Understanding Battery Voltage Sag Under Load
Why voltage drops on DC cables and heavy loads, and how to keep inverters online.
You measure 12.8 V at rest and watch 11.2 V under load—and wonder if the bank is dying. Often it is simply sag: resistance in cells, connections, and wire carrying current.
Ohm's law on the bench
Voltage drop equals current times resistance. Double the amps on the same cable and drop doubles. Long thin runs are the usual villain in DIY 12 V systems.
When inverters trip
Low-voltage disconnect protects the battery from over-discharge but also reacts to sag. A brief compressor start can dip terminal voltage enough to reboot electronics even when average state of charge is fine. Shorter cables and heavier gauge fix many "mystery" shutdowns.
Measure correctly
Measure at the battery terminals under load, not at the far end of a skinny extension. Compare to load current at the same instant.
Design targets
Many installers aim for three percent or less DC drop on critical runs. High current 48 V systems help because current is lower for the same watts.
Sag is physics, not drama. Calculate drop before you blame cells—upsize wire, tighten lugs, and watch starter loads with a clamp meter before replacing a bank that still has life.