Will a 12v alternator/generator connected in parallel to the vehicle alternator cause damage long term?

Question

I am a computer systems administrator and I've don't basic work on my car but haven't done anything significant with car electrical.

I have an overland/adventure truck I'm building with a battery bank and DC DC charge controller that draws power from the car and solar. I can forsee a situation where I need to charge my batteries and don't want to put wear on my cars engine.

My solution is to get/build a 12v output generator with electric start that will be automatically turned on when the batteries are low, so I need to connect this behind the DC DC charger on the car alternator side to handle the input amperage

On a basic level, will connecting the 12V 50A+ generator output to the car battery terminals where the DC DC charger does damage the car electronics in any way over long term use.

In an accidental emergency will both alternators running in parallel cause any damage and how could I prevent this without installing a physical switch.

Answer 1

It shouldn't cause an issue with the power kept in parallel.

Why not use a conventional small generator using a power inverter, keeping these two separate systems? Your vehicle's 12v battery will not power the system for very long. Plus, pulling power in and out like you're planning will cause degradation of the car battery in short order. Regular car batteries are not made to be drawn down to lower voltages as would be needed to run your electronics for any amount of time.

You said you'll be using a "battery bank", but I suspect you're talking about connecting this directly with your car battery. This really isn't a good idea for a few reasons, but the main thing is you want to keep them separate so that you never draw too much power from your starting battery. It could leave you stranded not being able to start your car's engine.

My suggestion would be to use at least two deep cycle batteries kept in parallel to power your electronics, then have a small generator done as you've suggested, which will push power through a battery charger to charge your batteries. The system itself will be much easier to make (everything is off the shelf), and you never have to worry about it depleting your car battery.