When we sell a battery, we recommend it not going below a 40% charge. All right, that's easy to figure out. A 12v (volt) battery holds 12v, 40% of 12 is 4.8 or 4.8v. That line of thought is incorrect.
12v batteries are fully charged at 12.6v and are depleted or 0% at 11.7 volts. Some energizers will function even when a battery is below 11.7v. However, this will cause damage to the battery. So a 40% charge is slightly over 12.1v. Clear as mud right? For 12v batteries we use the chart below for determining charge percentages.
How do you determine the voltage of your battery? Use a Battery Charge Meter to check the voltage. Use a ReCharger for 12v batteries to charge batteries. If you have a 12v voltmeter (not a fence tester), test your battery after charging. Try to avoid going over 13-14v charge on a 12v battery. Overcharging is similar to blowing up a balloon until it pops. You may continue getting air into it, but eventually it's going to pop. Our battery won't "pop" but overcharging does cause damage and will reduce its usable life. -