying that you're charging these in parallel?
>
> It might be helpful to answer this question.

>>> I can only recommend getting a half decent charger that detects the end
>>> point, not just a timer variety of charger.
>>
>> It does that. If necessary it conditions the cells first. There are 4
>> separate channels that can be adjusted individually.
>
> Or provide details of the charger?

Nitecore Digicharger, as recommended in this group when I asked.

They are Pros - 2.4Ah - good for 'up to' 500 charges. Mine, allthough several years old, get recharged every 18 weeks, so haven' been charged many times (<3x/year.

I was using 150mA then got impatient and went to 375mA; that's when the trouble started.

There are 6 models that take 4 batteries. I've looked at a couple and they charge at much higher currents than 150mA.

I believe the Nitecore Digicharger (which you mentioned elsewhere in this thread) doesn't condition batteries first.

As an test, I wonder how complete cells get charged if you charged at 750 mA. (Use only 2 cells in slots 1 and 3.)

that is correct. It will go to 750mA for a start then drop back to 350mA. It will do that on all 4 slots - as I have said several time the slots are completely separate.

When it's 350mA, pressing and holding the lower button for about 1S will change it to 150mA. (Holding in the top button for a couple of seconds turns off the backlight).

OK, sort of solved the discharching. I've a couple of puck lights that take 3 AA cells. One is at the side of the ped and the other in the bathroom. They save using clacky pull-switches during the night. Eneloops in, leave bathroom one on all night, bedroom on all day. Couple of days or so they go to around 1V. Into charger, let them run at 350 mA for a couple of minutes then down to 150mA. 17 hours they're charged (no trickle current) and are at 1.48V. Next is to try them in a brush.