Compatability of RV system with solar

[Sockeye]

Hello to all. I have searched for days on this forum and elsewhere online to find an answer to my questions, to no avail.

Background:
I have a typical 2015 vintage bumper-pull RV parked off-grid for summertime recreation use. AC power comes from a generator. AC power is converted to 12V DC to charge the house batteries. Unfortunately the manual for the device that converts the AC to DC power is in the trailer far from where I am now, but I assume it is a fairly simple onboard battery charger. I also assume the 12V batteries in the front of my RV are connected directly to the onboard charger and then through a fuse bank to the 12V appliances in the RV. I am planning to add a 400W solar system to eliminate, or at least substantially reduce the use of generator power. With a solar system in place, when I connect the solar system to the RV batteries, the batteries will then be connected to both solar power and the DC power from the RV onboard charger (when the generator is running).

Questions:
What will happen when the solar system is powered-up by sunlight and the onboard charger is powered-up by generator power?
How do these two systems "decide" which one charges the batteries?
What if I decide to go with LiFePO4 or AGM batteries and the charging voltages from the onboard charger are not suited for them?

I'm thinking I may need to replace the onboard charger. This must be a common situation, yet I can find no mention of it anywhere.

Thanks for your interest.

 

[Samsonite801]

Questions:
What will happen when the solar system is powered-up by sunlight and the onboard charger is powered-up by generator power?

It's ok most of the time to have 2 chargers running, they will eventually both switch into Float stage and not overpower each other.

There have been some rare occurrences where 2 chargers wouldn't always play well together, so I just recommend to hook them up and do some testing, and monitoring with a voltmeter for a charge cycle to make sure they are performing fine together.

How do these two systems "decide" which one charges the batteries?

If one charger is say, charging bulk (CC) or absorption stage (CV) voltage, something higher than float, the other charger might back down to Float, or try bulk, or absorption until they hit their limit triggers, eventually they both back down based on their normal triggers in based on their programmed curves.

Many times, you can read in the manual for the charger and solar charge controller to see what those triggers are.

What if I decide to go with LiFePO4 or AGM batteries and the charging voltages from the onboard charger are not suited for them?

This is where you need to look in the manual for both the charge controller and battery charger, see if they support the chemistry you want to charge, and how to select which battery chemistry. Usually the better charge controllers have a custom curve (if they do not specifically have a profile for LFP chemistry), but likely your regular battery charger does not support LFP, and you may need to buy a 120v to 12v charger that does have a profile for LFP (or AGM), or a custom profile where you can set your own triggers for bulk, absorption, and float voltage...

I'm thinking I may need to replace the onboard charger. This must be a common situation, yet I can find no mention of it anywhere.

Yeah, if you switch to LFP chemistry (or even AGM), your existing charger will likely over-volt especially on float (and bulk/absorption) it if it is made for only SLA chemistry.

Thanks for your interest.

Just doing my part to try and help, and pass time during the darkness of the cold Winter months hehe... Peace be with you bro...

 

[Rocketman]

With 400w of solar make sure your SCC (solar charge controller) is an mppt - the pwm style will lose a lot of that solar.

I like Victron - a smart solar mppt 100/30 will handle 400 watts. One advantage of higher quality equipment is you have full control of the voltages for bulk,absorbing, and float. Plus it will gives you good data -

Good luck

 

[HRTKD]

@Sockeye , it's nitpicky for me to say this but it may be helpful at some point. In most RV's, the converter is wired directly to the main distribution panel. The battery is also wired to the main distribution panel (with a cutoff switch somewhere in the circuit). Replacing the OEM converter with one that understands how to charge a LiFePO4 battery is fairly easy since the connection between the converter and the main distribution panel is just a few inches. The thing that may trip you up is that an OEM converter is often wired directly into a circuit breaker (no plug). The aftermarket converters come with a plug.

In my original 12v system I had a PWM solar charge controller wired directly to the lead acid battery on the tongue. The OEM converter was a 45 amp unit as I recall. When I fired up the generator, the converter automatically turned on and started charging. When that happened, I watched the solar charge controller display and it went from 4 amps down to zero. Like it was deferring to the converter. There was no problem there.

 

[Sockeye]

Thank you all very much for your very direct and helpful replies. You verified what I was thinking. I understand everything you said and will use your advice to help design my system. When I get it operational, I'll report on my experience. That should be in mid-June.