Last time I talked about this project, I left you hanging with the lead acid batteries from the old house power system gone, and the generator connected to the chassis batteries. But I didn’t have anything hooked up to power the remaining 12V house loads–things like the furnace, water heater, water pump, slide out motor, and a few lights.
There are lots of ways to approach this, but first it’s worth knowing how much 12V power I really need:
- Water heater: just control logic here, no actual heating. It isn’t enough to worry about.
- Furnace: about 8 amps running at 12V.
- Water Pump: 4 amps just before reaching the high-pressure cut-out (in other words, when it’s under the heaviest load)
- Slide out motor: 30 amps while in motion, more at the beginning when it’s sort of stuck in place.
- Lights: since they’re all LED, not more than 10 amps.
- Awnings: Just control logic here, and from what I can tell the only reason they use 12V at all is to power the on/off switch. I may be able to eliminate the need for 12V at some point in the future.
- A couple of power outlets (10A max each), an antenna amplifier, and a small sound system.
Other RVs may have other considerations:
Leveling jacks: I have them, but they’re driven by the chassis/starting batteries.
- 2-way (LP, 110VAC) refrigerator: The control board runs on 12V on these. Very little power needed, but the refrigerator doesn’t run without it. If you have a 3-way (LP, 110VAC, 12VDC) refrigerator, you’re using 12V power not just to control the other two, but also to provide cooling–which is a significant demand.
- Generator: Most RVs are configured to start the generator from the house batteries. I talked about why I don’t like that setup here.
When you look at the list of things that need 12V DC power to run, almost all of it is stuff that only needs to run for short periods of time. So while it might work, I really don’t want a big DC power supply running all of the time.
I’ll admit I didn’t have it all settled when I pulled the old batteries, but I had enough of a plan to get by. The slide-out was the biggest thing to deal with, and while I could in theory have wired it to run off of the chassis batteries like the generator and jacks, power for its motor and all of the lights in the slide out were fed in together–I didn’t want to try getting another set of wires into the slide out.
So the short term solution was pretty crude, but it’s still working just fine. I picked up a cheap 12V lithium jump starter, that’s able to be charged while the main leads are energized. It’s also not one of the ones with the smarts to limit current until the car’s battery is charged to a certain point, so it’ll work with no other battery in the system.
To test it out, I clipped the leads and connected them directly to the slide-out’s controller:
This configuration has no trouble running the slide out through its full travel multiple times. It’s micro USB charging port is more than enough to charge it and run the few 12V LED fixtures in the slide out.
For the rest of the 12V stuff, I’m using a 300W adjustable power supply. I’m not quite happy with it, as it’s not nearly as efficient as I’d like, so there’s not much point in dwelling on it at this point. It works fine, but I’ll post back when I have something better in its place.
With an extra set of leads, I now no longer carry the bulky SLA battery pack as insurance against a dead toad battery. It just isn’t necessary any more–the lithium jump starter does a better job.