An important consideration is the energy store of the battery that you are going to charge. This is rated in units of Ampere-hours. This is the total amount of energy the battery is able to store. A typical auto battery has about 50 amp hours and would take 10-amp hour 12 volt battery charger approximately 6 hours to recharge it to full capacity assuming the battery was completely dead. A Marine Deep Cycle Battery is rated at 100 amp hours and would thus take a 10-amp hour charger 11 hours to charge to full capacity a discharged cell. Here is a rule of thumb to calculate the charge time. Take the amp hour battery rating, divide it by the charger rating and then add 10 percent. Another consideration to take note of is the desired use of the battery output. If you are going on use the charger on a trolling motor battery, then a fast and powerful charger will do the trick while a low current charger will work fine for batteries that will be used off season like in a classic car or a motorcycle or an aircraft.