Electronics > Beginners
Notify when batteries getting too low
fixit7:
I have 2 18650s powering my bike headlight.
The light fails pretty suddenly when the voltage gets to about 2.5 volts.
I was thinking of building a low current monitor the uses a momentary switch to check the voltage without having to take the batteries out of the holder.
Are there other options?
Thanks.
Audioguru:
An 18650 battery cell is lithium. It is destroyed when its voltage is lower than 3V and yours are probably in series so if they produce only 2.5V then they have only 1.25V each.
fixit7:
No, that was voltage of INDIVIDUAL cells.
You should not make a lot of assumptions.
SparkyFX:
Search the net for "18650 discharge curve", you´ll find various examples for the capacity/voltage relationship. As these curves are rather flat and then go steep, lithium capacity sensing is either imprecise (given all other factors, like ambient temperature) or contains some form of computational model - probably too big of a deal for a bike lamp.
You can of course test by adding additional load and if the lamp still works, the battery holds sufficient charge.
Spare batteries or premature charging might be more practical in the end.
sleemanj:
A simple cell monitor device
https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20190614205829&SearchText=lipo+cell+monitor
cycles through each series cell and displays it's voltage, typically handle 2-8 cells
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