Well, this depends on the size of the battery as well as the power of the charger. While some batteries charge very quickly, others might take some time to get fully charged. However, on average, it should take somewhere between 4 and 6 hours to charge a Lawn mower battery. Charging time might also depend on the type of charger that you are using. Further, there are usually three ways of charging – fast charge mode, timed charging, or trickle mode. Some experts are also of the opinion that taking the battery out charges the battery more efficiently.