Hi Attila:

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.

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
The lesser of evils is still evil.

-------- Original Message --------
On Fri, 16 Sep 2016 11:37:23 -0700
Brooke Clarke <bro...@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.
Namely:
* 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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