DIAGNOSIS A flashing red battery icon usually means the mower's computer has detected a problem with one or more of the battery packs, like a voltage imbalance, temperature issue, or a communication fault with the Battery Management System (BMS).
FIX First, try reseating all the batteries. 1) Turn the mower off. 2) Remove all battery packs from the compartment. 3) Inspect the battery terminals and the mower's connectors for any debris or corrosion (clean with a dry cloth if needed). 4) Reinstall each battery, pressing firmly until you hear a distinct click and the status light on the battery turns on solid. 5) Turn the key on and see if the flashing red icon is gone.
IF THAT DIDN'T WORK The next step is to isolate if one specific battery is causing the fault. With the mower off, install just one battery at a time and turn the key on. If the flashing red icon appears with a particular battery installed (even if that battery shows a full charge on its own light), that battery likely has an internal BMS fault and needs service.
HEADS UP Don't keep trying to force the mower to run with this error. It's a protection mode. Continuing to try could lead to further imbalance or prevent the batteries from charging properly.