Fliteboard (Ultra L2) battery readings

I've noticed that my hand controller battery readings are completely at odds with the battery charger (basic version) readings - today I was out, got a warning it was at 50%, got it down to like 20%. Went home, put the battery on charge and the battery charger says it's still at 60%. The board lights at the back mirror the charger readings but I don't want a flat battery when i'm a km offshore.

Which reading is correct?