NiCAD vs. NiMH Batteries in Woodstocks

[rolfmwa]

I am about to take delivery of yet another HP21 beauty. Rather than using standard AA 1.5 Volt batteries I want to use rechargeables as the calculator comes with an original power unit.

Will it be ok to use 1.2 Volt NiMH batteries rather than NiCADs or do I miss something here?

Any feedback appreciated.

[Zaphod]

If you're charging them outboard on a proper Nimh charger then no problem ...

But don't use your HP power unit to charge them , it's obviously a different chemistry with a different charging scheme.

[Dieter]

(05-04-2018 10:25 AM)rolfmwa Wrote:  Will it be ok to use 1.2 Volt NiMH batteries rather than NiCADs or do I miss something here?

NiCd and NiMH both provide the same output voltage (which is why the newer NiMHs usually were able to replace NiCd when they became commonly available almost 30 years ago). Since calculators usually do not take advantage of special NiCd features (behaviour at very low temperatures, high current discharge etc.) I don't see any problems here. I am using NiMHs in my calculators without any problems.

However, the capacity of current NiMHs is much, much higher than the usual 450 mAh of NiCd-AAs in the Seventies. Even the often recommended Sanyo/Panasonic Eneloops which do not feature an extremely high capacity (while offering other, more important benefits) deliver 2000 mAh which is more than 4x the original capacity. This means that if you want to use the built-in charging circuitry you will have to charge NiMHs about 4x the original charging time. Of course you can also apply the same time as before, but then the battery will only deliver the same 450 mAh as before. ;-)

That's why I would generally recommend an intelligent charger for external charging. The Woodstocks' battery holder can be modified so that the two cells can be removed. Among other benefits such a charger (e.g. the BC700) takes care of the batteries and stops charging automatically as soon as the charge is completed. Common charging times are just a few hours.

But... this topic has been discussed here several times and in detail. You may search the forum for more information on this.

Dieter

[Dieter]

(05-04-2018 12:14 PM)Zaphod Wrote:  But don't use your HP power unit to charge them , it's obviously a different chemistry with a different charging scheme.

Both NiCds and NiMHs are charged with constant current, the charging method is the same. However, due to the much higher capacity the charging time or charging current have to be adjusted. That's why I'd stronly recommend an external charger. See my first post in this thread.

Dieter

[burkhard]

(05-04-2018 12:25 PM)Dieter Wrote:  

(05-04-2018 10:25 AM)rolfmwa Wrote:  Will it be ok to use 1.2 Volt NiMH batteries rather than NiCADs or do I miss something here?

That's why I would generally recommend an intelligent charger for external charging. The Woodstocks' battery holder can be modified so that the two cells can be removed. Among other benefits such a charger (e.g. the BC700) takes care of the batteries and stops charging automatically as soon as the charge is completed. Common charging times are just a few hours.

Actually, even beyond the missing battery scenario: I still won't even use the Woodstock charger myself. Put it in a box with a tag that says "HP Woodstock calculator (25C or whatever) charger Faulty Design -- Do NOT plug into calculator! For historical purposes only". That way, someday your heirs don't blow up your treasure(s) trying to power it (them) with a missing battery.

Failing such measures, within a couple of generations, every one of the bloody things will be smoked.

Considering the poor design of the Woodstock charging circuit, I don't charge any batteries-in calculator. If there's a cell fault, or it gets jostled while on the charger, the calculator is toast.

burkhard

[rolfmwa]

Guys! Thanks a lot for your replies I really appreciate your knowledge and help.
I think I won't be using the charger as there is a real danger of killing the machine.
Thanks again
Rolf

[John Keith]

(05-04-2018 12:28 PM)Dieter Wrote:  Both NiCds and NiMHs are charged with constant current, the charging method is the same. However, due to the much higher capacity the charging time or charging current have to be adjusted. That's why I'd stronly recommend an external charger. See my first post in this thread.

Dieter

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.

Again another good reason to use an external charger made for NiMHs.

John

[grsbanks]

(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.

Hence trickle-charging NiMH batteries once they're full.

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 and NiMH can be charged at a much higher rate anyway. 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?

[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