Toyota 2009 Avalon Owner's Manual page 357

Before recharging
n
When recharging, the battery produces hydrogen gas which is flammable
and explosive. Therefore, before recharging:
If recharging with the battery installed on the vehicle, be sure to discon-
l
nect the ground cable.
l Make sure the power switch on the charger is off when connecting and
disconnecting the charger cables to the battery.
4-3. Do-it-yourself maintenance
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