Build A Battery Level Indicator

This circuit uses a LM339 pdf, a quad comparator. LM339 pdf can work on single or dual supplies, it has a open collector output that can drive 15mA, low power consumption. When you measure the open circuit voltage of a battery with a high impedance DMM (10M), the value may be a bit misleading. Apply a dummy load to bleed the battery a bit so that proper readings can be taken on Load. The load below is a 100 ohms wire-wound fusible ceramic resistor which will heat a bit when you test 12V batteries.