Firman controller displays E12 and the generator will not start; battery icon may also be lit.

Safety first: Turn OFF the generator and isolate the battery before testing.

• Measure battery voltage: With a multimeter, check the battery at rest. If below 12.2 V, charge the battery fully and retest. Replace if it fails to hold charge.
• Inspect charger: Firman standby generators typically include a built-in battery charger powered by utility AC. Verify the charger is plugged in and its fuse is intact. With utility power present, measure output voltage at the charger leads (usually 13.2–13.6 V DC).
• Check wiring: Ensure the charger leads are securely connected to the battery terminals and that there is no corrosion.
• Test charging during run: If the generator can be started manually, measure battery voltage while running. It should rise above 13 V. If not, the engine-mounted charging alternator or rectifier may be faulty.

Replace the battery charger or charging alternator as needed. After correcting, clear E12 and verify that the controller no longer reports low battery.