How to check how much energy is left in a battery?
Everyone: Measure the open-circuit voltage. Monitor discharge current over time and compare it with the calibration curve.
Flooded lead-acid users: Just open the cell and see how much unreacted materials is physically still there. Be careful not to spill the sulfuric acid.
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@niconiconi I don't understand how you'd measure that optically. I remember seeing densitometers that were used to measure lead acid battery charge level by measuring electrolyte density.
@robryk@qoto.org Some batteries use colored floating markers as a cheap meter.