[volt-nuts] Ni-Cad vs. Ni-MH "Trickle Charging"

[Brooke Clarke]

Hi Chuck:

There's a fundamental difference in the chemistry of these two 
batteries.  The Ni-Cad is what's called endothermic so the temperature 
remains relatively constant during charge, but the Ni-MH is exothermic, 
meaning that the battery temperature is rising along with the state of 
charge.  So you can leave a Ni-Cad on charge at a 0.1C rate and it will 
be charged in a half to full day and the temperature will be warm but 
not hot,. BUT, if you put a Ni-Cad on 0.1C charge it will get very hot 
and degrade the battery and might even catch fire or explode.  Hence the 
constant rate recommendation of C/250 (or lower).  It's not to charge 
the battery but to keep it from discharging.  Some other charge 
mechanism needs to be used in addition to the constant rate for charging.

Ref Handbook of Batteries, third edition, Linden & Reddy, pg. 29.22.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.End2PartyGovernment.com/


Chuck Harris wrote:
> On the contrary, the manufacturers of NiCd cells consider 0.1C to be 
> the highest
> rate that can be applied indefinitely to a NiCd cell.  It is by 
> definition the
> trickle charge rate.  At the 0.1C rate, the cell will be fully charged 
> in 14 to 16
> hours.... The extra 50% allows for losses in the process.
>
> If you charge at a C/40 rate, it will take 64 hours to fully charge 
> your pack,
> which is too long for most applications.
>
> C/250 to C/300, which are the maximum indefinite term charge rates 
> that modern
> NiMH cells can take are ridiculously long.  If you want to "trickle" 
> charge at
> that rate, you will need a charger that can boost the cell up to full 
> charge
> at a higher rate, and then drop down to the C/250 to C/300 rate as a 
> maintenance
> charge rate.
>
> -Chuck Harris
>
> shalimr9 at gmail.com wrote:
>> 0.1C is not trickle charge, it is the normal charge rate for 14 hours 
>> recharge
>> time. Most NiCad and NiMH batteries will not like that too much as a 
>> trickle
>> charge rate and will get and stay warm, at least those that I have 
>> had experience
>> with. The batteries in the HP 8116 function generator were trickle 
>> charged at that
>> rate and cause extensive damage when they get tired of it. I have had 
>> better luck
>>
>>
>> Trickle charge would be 1/10th or 1/20th of that.
>>
>> Didier KO4BB
>
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