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Re: [OM] Chargers and max voltage

Subject: Re: [OM] Chargers and max voltage
From: HI100@xxxxxxx
Date: Sat, 3 Mar 2001 14:33:48 EST
 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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