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Can I indefinitely charge NiMH batteries through a current source that limits just below the battery voltage?

More specifically, the battery is a 400mAh 14.4V pack and the supply is 15V but after a small resistance (25 ohms DCR of an inductor), the current source pass transistor and a Schottky diode the current source will stop supplying current when the battery terminal voltage reaches ~14.2V. Meaning the current source will supply about C/11.8 (34mA) until the battery charges enough to where Vbe of the pass transistor decreases such that current settles on something that maintains some voltage (hopefully in the upper 13V range but I'm just doing LTSpice at this point).

So can I leave the battery under the above described condition indefinitely or will the slight positive pressure reduce battery life?

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  • \$\begingroup\$ Please provide a schematic \$\endgroup\$
    – mhaselup
    Commented Oct 13, 2020 at 3:25

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At first, NiMH cells did not have a catalyst to recombine H2 and O2 liberated during even mild overcharging, and seals would rupture from gas pressure. Most NiMH now contain a catalyst to reunite the gases, releasing heat and water (as do catalytic lighters).

As long as the charge rate is low (less than C/40... I use C/100), a NiMH cell with catalyst can be left on maintenance charging indefinitely, so in that case, current limiting is better than voltage limiting. Current limiting also protects a battery from further damage should a cell short: a 14 V battery with one dead cell could not charge above 13 V, but would receive full current from a voltage-limited-only charger.

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  • \$\begingroup\$ Thanks @DrMoishe. So I suppose my method is worth a try then. In my scenario voltage limiting is current limiting since the 14.2V limit of the current source is below the nominal voltage of the pack. A shorted cell would be a problem. But the whole pack would just be discarded anyway. So as long as there's no danger of something bursting open and making a mess (which seems unlikely at C/11.8) I think I can overlook that for now. \$\endgroup\$
    – squarewav
    Commented Oct 13, 2020 at 5:36
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    \$\begingroup\$ Hi DrMoishe, regarding: "Most NiMH now contain a catalyst to reunite the gases" User Russell McMahon has done lots of research. In various answers e.g. here, here, here & here he discusses NiMH batteries & reports that modern NiMH (especially larger-capacity) cells are more likely not to have the catalyst that would allow long-term trickle charging, and would be damaged by that. Any thoughts? \$\endgroup\$
    – SamGibson ♦
    Commented Oct 13, 2020 at 14:45
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    \$\begingroup\$ @SamGibson, thanks for the caveat! Though I haven't done research, my experience is with "AA" (HR6) ~2,000 mAh cells only, and all brands I've used, including low self-discharge Eneloop, withstand 1/100 C for years with no noticeable degradation. Of course, that does not apply to other cells. \$\endgroup\$
    – DrMoishe Pippik
    Commented Oct 13, 2020 at 17:47
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    \$\begingroup\$ @squarewav, two caveats: 1. A comment above warns some new cells do not have the recombination catalyst. 2. Even with catalyst, continuous charging at C/12 could burst cells, but is not likely to cause a large leak of alkaline electrolyte, and is not a fire hazard, at least for smaller "flashlight" size cells.. (Overcharging Li cells, on the other hand, is far more dangerous.) \$\endgroup\$
    – DrMoishe Pippik
    Commented Oct 13, 2020 at 17:54
  • \$\begingroup\$ Interesting comments. I'm using 2/3AAA 400 mAh so I might still get away with this. But long term it sounds like I should move to a PIC so that I can code around problems. \$\endgroup\$
    – squarewav
    Commented Oct 14, 2020 at 2:19

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