[time-nuts] HP-105B Battery Replacement?
brooke at pacific.net
Sat Sep 17 18:18:35 EDT 2016
The difference in chemistry I got from "Handbook of Batteries" 3rd ed, 2001. Your comments about modern chargers are
correct, but this thread is about the HP 105 which uses what we both might call an old fashioned charging circuit.
The lesser of evils is still evil.
-------- Original Message --------
> On Fri, 16 Sep 2016 11:37:23 -0700
> Brooke Clarke <brooke at pacific.net> wrote:
>> The chemistries are very different. Ni-Cad is endothermic whereas Ni-MH is
>> Exothermic. This is why chargers for Ni-MH
>> have a mandatory temperature sensor. This is one of the reasons I say Ni-
>> Cad cells batteries are easy to charge.
> Both NiCd and NiMH behave the same way chemically. Both reactions
> are exotherm when the batteries are full, i.e. the electrical
> energy you put into them cannot be "absorbed" chemically and thus
> is dissipated through heat. (I'm not sure whether it's correct to
> talk about exotherm/endotherm in this kind of setting, i'd appreciate
> if someone with chemistry knowledge would correct me) The reason why
> NiMH charger "need" a temeperature sensor is, because the classical
> fast-charger for NiCd uses the negative dV/dt slope when the battery
> gets full to detect end of charging, but the peak is much less
> pronounced with NiMH than with NiCd (factor 2-5 IIRC). Hence people
> were adviced to use only NiCd fast-chargers which had a temperature sensor.
> Slow chargers (i.e. 0.1C chargers) do not have this problem, though
> you shouldn't leave the battery on for days (NiMH is a quite bit more
> sensitive when it comes to overcharging). "Modern" fast-chargers for
> NiCd/NiMH chemistries have adjusted their dV/dt trip point to reliably
> trigger with NiMH. Additionally all better chips (probably all chips, today?)
> use pulse charging where the battery is measured during a short no-charge
> period to more accurately measure the batteries condition.
> NiMH is a good replacement for NiCd if you can live with the drawbacks.
> * slightly trickier charging (but that's the problem of the charger)
> * more sensitive to over/under charging
> * higher self-discharge
> On the positive side, you get a greatly reduced memory effect (to the
> point where a lot of people say it doesn't exist).
> Attila Kinali
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