Resurrecting an old topic: Batteries for a Commodore 4190


Older Discussion

In the link above, someone asked about replacing the original batteries in the Commodore 4190 models. Documentation on the web suggests 6V but inside it looks more like room for 3 AA-ish size batteries or 4.5 volts.

What have people used? I have a co-worker who is now the proud owner of this model, so he'd like to get batteries to compute on the go! :-)


I don't have a 4190 but several other Commodore calculators in the same size case. I've modified theme to use a 3 x AAA plastic battery holder filled with either lithium (or alkaline) AAA cells.


So it works off 4.5 volts?


The answer is not straightforward. If you were to use the calculator without batteries, then 4.5V should work fine.

However, to charge the batteries, you need some voltage overhead (hence the 6V charger). If you are not using an original charger this is not straightforward either.

The original chargers were unregulated and of a relatively low current output (100mA to 200mA). But it is in the "unregulated" that the trick lies. When the battery is is low it draws a higher current from the charger, and because it is unregulated the output voltage drops thereby automatically serving as a limiter. Now when the battery gets full, the voltage increases and now because the charger is unregulated, the current it delivers drops - thereby once again acting as an automatic limiter.

In contrast, modern regulated power supplies will attempt to keep the voltage the same and deliver their maximum current if necessary (usually several times higher than the original power supply). This can have the effect of giving a high current to the battery when it is low and not reducing the current to a safe limit (usually <0.1C) when the battery is charged. An excessive current when the battery is full or faulty can lead to destructive overheat.

I recommend either using an original charger (in GOOD condition) or building in a current limit circuit (keeping it to ~0.1C) into the calculator for the battery charging. I usually do the latter and include a PTC fuse.


Good news is... the charger is an original 220v European charger. He's using a converter plug thingy on the end to plug it into a US 110v plug.


That is good. As the new battery will probably have a higher capacity than the original, all that is needed is a longer charge time to fully charge and make use of this extra capacity.

Edit: to answer your other question: yes, they originally used 3xAA NiCd batteries.

Edited: 11 May 2012, 6:19 p.m.


I have a 4190R and the modification suggested by Katie works just fine and simplifies future maintenance.


Years ago I tried to replace the rechargeables with regular alkaline AAs in a battery pack. There wasn't enough room inside for the battery holder, so I ran wires and taped the batterie to the back. It was a temporary ugly fix, but I still have it that way and it works fine with the 4.5V.

I couldn't afford an HP at that time, so this is the calculator that got me though engineering school and years of work. It is a great machine but no internal order of operations so the () were needed a lot of the time. Also not programmable, but I never really missed that.


The 4190R ran from three NiCd cells, i.e. 3,6 V, originally. I know, I still have mine.

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