It is common for a camper to have different house battery charging sources:

• solar
• alternator
• shore power
• generator
• etc

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.²⁾ In general, the charging source with the highest voltage setpoint will “win”.

Exception: some commercial products integrate charging from multiple sources into a single unit so there is no “competition”. See Combination DC-DC chargers.

Most charging involves holding³⁾ a particular voltage setpoint.⁴⁾ 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.⁵⁾ The Xantrex charge controller has been configured by you to start Absorption at 14.7v.⁶⁾ 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:⁷⁾

• 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 then 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).⁸⁾
• 14.6v in the parking lot as you stop at an Oasis water kiosk. The Morningstar stopped contributing at 14.4v⁹⁾ 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 complete¹⁰⁾, then drop back to Float voltage (Vfloat).¹¹⁾

So how do we make them all charge all the time?¹²⁾

Barring heroic measures¹³⁾ 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.¹⁴⁾ 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. Even if they contribute unequally it is a non-issue because the voltage setpoint is being held. If a big load is added or conditions deteriorate it will be “all hands on deck” as in the Bulk stage scenario above.

Note: there are high-end controllers/chargers which communicate with each other to coordinate charging. That feature is beyond the scope of this article.

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.

Some of these are observed, and some are theoretical.

• the vagaries of voltage sag across the system may result in the various charging sources seeing slightly different voltages and therefore contributing in unpredictable ways.¹⁵⁾ Voltage Sense wires can help with this issue.
• Multiple non-communicating controllers set up to trigger Float from endAmps¹⁶⁾ may enter float early since they are providing only some of the charging current.
• Multiple sources may inadvertently exceed the bank's recommended charging current, or your preferred current. Example: a lithium battery wants 50A max but each charging source can make 30A. This might result in overcurrent (60A) during Bulk.
• theoretical: low-frequency PWM controllers in parallel with MPPT controllers may confuse the latter with an unstable reference voltage.
• some systems may throw a warning if chassis voltage > alternator output voltage¹⁷⁾