Care and Feeding of Topcat NiCads


I am proud to have been able to zap dendrites and resurrect two of three Topcat batteries I had gathering dust. I want to treat them with kindness and get some more use out of them, so I am interested in charging parameters.

Both of these packs have been re-celled by prior owners, and I don't know the capacity of what's in them, so I will probably aim for the undercharging side, given that it seems that overcharging (i.e., cooking) NiCads is more damaging than undercharging. Would a max of of 1000 mAh per cell (4000 mAh per pack) be a fair estimate? (In contrast, my much newer packs from waterhosko at TAS are 1500 mAh per cell and 2200 mAh per cell.)

The Topcat charger (which is also used for the 82143 and 82162 printers) is rated at 7V and 3 VA power, working out to a current of of 375 mA. This means that charging the 4000 mAh Topcat battery pack for over 11 hours puts us into overcooked territory. This seems a lot less than some of the charging times I have seen recommended.

Is my math off? Am I overestimating the capacity of my battery packs or the actual charging current? Am I wiserto assume that undercharging is better for battery life than over charging?

This my seem all so basic to many, but I would really appreciate guidance.



Edited: 31 May 2012, 10:47 p.m.


Your electronics is off, not sure about your math.

Let's say the 4 sub-C nicads cells are each 1000mAh in capacity. Since that are in series the voltages add but the capacities remain the same, the battery pack is nominally 4.8 volts with a capacity of 1000mAh.

As far as charging is concerned, the rating on the AC adapter (actually just a transformer, there's nothing else in that black box) is pretty meaningless. The charging rate is determined by many factors, the rating on the transformer is the voltage it can supply at the rated current -- but the charging circuit will not charge at that current. Because the charging circuit in the Topcats are not regulated, the only way to know the charging rate that occurs in the TopCat calculators is to measure it by putting a current meter in series with the battery pack when charging.

Nicad's are very robust and can be charged over a broad range of rates with no damage. If you don't want to bother measuring the current, or the voltage across the battery (fully charged nicads should be about 1.45 volts per cell) just go ahead and charge it in a Topcat for 10 to 20 hours and don't worry.


Hello all.

While we're on the subject, I would ask for a memory refresh. I have an HP-35, 45, 55 and 67 which all have the three battery pack assembly equipped with 1000mAh batteries. So, at the first light of the low battery beacon, how long should I recharge my Classic from the original HP charger?


Edited: 31 May 2012, 11:40 p.m.


The original cells were 600mAh or so and HP recommended charging for 14 hours for a full charge, so charge for 66% more time. The manual also mentions that you don't have to worry about overcharging.


Thanks. So, for the 14h with the 600s, I should let the charge go for 23 1/2 hours or so on the 1000mAh set, right?


Somewhere around there. But you can leave it plugged in for days, weeks, months, even years. I've done that with no problems.


Thanks, Katie. I honestly thought that the capacities where additive. Given your correction, I agree it makes even less sense for me to try to estimate charging parameter from numbers etched on the AC transformer.

I do confess that I did manage to cook one of these packs myself, and I know it is because of overcharging--basically, running the calc (or printer) plugged in most of the time. I have learned since that this is not recommended, given that the charging circuit in the 97 and printers does not down-regulate to a trickle charge like it does in some newer devices. I would qualify your statement that NiCads are robust--basically, I have learned recently (and really sheepishly too) that they last a long time if not mistreated. I am embarrassed to admit that, until recently, I thought the cells getting very warm to the touch while charging was desirable!

I am a bit of an obsessive, so I just may invest in a multimeter...

Thanks for your informative reply.



I am a bit of an obsessive, so I just may invest in a multimeter...

A very worthwhile investment and necessary if you play around with old calculators and battery packs. You don't need to buy a top of the line Fluke meter, an Extech for around $50 would likely do anything you'll ever need well enough.

I'm curious about your "zapping". What did you use for your high current source? I use some really large value capacitors and for this and it usually works well if the cells aren't too old.


I was a bit spooked at the idea at first, so I looked for a reportedly effective method that seemed to be relatively safe and didn't require a car battery or arc welder, both of which scare me a bit.

The most approachable method (for power drill packs) entailed hooking up two or three health packs in series, attaching the dead pack to the series negative to negative, and "zap" by repeatedly taping the positive lead to the positive terminal of the dead battery rapidly for a few seconds, letting things cool, then charging normally.

My variant entailed a 16V, 4A power adapter for a Duracell rapid charger (which basically destroyed a few NiMH cells before I clued in that faster was not better). The negative terminal is the outside of the barrel of the plug, so I wrapped the negative lead around and taped in place. I taped my wire leads, neg to neg and pos to pos, onto the terminals on the pack. I then twisted the wire of the positive lead to a fine point and did the rapid tapping thing by moving the wire end in and out of the barrel of the plug. I only did this for a few seconds--the wires got warm in my hands and the battery warm to the touch quite quickly. I let the packs cool and I have been charging them and using them normally since.

I have seen variations involving harvesting capacitors from disposable cameras, and I am sure they work great in tougher cases that need a bigger zap. But judging from how warm my packs got, and so quickly, I could help but think that I was dealing with some significant current for the job at hand.


I've done this many times with a 10,000uF/75V cap connected to a power supply through a 100 ohm power (10W or so) resistor. I connect a voltmeter across the cap so I can tell when it's ready for another discharge and, once discharged into a cell, I can see if the cell shorts have cleared yet. I use some sharply pointed test leads to deliver the jolts to the cells, even through plastic/heatshrink coverings if necessary. I work through the pack one cell at a time, usually starting with about 15VDC and going up to as much as 25VDC for some 'tough customers'. It's good to wait a few minutes and see if any shorts return and then, if not, it's off to the charger.

I've revived many NiCad packs this way and if you give them a good charge and use them regularly they will often last/work for quite a long time without problems. If they do run down though they often develop shorts again very quickly. I guess when they have a good charge they can self-open.



This is almost exactly how I do it, although with larger capacity, lower voltage capacitor. Usually I use one that's 82,000uf at 20 volts. I charge it up via an external power supply with a settable maximum current. I can watch the current into the capacitor and know when it's ready for another zap -- just a few seconds.


I would suggest you insert some resistance between the supply and the capacitor to limit the transient current when the cell is suddenly connected. Current limiting circuitry in many power supplies is not instantaneous and damage can result. Unfortunately I know this from experience. :(



When I saw your 100 ohm resistor I thought "yup, that's the right way to do it", I've blown a power supply too (fortunately it was just the TO-3 type power transistor).


I've never had any issues with current limit kicking in many years of use but I use a pretty good quality Amrel LPS series supply. I could see that with some others a slow-kick in would be an issue, I've got a new BK 9110 supply too and although I like it the current limit is slow to kick in.


Rechargeable cells have gone through 4 generations wsince the NiCad cells. NiCads adapt to their usage, in stored calculators they will continue to lose capacity. The latest ENELOOP NiMh cells are rated 2000mah capacity and hold their charge for over a year and have no memory of past usage. See
NiCads should be disposed of in a hazardous waste collection. Sam


Sam, I have a drawer full of Eneloops AA and I use them in pretty well anything that will take them. They are especially useful in photography flashes as they hold the charge much longer and endure the high drain more than standard NiMH cells. I also have a handful of Eneloops pre-charged Duracell cousins, which are going strong years after I have toasted standard Duracell and Energizer NiMH cells in cheap chargers. Eneloops can also be had for cheap and in bulk--there are a couple of good vendors via TAS.

I would love to be able to find Eneloops-type NiMH cells with flat top and with or without tabs in the AA or sub-C size. I have ordered some standard high-capacity NiMH sub-Cs and will try to fashion a Topcat battery pack, but I think I have heard around here that NiMH's don't charge well in the Topcats or the printers, so I may have to fashion a custom charger to charge the pack outside of calc. I think it is important to note that when Randy of refurbishes Topcat packs he uses NiCad cells. Likewise, Mark Hoskins (formerly waterhosko and now samartius over at TAS) provides a NiMH classic pack, but only NiCd versions of the Topcat and Spice batteries. Maybe these guys know something we don't...


Yes, and for me it was my antiquated (but beloved) Heathkit Variable DC Power Supply I bought and built around the same time I got the HP-35. It's still awaiting repair and I'm thinking I may need to modify a few things to substitue silicon transistors for the original germaniums. I have a number of more modern supplies now (including a beautiful HP6227B, love the meters!)but I treat them all more cautiously these days.


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