Testing the voltage and recharging a battery
When a battery stops performing – more often than not it is simply flat. When you use/discharge a battery, the voltage drops. Any voltage below 12.5V is flat and needs recharging.
Connect the battery to a charger (10% of the Ah of the battery).
Leave to charge fully.
Disconnect and let the battery sit for 24 hours, this lets the ‘surface charge’ run off.
Test again. You should then see the battery at circa 12.7V.