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Charging from multiple sources

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].

concurrent charging

So what happens when multiple chargers are running at the same time? Nothing dramatic, as it turns out.1)

In general, the charging source with the highest voltage will “win”.

why one source wins

Most charging involves holding2) a particular voltage setpoint.3) If a charging source 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 400w of solar panel (2x200w) 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.4) The Xantrex charge controller has been configured by you to start Absorption at 14.7v.5) Normally the controllers would be set to the same voltage but we will make them different to illustrate how it works.

Let's charge that battery. Here is the battery voltage along the way:

  • 12.2v at daybreak, approximately 50% Depth of Discharge. Charging is in the Bulk stage and all three sources are contributing; the battery bank voltage is lower than what they all want to provide. The alternator is contributing the most because it can produce the most current and the panels aren't getting much light yet.
  • 13.9v a few miles down the road. Voltage is climbing but all three sources are still charging. The sun is higher in the sky so the panels are making more power now.
  • 14.1v by the you get to a neighboring town. Alternator charging ceased at 14.0v; it has plenty of current but no voltage “pressure” to force that current into the battery. Your battery bank is now being charging only by solar (the two separate controllers).6)
  • 14.6v in the parking lot as you stop at an Oasis water kiosk. The Morningstar stopped contributing at 14.4v7) so we are down to just the Xantrex.
  • 14.7v on your way back to camp. This is Absorption setpoint (Vabs) of the Xantrex, the highest setpoint of all three. The Xantrex will continue the Absorption stage by itself until that stage is complete, then drop back to Float voltage (Vfloat).8)

how to make them cooperate

So how do we make them all charge all the time?

Barring heroic measures9) 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.10) 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.

serial charging

Serial in this case means “one after another”. Sometimes the charging is done in a particular order for best results.

  1. a high current (but fuel expensive) source handles Bulk stage that takes the battery to about 80% SoC.
  2. a low current (but free to run) source handles the much longer Absorption and Float stages.

In most scenarios the high current source is either alternator charging or generator charging. A huge influx of power gets the battery off that deeply discharged point early in the morning. You will know when you have finished Bulk stage when the battery starts accepting less and less current. Since running the alternator or generator uses fuel and causes wear and tear, it's time to shut them down and use the next source.

The low current source is almost always solar. Solar is free to run and is not subject to wear and tear; it is perfect for the hours it will take to finish Absorption.

The only caveat, for advanced users, is if one of your controller uses end-amps to signal the end of Absorption, it may get confused and drop out early. Also see these potential isolator gotchas.
or trying to reach
current-terminated Absorption excepted for purposes of this discussion
i.e. Vabs == 14.4v
i.e. Vabs == 14.7v
exception: see this gotcha
Absorption is the stage where the controller provides as much current as it takes to hold Vabs, 14.4 in the Morningstar's case. It takes zero current to hold 14.4v when the bank is 14.6v, so the Morningstar just sits there. It might drop into Float.
where the Morningstar has probably been waiting, twiddling its thumbs
exotic alternators/regulators, DC-DC chargers
including Absorption termination setpoints like time or current
electrical/12v/multipoint_charging.txt · Last modified: 2020/10/11 19:48 (external edit)