Charging NiMH battery pack

Jim Wilkins · 2021-06-17T11:52:51+00:00

I bought a $35 pair of replacement battery packs for my 14.4V DeWalt drill that contain NiMH instead of NiCad cells like the original. Amazon reviews claim the original DW9118 charger handles them without problems, but having been a battery test tech I wanted to know more.DeWalt says their chargers need a true sine input so for remote job site solar+battery use I bought a 300W Bestek inverter which gets decent reviews. Mine shows a nice 113V sine wave on a scope and cuts off at 350W. Some users wrote that a modified sine inverter blew their charger's fuse or worse. The AC input feeds a capacitor rather than a transformer.I recorded the voltage and current while charging the old NiCad and found that the charger ignores the small negative steps as each cell tops off and begins generating oxygen, instead it cuts off the current once a minute and measures the battery voltage. Charging ends when the zero-current voltage reaches 17.0V, or ~1.42V per cell. The charging current of 1.3A raises the voltage almost to 18V before the individual full-charge cell drops begin, ending at 17.65V. When the NiCad pack was new (or new-old-stock) it measured 17.13V at full charge.Internet sources suggest without firmly stating that constant-voltage charging to 1.4V~1.45V per cell is acceptable, though the last part of the charge is slow.So does anyone have hands-on experience with replacement drill battery packs that use NiMH cells instead of the original NiCads?

Jun 17, 2021 26 Replies

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Jim Wilkins 5 years ago

Okay, that's different; the microprocessor Z86C04, is one of the ROM-on-board Z8 things, so that's not a generic replaceable part. Last bad charger I fixed just needed a fuse and two or three semiconductors (diodes and fuse taken out by a switch MOSFET).

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I ordered a clone NiMH charger that I can rewire to record the voltage and current on the bench, and then use as-is at a jobsite if it proves to not overstress the battery. My homebrew charger designs wouldn't rapidly or fully recharge the battery.

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