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It is common for a camper to have different house battery charging sources:
Some solar installs will even have separate charge controllers running their own panel[s].
So what happens when multiple chargers are running at the same time? Nothing dramatic, as it turns out.
In general, the charging source with the highest voltage will win.
Most charging involves holding1) a particular voltage setpoint.2) If a charging sources sees the bank voltage is above its own setpoint it thinks the job is done; it sits quietly.
Example. Pretend you have a bank in a van with alternator charging and a large and small solar panel on the roof. Each panel has a dedicated charge controller because the panels are of wildly different specs. This gives us three charging sources if you are driving down the road at dawn.
The alternator is just a plain alternator and puts out 14.0v all the time. The Morningstar charge controller is factory set to start Absorption at 14.4v.3) The Xantrex charge controller has been configured by you to start Absorption at 14.7v.4)
Let's charge that battery. Here is the battery voltage along the way:
So how do we make them all charge all the time?
Barring heroic measures7) the alternator is going to stop contributing at some point during Bulk charging. That is ok, the alternator provided a ton of current while it was charging.
But if you want to the charge controllers (and some converters) can be convinced to stay roughly in sync and keep charging together. This is done by configuring the Absorption and Float setpoints on the controllers to be the same.8) There will be short periods where one controller makes a change before the other, but for most of the time both will be contributing at the same time.