How long will a NiMH battery pack last in a NEC PC-8201 or PC-8201a?

The NEC PC-8201, PC-8201a, and PC-8300 are a bit dif­fer­ent from their Kyo­cera cous­ins (Tandy Mod­el 100, Oliv­etti M10, Kyo­cera K‑85) in that they have the abil­ity to use replac­able bat­tery packs rather than just insert­ing Alkaline AA cells behind the bat­tery door. NEC even released an option­al rechargeable bat­tery pack.

Lately I’ve con­ver­ted a couple of the stand­ard NEC bat­tery packs into packs cap­able of using rechar­gable Nickle Met­al Hydride bat­ter­ies. The pro­cess is simple, and out­lined quite well in this brief art­icle.

Basic­ally, add a jump­er and a res­ist­or (or couple of res­ist­ors in series) to the pack and it’s ready for rechar­gables. It’s a one-way trip, you can­’t use Alkaline cells in the pack after the conversion.

The res­ist­ors are needed to con­vert the voltage up to the expec­ted val­ues — rechar­gable AA bat­ter­ies only deliv­er 1.2 volts each, rather than the expec­ted 1.5 volts of Alkaline cells.

The jump­er tells the com­puter to trickle-charge the bat­ter­ies when the sys­tem is plugged in and turned off. One thing to note, these old pup­pies don’t have soph­ist­ic­ated char­ging cir­cuits — just dumb trickle charge — so don’t over­charge them; about 48 hours accord­ing to the NEC User­’s Guide:

On to the code

As was doing this, I wondered just what the run-time of a bat­tery pack would be. So I came up with is a simple BASIC pro­gram to run the com­puter bat­tery down, and record the time that it dies and cal­cu­late the run-time.

Not being a com­puter sci­ent­ist or elec­tric­al engin­eer­ing wiz­ard, I’ve made a lot of assump­tions and prob­ably some mis­takes, but this seems to work for me. Feed­back is appreciated 🙂

Basic­ally (ha!), it’s an end­less loop that:

• writes the cur­rent iter­a­tion of the loop to the screen (until the sys­tem time minute value changes)
• once every minute it writes to a RAM file; the sys­tem date, start time, cur­rent time, iter­a­tion num­ber, and cal­cu­lated run-time of the bat­tery pack
• loops to the next iteration

The idea is that the com­puter will do some work (write to the screen, and look at the time, etc). Every minute it’ll write what it knows to a file. If the power level gets low enough and causes the sys­tem to shut­down, then the last saved file will have all the data and the bat­tery pack runtime length.

I don’t have any com­plex cal­cu­la­tions in the code as my ini­tial pro­ject scope was to just get some­thing work­ing and give me a run-time value. I might add them to a future ver­sion, maybe select­able from the start screen.

Both of the units tested had dif­fer­ent memory con­fig­ur­a­tions, and the PC-8201a also had Steph­en Adolph’s REX# and a Bank2 32kb SRAM card, so the res­ults are expec­ted to dif­fer a bit.

I’ve been dab­bling a bit with Git­hub, so I’ve saved the pro­gram files to a pub­lic Git­hub repos­it­ory. Feel free to Clone, Fork, or just check it out. I’ll be updat­ing and main­ta­ing this repo as I do improvements.

Oh, and as the image shows, in my loaded 8201a, the pack las­ted almost 33 hours. I lost the num­ber for the PC-8201 due to a cold reset but will update this post when I run it again.