Re: Trickle charging NiCads

Phil Allison wrote:
<ggherold@xxxxxxxxx
"Phil Allison"
"pimpom"

I'm ok with designing the charging circuit to any degree of
precision as far as current regulation is concerned. I just want to
avoid a drastic reduction in useful battery life by constantly
overcharging it, while keeping the circuit simple by omitting a
complex full-charge detection.

Perhaps this is a good time to explain the intended application. I
want to
make an emergency light using white LEDs that will turn on
automatically in the event of a power failure. Nothing new in that.
But this unit will be connected in parallel with a normal house
light that's usually kept on
every night from about 5 or 6 pm to 1-3 am.

The emergency light will sense the state of the light switch even
in the absence of mains power and turn on only if the switch is in
the 'on' position - a fully automatic fit-and-forget operation.
I've designed and tested the circuit but am not sure what level of
trickle charging would be
a good compromise.

** Most NiCd makers suggest a C/50 rate for constant trickle
charging. But why use NiCds at all??

NiMH cells have lower self discharge rates and less issues like the
formation of "dendrites" that plague NiCd cells left on trickle
charge.

I was going to suggest a different battery type also. Why NiCds? If
you don't mind the weight there are also lead acid gel cells. I
haven't done much (read anything) with NiMH. But I thought I read
that they didn't want to be trickle charged. They wanted pulses... I
guess you can make a trickle pulser.


** Trickle pulsing is probably a very neat idea - funny I have
never seen it done other than in the standby-by mode of a pulse type
fast Ni-Cd charger.

The OP could set up a 555 timer, in astable mode, to produce a 1
second pulse each 30 seconds and have a CCS deliver say 500 mA to the
battery during that pulse. Averages out as 16mA or C/50.

OTOH - emergency systems nearly always use Gell Cells and with good
results. Constant voltage ( current limited) charging is the go with
them and is simple to implement


Thanks for your inpuits, everyone. NiMH, lead-acid gel, pulsed and timed
charging are all technically sound ideas. But ATM, I'm committed to the idea
of using one of those compact 3.6V NiCd packs that come with cheap Chinese
import gadgets such as a US$2 rechargeable flashlight.

My circuit uses 1 low-power transistor, one-half of LM393, 3x1N4007, 1 zener
diode, 8 resistors and 2 caps on a 1"x2" pcb, and I want to keep it no more
complex than that.


.

Relevant Pages

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