When is the best time to charge lead acid batteries ?

Generally speaking, it is the best time to charge lead acid batteries when the remaining capacity is about 20%. If possible, it is also good to charge when it is about 40%. Please keep in mind that there will be a big risk of batteries damaged if you let batteries drain and 100% discharged. Charge your batteries regularly after each ride if the remaining capacity is less than 40%. It will help you keep the lead acid batteries in sufficient voltage always and prolong lifetime of batteries a lot.