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2-stage vs 3-stage battery chargers

by RV Forum staffer Lou Schneider

Early RV converters did a poor job of charging the house batteries. If the battery was simply connected to the converter's main output, it would quickly overcharge and boil off water, ruining the battery. Some chargers had a separate charging output, but electronics of the time restricted it's output to 5-10 amps. And it would still overcharge the battery, making it lose water.

As electronics became more sophisticated, it became practical to design a converter/charger that more accurately monitored the battery's state of charge. This lead to the design of a smart, multistage converter/charger that could charge a battery at a fast rate but not boil off water when the battery was through charging.

When a battery starts charging, it draws a lot of current - as much current as the charger can supply. This is known as the Bulk mode of the charging cycle.

Ohm's Law says Power = Voltage x Current. If the charger is running at it's full output, and the Current it can supply is the limiting value, the Voltage will be less than the normal.

Once the battery gains a partial charge, it's demand for current starts tapering off. This lets the charger's voltage rise to it's normal value.

Two Stage Chargers work this way - they start out in Bulk Mode where the current is limited by the capacity of the charger. Once the current tapers off and the voltage rises, it's capped at the Float value of 13.6 volts.

If you put a higher voltage on a battery in the middle of it's charging cycle, it will continue to charge at a higher rate than it would if the voltage was limited to the Float value. This is known as Acceptance Mode. But the voltage can't stay this high forever, because once the battery reaches full charge it will start boiling off water unless the voltage is reduced to the Float level.

Three stage chargers start out in Bulk Mode and let the voltage rise to the Acceptance level while the battery charges. Once the battery is fully charged the voltage is reduced to the Float level to avoid water loss.

Float voltage is 13.5 - 13.6 volts. Acceptance voltage can rise as high as 14.1-14.5 volts.

A dedicated charger is one that is connected only to the battery, not feeding anything else. It can sense when the battery is fully charged by monitoring it's output current. When the current drops close to zero, the battery is full and the charger turns off or goes into Float Mode.

But in an RV application, the converter is also feeding other 12 volt house loads. If you're using 12 volts elsewhere, for lights, etc. the current coming out of the converter may never drop to zero, so the voltage can stay high longer than it should, overcharging the battery.

Progressive Industries gets around this problem by limiting the time it's Charge Wizard stays in Acceptance Mode to a maximum of 24 hours, then it drops to Float Mode. In my opinion this is a good solution to the overcharging problem.