![]() ![]() One point of contention is: just what does “0%” mean? At what point is an AGM battery empty? You’ll also find a lot of variation and disagreement. If you search for “AGM battery state of charge chart”, you’ll find a lot tables and some graphs similar to this one. Well, yes – assuming that the graph is correct. So, if we disconnect our AGM battery, let it rest for a few hours, and then measure its voltage and get a reading of 12.00 V (for example), the start of charge must be around 45%. In this example, when the SoC is 50%, the at-rest battery voltage is 12.05 V. In theory, the at-rest battery voltage depends on the state of charge, as shown in this example graph: For this reason, some smart chargers periodically stop charging to measure the battery voltage before continuing. Even when you remove the charger, the battery voltage will remain high as the surface charge begins to dissipate, and then fall toward its true level over time. But what you’re seeing is the voltage delivered by the charger, not the true battery voltage. ![]() To charge a lead-acid battery, the charger applies some charging voltage to the battery and, if you measure the battery voltage at that point, you might conclude that the battery is fully charged. This is because the charging process occurs at the surface of the battery’s lead plates, creating a surface charge which takes some time to diffuse through the battery. This is the voltage measured when the battery is not connected to anything that’s drawing any current and the battery has not been used or charged for at least a couple of hours.įor an accurate reading, it’s important to rest lead-acid batteries after charging them. AGM batteriesĪbsorbent glass matt (AGM) is the most common type of deep-cycle lead-acid battery.Īs for any battery, the voltage of an AGM battery is highest when it is fully charged, and falls as it discharges.īy “voltage” we really mean the “at rest” voltage. But similar considerations apply to all battery types. We’ll focus here on the 12 volt rechargeable batteries most commonly used with recreational vehicles and when camping. Why “estimate”? Isn’t there a way to be sure? Unfortunately, no, not exactly. ![]() But first you must estimate how much charge the current battery is holding. What you really want to know is how much longer your battery will last and whether you can safely draw more power from it or if you need to cut back, or if it’s time for a recharge. When using a rechargeable battery, you’ll want to know what its state of charge (SoC) is – how charged is it? ![]()
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