It is said:
with a charged battery , over 12,6v battery is ok
between 12 & 12,6v you have to charge again
under 12v you have to change the battery
if battery voltage is from 11,5 to 12,6 charge 1,8A during 5 to 10 hours
if the battery voltage is under 11,5v charge 1,8A during 20 hours
charging voltage must be greater then battery voltage: 13 to 13,2v but less then 15,5v (5000tr/min)
If it can help for instance ...