NiCAD vs. NiMH Batteries in Woodstocks

[Dieter]

(05-04-2018 10:12 PM)John Keith Wrote:  Actually, there are other more subtle differences in chemistry between NiCd and NiMH batteries. Long-term "float" charging, e.g. leaving calculator connected to charger after batteries are fully charged, will eventually damage NiMHs. NiCds on the other hand are not damaged by long-term unregulated charging unless the current is excessively high.

The datasheets I have read state an allowed max. trickle charging current of 0,03...0,05 C, both for NiCd and NiMH. Do you have other references that recommend something different?

But – the essential point here is that NiMHs are available in low self-discharge versions, for instance the Eneloop series. These batteries, once charged, can be stored for months or even several years without significant capacity loss. Charge them now, put them in a drawer, get them out by Christmas 2019 and they are ready to use. Which means that trickle charging is obsolete.

(05-05-2018 06:50 AM)grsbanks Wrote:  Hence trickle-charging NiMH batteries once they're full.

As mentioned above, this is not required at all with the right choice of batteries.

(05-05-2018 06:50 AM)grsbanks Wrote:  This said, the capacity of NiCd batteries of the day was roughly 450mAh and you're supposed to charge them at 1/10 charge capacity for 12 hours, so 45mA for 12 hours. By the time NiMH came on the scene, capacities had improved somewhat

...to about 750 mAh. In the 90s NiCds with 1000 mAh became available (e.g. Panasonic's P100AAS).

(05-05-2018 06:50 AM)grsbanks Wrote:  and NiMH can be charged at a much higher rate anyway.

Not at a higher rate (i.e. relative charging current), but at higher currents. These are proportional to the capacity, so 4x the capacity means 4x the charging current. What 50 mA did forty years ago now requires 200 mA with a current NiMH cell.

(05-05-2018 06:50 AM)grsbanks Wrote:  Surely the 45mA that a charger designed for period NiCd cells spits out is barely trickle-charging the NiMH cell, meaning that there's little to worry about regarding over-charging anyway, right?

Let's see: charging a conventional 2500 mAh NiMH battery at 45 mA means a relative charging current of 0,018 C. Which is about half the usual trickle charge current that can be applied permanently without damaging the battery (according to my data, please check with your particular battery type). But I wouldn't try charging an empty battery with such an extremely low current: it would take about three days. #-)

So the recommended method still is: get a set of Eneloops and an external charger with "smart" charge control. Avoid the calculator's internal charging circuitry for several reasons.

Dieter