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I've heard that discharging a lead-acid battery to 0% is bad and wears it out. I have a 10Ah 12v lead-acid battery, is there a circuit to tell me when the battery is low?

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Fortunately, there is. You can find diagrams of the discharging process of a battery type, search for "lead-acid discharge graph" or so. This returns graphs like this one:

enter image description here

In the datasheet of your battery you can find the exact graph for your battery.

Say you want a circuit to detect 80% discharged, just check in the graph, the voltage of a lead acid battery with 80% discharged is about 1.9V.

You can check this voltage with a multimeter, or use a comparator circuit or a microcontroller with ADC to make a dedicated device for this.

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  • \$\begingroup\$ I believe 1.9 V is for a single cell. The OP's 12 V battery will have 6 cells and therefore an 80% capacity voltage of 11.4 V. Google for "battery level indicator circuit" \$\endgroup\$ – RedGrittyBrick Apr 30 '13 at 13:34
  • \$\begingroup\$ @RedGrittyBrick you are totally right. Most of the time, the battery has a datasheet where the battery-specific graph is given. \$\endgroup\$ – Keelan Apr 30 '13 at 13:36

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