NEC PC-8201a Adventures – Part Two: Rebuilding the NiCad Battery Pack

These were great batteries, 40 years ago.

This was a good task to start on. It’d been years since I’ve done any ser­i­ous elec­tron­ics work, and my pre­vi­ous Ardu­ino pro­ject, while suc­cess­ful, reminded me just how much I’d for­got­ten about elec­tron­ics over the years. And my skills had mostly van­ished too. So a nice easy pro­ject was just the thing.

About those bat­ter­ies. They were amaz­ing things back in the day. But when I opened the bat­tery pack, it was revealed that they’ve done what all bat­ter­ies do over time. Run out of charge and leak.  The cor­ro­sion was­n’t as bad as it could have been and the plastic pack­age was in great shape — but the cir­cuit board in the bat­tery pack was bey­ond hope. 

First step was to remove the res­ist­or and the con­nect­or. A few minutes with a hot sol­der­ing iron and some desol­der­ing braid did the trick.  Then clean the con­tacts and leads with some CLR, fol­lowed by a good rinse and they’re ready to be used on the new board.

I had some old perf­board / pro­to­typ­ing board avail­able so it was a rather simple thing to carve out a new one with a Dremel tool cut­ting disk. *PROTIP — CUT PERFBOARD OUTSIDE. THIS STUFF STINKS!!*

Then, rebuild the simple cir­cuit to match what was on the ori­gin­al board. The power con­nec­tions (Left (-), Right (+)) will be made when I solder the bat­tery pads to the board.

If you look closely at the res­ist­or, you’ll notice that I cracked the ceram­ic(?) coat­ing and some chipped off. Since I had some replace­ments on order (though not the exact value so I’d be link­ing three in series) I figured ‘what the hell? If it does­n’t work, I’ll just do the series thing’.

But before sol­der­ing it on, I did test it and the value matched what it should be accord­ing to the NEC PC-8201a Ser­vice Manu­al. So maybe I won’t have to do the ‘series thing’.

The bat­ter­ies in the NiCad pack wer­en’t meant to be replaced, so they were soldered togeth­er in series to form the 4.8V pack­age. My goal was to make the bat­ter­ies replace­able, which means I needed to fab­ric­ate the connections.

Rather than order­ing the parts from eBay or wherever, and them tak­ing 30–60 days to arrive, I man­aged to sal­vage the springs and plates from some dead sol­ar-powered side­walk light­ing units.

They were a bit big, but I had some tin snips and they were soon cut down to size. Then run the Pos­it­ive and Neg­at­ive leads, a little solder and some stripped sol­id-core cat5 strands to make the seri­al con­nec­tions, and the bat­tery pack cir­cuit was complete.

Hav­ing the res­ist­or in the cir­cuit makes our 4.8V pack into a 5.5v pack, which is with­in the input voltage range of the PC-8201a. The stand­ard, non-rechar­gable, AA bat­tery pack that was avail­able for the unit did­n’t have the res­ist­or, and reg­u­lar AA cells are 1.5V so that x4 gets you a nice, tidy 6V input voltage.

Next up, a good wash up to clean off 40 years of grime!