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I'm trying to save an inherited Stiga lawnmower with a battery pack that I thought was dead. It's a 20 cell 18650 pack at 36V. When newly charged, the running time is just a few minutes before the mower comes to a crawl. This particular battery pack seems really hard to find so I took it apart to see if I could save it.

It's a bare bone battery pack, no bms, just three charge indicator leds with comparator and a thermistor.

I checked the cells with a liitokala lii-600 and they're all well over 90% capacity (around 1250 mAh from spec'd 1300 mAh) so I'm considering the thermistor now. When thinking about it I kinda remember that after the lawnmower came to a stop and I checked the battery - the level indicators was still at two or three leds out of three after cooldown.

I have no reference for the thermistor. At roomtemp it's at 10k and when I squeeze it, it goes down to 8k.

Could this be a bad thermistor or could the cells be degrading with too high temperature even if the capacity still seems fine? The thermistor is just pinned out to the machine itself.

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Sounds like a standart 10k NTC thermistor. Based on you description the values also seem to be okay.

See this chart from a random thermistor: https://f01.justanswer.com/Poolservice123/1ea3dfc8-6a7a-46c2-a991-13c92210166a_TempVsResistChart.pdf

Maybe the internal cell resistance is getting too high as they are getting older. This would limit the delivered current and would also lead to an early cut-off.

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