constant current circuit

[Winfield Hill]

Tim Hubberstey wrote...

For years I've used the following circuit for charging NiCd cells.
It is very simple and has the added advantage of providing visual
indication of charging in progress. The regulation isn't great but
it's good enough for battery charging.

Any non-Darlington NPN with a reasonable gain (>20 or so, less gain
= poorer regulation) can be used as long as it can handle the power
dissipation (0.8 W for the values shown). The values shown assume a
LED voltage of 1.6 V @ 15 mA. The 4.7 ohm resistor sets the charging
current, adjust as necessary. You can "turn it upside down" and use
a PNP if you want one of the battery terminals ground referenced.

+5 V ---+------------> +
|
/ 200 mA to Battery, 1-2 NiCd cells
\
150 / +---> -
| /
| |/
+-----|
| |\
Red | V
LED | |
_|_ \
<= \ / /
_V_ \ 4.7
| |
GND ----+--------+

Very nice Tim, but what happens after the NiCds are fully charged,
stop accepting charge and start to heat up? I sometimes plop four
of my camera's NiCd or NiMH cells into my Radio Shack charger and
leave, only to realize a weeks later that they're still in place.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[Tim Hubberstey]

Tim Hubberstey wrote...

Very nice Tim, but what happens after the NiCds are fully charged,
stop accepting charge and start to heat up? I sometimes plop four
of my camera's NiCd or NiMH cells into my Radio Shack charger and
leave, only to realize a weeks later that they're still in place.

Well, my assumption (perhaps incorrect) is that anyone specifically
asking for a constant current charging circuit is aware that you can't
continuously charge NiCd cells at a rate greater than C/10 without
damaging the cells. If you want to be sure you won't damage them, charge
at C/15 to C/20 and accept a longer charge time, or use a more
intelligent charger. After all, the OP asked for a "simple constant
current circuit", not a bullet-proof charger.

My understanding is that you shouldn't charge NiMH cells using constant
current because they are much less tolerant of overcharge. You need to
use an intelligent charger or keep the charge rate way down to something
like C/100, IIRC.