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wall transformer for Nimh batteries

R

ray

Jan 1, 1970
0
I have a flashlight handle that uses 2 1700 mAH Nimh batteries in
series.
How strong of an unregulated ac/dc wall transformer can I use safely
to recharge the handle?
It must be safe to leave plugged in for days at a time.
Will a 3vdc 200 mAH output transformer overheat the batteries or cause
damage?
Should I stay at 100 mAH to be safe?
What is the strongest transformer I can use?
 
M

mike

Jan 1, 1970
0
ray said:
I have a flashlight handle that uses 2 1700 mAH Nimh batteries in
series.
How strong of an unregulated ac/dc wall transformer can I use safely
to recharge the handle?
It must be safe to leave plugged in for days at a time.
Will a 3vdc 200 mAH output transformer overheat the batteries or cause
damage?
Should I stay at 100 mAH to be safe?
What is the strongest transformer I can use?

It's generally bad to leave batteries on continuous charge. The reason
you see it done so much is because it's cheap, and the vendor doesn't
care if your batteries last a long time.
Rechargeable batteries in a flashlight are problematic because the
voltage drops off suddenly. If the cells aren't perfectly balanced,
the weak one will go into reverse charge rather quickly. That weakens
it further and the whole thing snowballs. Turn it off at the first
sign of light dimming.

If you use it heavily sometimes and none other times, you have a
problem. You need a better method of charge control. If you use it
rarely, you should look into using the SMALLEST, not the largest current
that will keep it up...remembering that it's still bad.

I've put my rarely used items on a timer. I charge them half an hour
per day at a rate that just puts back the self-discharge plus average
use...which is typically zero. Had fewer battery failures since.

mike



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A

Activ8

Jan 1, 1970
0
It's generally bad to leave batteries on continuous charge. The reason
you see it done so much is because it's cheap, and the vendor doesn't
care if your batteries last a long time.
Rechargeable batteries in a flashlight are problematic because the
voltage drops off suddenly. If the cells aren't perfectly balanced,
the weak one will go into reverse charge rather quickly. That weakens
it further and the whole thing snowballs. Turn it off at the first
sign of light dimming.

If you use it heavily sometimes and none other times, you have a
problem. You need a better method of charge control. If you use it
rarely, you should look into using the SMALLEST, not the largest current
that will keep it up...remembering that it's still bad.

I've put my rarely used items on a timer. I charge them half an hour
per day at a rate that just puts back the self-discharge plus average
use...which is typically zero. Had fewer battery failures since.

mike

I kinda like those flashlights with the flip out plug. You just plug
them in and leave 'em. For my flashlights that have rechargables, I
just use a charger. I also have a Craftsman lantern (not like a
Colman lantern - it's a flshlight style lantern) that has a wall
wart stored inside to recharge it.
 
T

Terran Melconian

Jan 1, 1970
0
How strong of an unregulated ac/dc wall transformer can I use safely
to recharge the handle?

You can't.

When you have an unregulated wall-wart transfomer which says 200 mAH on
it, this doesn't mean that it will current-limit at 200 mAh. This means
that when you attempt to draw more than 200 mAh from it, it will blow a
fuse or catch on fire.

In order to recharge your batteries, you need an actual current-limited
supply. For NiMH, there are two charge rates which seem to be standard
across manufacturers. For short-term charging (it's on a timer and you
stop after about 12 hours), take the capacity of the battery, divide by
10 (in appropriate units), and charge at that current. This is called
C/10 (C is the capacity of the cell). For leaving it on charge forever,
use C/300. This will, of course, take >300 hours to recharge from a
fully discharged state (that's about two weeks), but you can leave it
for months on end and not damage the batteries. Usually these two
approaches are used together.

You *can* charge faster than C/10, but you'll need more sophisticated
electronics and/or a temperature sensor to tell when you're done.
 
R

ray

Jan 1, 1970
0
What are implications of using a basic unregulated 3 vdc 100 mAH wall
transformer to charge the handle that has 2 AA 1700 mAH Nimh
batteries?
Even if this isn't the optimal way to charge (and I know it isn't), as
long as charger is used a maximum of 15 hours, is there any major risk
to doing this?
If the charger was left plugged in longer, because the charge is less
than C/10, isn't the battery chemistry ok to accept this slight
overcharge?
 
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