Diagnosing battery parasitic drain

A house bank losing charge overnight with no obvious load usually has parasitic draw. Shut off all breakers, then measure current in the negative main cable with a clamp meter. Anything above 100 mA (0.1 A) is a real draw worth chasing. Common sources: bilge pump float switch stuck partially closed, CO detector on constant standby, stereo memory keep-alive, chartplotter memory, refrigerator that appears off but has a control board drawing current, inverter in standby mode (can pull 300-800 mA), shore power charger backfeeding when unplugged. Isolate by pulling one fuse at a time and watching the clamp. On a 400 Ah AGM bank, 200 mA of continuous draw = 4.8 Ah per day = 1 percent per day = noticeable within a week. Lithium banks with BMS pull 20-40 mA baseline, that's normal. Bank aging (>4 years on AGM, >8 on lithium) also mimics parasitic drain because capacity is silently gone.