NiMH battery charger; on a timer for "always charged" batter

[Poor Impulse Control]

I can't seem to find an NiMH charger that will keep a bunch (preferrably at
least 8, if not 16) of NiMH cells topped off and ready to use. I have a
couple of 4-cell Radio Shack chargers that seem to work OK, but unless I use
the batteries with a few days of charging them, I always get them with
incomplete charges.

This stymies my attempts to stop using disposable cells, and has completely
eliminated my wife's cooperation with that desire.

How much abuse would NiMH batteries get if I hooked a few chargers (to reach
my preferred "always charged" cell count) up to a household timer and had
the timer cycle "ON" for the charging period and then "OFF" again? By power
cycling the charger, I would get it to start charging the batteries,
preventing the problem of discharched batteries sitting in the charger.

My concern is that without a charger capable of "top-off" charging, I'd kill
the cells real quickly by putting them through too many unnecessary cycles.

If anyone is aware of a trickle charger/maintenance charger, especially for
AA batteries, I'd love to hear about it.

 

[CWatters]

"Poor Impulse Control" <usenet@grasslake.net> wrote in message
news:1155ftu2pqnos2b@corp.supernews.com...

My concern is that without a charger capable of "top-off" charging, I'd
kill
the cells real quickly by putting them through too many unnecessary
cycles.

It's not usually cycling that kills them it's overcharge at high currents or
over/reverse discharge.

 

[Doug McLaren]

In article <1155ftu2pqnos2b@corp.supernews.com>,
Poor Impulse Control <usenet@grasslake.net> wrote:

| How much abuse would NiMH batteries get if I hooked a few chargers (to reach
| my preferred "always charged" cell count) up to a household timer and had
| the timer cycle "ON" for the charging period and then "OFF" again? By power
| cycling the charger, I would get it to start charging the batteries,
| preventing the problem of discharched batteries sitting in the charger.

R/C guys do this all the time.

You've got a 50 mA wall-wart charter, and 500 mA NiCd cells. You just
flew your plane, and don't know when you'll fly it again. So you put
the light timer to charge for the next 16 hours, then turn off. Then
every day after that, it charges for one hour then turns off.

At low charge rates, NiMH batteries work just like NiCd cells, except
that the self discharge rate is about 3x as much (as you seem to have
noticed.) So the same procedure would work, though one hour at C/10
might not be quite enough each day to counteract the self-discharge
rate. And too much charging will wear out the cells.

As for which charger to use, whatever wall-wart that gives the desired
C/10 current at the appropriate voltage will work. The R/C chargers
seem to create a voltage that's a good deal higher, and then just use
a resistor to limit the current that actually flows, creating an
approximation of a constant current source.

And ultimately you should consider what's being done with these NiMH
cells, and wonder if NiMH even makes sense. NiCd cells will have
about 1/2 the capacity, but will last 2-3 times as long between
charges if not used. You can get AA NiCds at Wal-Mart, Lowes or Home
Depot in the solar powered light section. They're cheap too -- $5-$6
for 4.

For an emergency flashlight or something else that you almost never
use, it's hard to beat alkalines -- with rechargables, the self
discharge rate will almost guarantee that you'll never have battery
power when you need it. Same goes for things that use very little
power like smoke alarms and remote controls.

--
Doug McLaren, dougmc@frenzy.com
Truly great madness cannot be achieved without significant intelligence.