chling@xxxxxxxxxxxx writes:
<< 1. Over discharge damage only happen to on-load discharge not self
discharge. Full self-discharge will not cause problem if you re-charge them
before use.
>>
CH is correct about this as far as the discharge is concerned but there is
another failure mode if the cells are left fully discharged, for extended
periods. This applies particularly to old cells. NiCd's are sealed with an
elastomer. Under charge the cells generate pressure internally that tends to
apply more pressure to the seals increasing their effectriveness. The same
pressure also tends to compress the seals over many charge discharge cycles.
Eventually if left fully discharge the electrolyte gets through the seals
and the cells slowly dry out losing capacity. If you examine old cells you
can often find crystals around the top at the seals showing evidence of
leakage. So old cells last better if left on a very low float charge or if
you recharge them every few months if unused. Most chargers do not have a
very low float float current at charge end. You can make your own float
charger with a resistor and diode and a low voltage wall wart power supply or
equivalent. Just select the resistor to give a current of say C/100 or C/200
where C is the battery capacity. So for example for a 1Ah battery (1000mAh)
choose a current of 10 or 5mA . Put the diode in series with the resistor to
stop the cells discharging if power goes off.
Regards,
Tim Hughes
>>Hi100@xxxxxxx<<
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