Step 3: Charge the battery at a 10-amp rate. 42 amp-hours needed by the battery divided by 10 amp charge = it will take around 4-1/2 hours to recharge the battery.he more discharged that a battery is, then the longer it will take to recharge it. Usually it takes several hours to effectively recharge a battery. If a battery is very discharged then it can take up to 12 hours or more to recharge it.