Re: NiMH battery compatibility with NiCd chargers
From: H. Dziardziel (hdzi_at_zworg.nospamcom)
Date: 06/29/04
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Date: Tue, 29 Jun 2004 20:15:35 +0900
On Tue, 29 Jun 2004 01:12:20 -0400, Andy Cuffe <baltimora@psu.edu>
wrote:
snip
>
>For best results, you need a charger designed for Ni-MH, but they seem
>to be more tolerant than many people would have you believe. I
>rebuilt the Ni-Cd battery pack in my cordless phone using some Ni-MH
>cells I had on hand and it still works well after 3 or 4 years of
>almost constant charging.
Unless you made any modifications to the charging current I
suspect the NiMhs, being about double the original NiCad's
capacity, are gently warming themselves at a continous C/20-C/10
rate.
Good qualty recent manufacture NiMh cells can handle that
overcharge for short periods but nevertheless gradually
deterioate. That may not have been noticed since their
original capacity was about double the NICads.
They will last about 4-5 years in a low load like the phone but
far less with high loads due to their internal resistance buildup
as a result of the continous overcharge. .
For maximum life Nimh must not be overcharged, nor trickle charged
and maintenance charged with care. They also die quickly with
heat yet are exothermic during charging especially so towards the
end of charge. Tricky buggers..
The NiCads are more rugged and handle overcharging better but were
also most likely being slowly cooked at about a C/10-C/5 rate in
the phone. Continous C/20-C/10 is generally fine for them (but
not for NiMh).
> I would think you would have more trouble
>with a 'smart' Ni-Cd charger than a simple charger because it's hard
>to say how the smart charger will handle the wrong type of battery. I
>have no seen any shorted Ni-HM cells yet. What I have seen is a very
>high self discharge rate in old Ni-MH laptop batteries. It's not
>unusual to see a laptop battery that will run for 2 hours when freshly
>charged, but if it's left sitting for even a few hours it becomes
>mostly discharged.
>Andy Cuffe
>baltimora@psu.edu
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