Replacing a LiPo battery with NiMh AAAs?

[RyanT]

Hey all,

I'm in a bit of a bind. It's Sunday evening and I need to fix a 10 year old commercial sensor that I don't have the parts for. The sensor had a "non-user-serviceable" LiPo battery that was soldered and then siliconed on to the board, which was then hot glued to the housing, which was then plastic welded shut. What a pain.

The battery was puffed up enough to crack the plastic housing and flex the multi-layer board, one of the leads was badly corroded, and the battery was completely flat. (My strong suspicion is that it was over-discharged and then attempted to charge again.) It was a slightly nerve-wracking trying to cut and pry the battery out without puncturing it.

Anyway, it's a 3.7v 250mAh LiPo, with no charging circuitry (that's built into the board, for all the good it did). I don't have a suitable LiPo battery on hand and won't be able to get parts for a couple of days. Would it be OK to use 3xAAA (1.2v NiMH) in series instead? I do not plan to charge them in-situ, just use them as a 3.6v power source until I can get a proper replacement. Or is the charging logic for LiPos OK for NiMH as well? (From what I remember, NiMH are a bit more tolerant, but I'm not sure.)

[IanB]

I think the operating voltage of the LiPo will be in the range from 4.2 V down to 3.7 V, while the operating voltage of 3xAA NiMH will be in the range of 3.9 V down to 3.3 V. So the NiMH may be a little under voltage, depending on the tolerance of the device. But it wouldn't hurt to try. Or you could try 3xAA alkaline as a temporary fix as long as the device doesn't try to charge them.

NiMH cannot be charged with a LiPo charger. NiMH require constant current charging with special end of charge detection logic. Using a CC/CV charging algorithm will be likely to overcharge them unless you set the CV voltage quite low (much lower than the LiPo voltage).