Cargo van to RV conversion - electrical

Hi… I’m new to solar and have been lurking here for a month or so, soaking up knowledge, principles, and ideas. This place is an awesome resource and I have learned a lot!

I plan on converting a van for occasional RV use. Usage will be dry camping, 2-4 days at a time, perhaps 30 or so trips a year.

I’ve analyzed my loads and have come up with 830 watt-hrs daily usage (roughly translating to 70 amp-hrs). For starters I’m contemplating two 100 a-h AGM batteries for 200 a-h of total capacity. I’m considering AGM for its low self-discharge rate (useful between trips), and knowing myself as one who is not into maintenance - the higher cost is a fair trade for me. 200 a-h capacity should yield a 65% daily SOC, allowing a decent number of charge cycles over the years for my anticipated usage. Most of my loads are 12v, but will have an inverter to handle a couple 120vac items.

For pv panels, I’m considering a single 250 watt unit or three 100 watt panels (series wired), flat-mounted on the roof. If I go with the 250 watt panel I may design in the ability to tilt forward or backward if flat-mounting produces insufficient power.

Needing to replenish 70 x 105% a-h daily, and considering CC inefficiencies along with non-ideal flat-roof collection yielding perhaps an overall 0.6 inefficiency multiplier, I come up with 5 hours to recharge via the 300 (3 x 100) watt panel option and 6 hours for the 250 watt panel. The battery manufacturer recommends charging at a rate of 0.4C (yikes, 80 amps for both batteries!). Obviously I’m not even close on that, so will just have to live with what I can reasonably produce from the roof.

At the end of the 2-4 day trip and back home I would immediately fully charge with shore power through a three stage charger in the van.

Question: Is 0.6 a reasonable inefficiency factor? Would 0.5 be better for planning purposes?

To augment and for days that are cloudy, I’m thinking about a second inverter, tied to the vehicle’s dual battery 220 amp alternator, that would drive the three stage charger (normally used when charging from home). That would top off the house batteries when driving from location to location. I realize that isn't the most efficient, but it could still charge while driving. Another possibility would be to tap into the 220 amp alternator directly at 12 volts (seems a shame there are at least 100+ amps available for other stuff), but would like to have a measure of isolation between the vehicle’s electrical system and the house batteries/system in the back - though it seems there would easily be enough for bulk charging, and then let the solar charge controller finish up.

Any thoughts or views on tying into the vehicle's alternator to charge while driving on cloudy days?

Would appreciate any comments or gotchas noticed. I’m aware there is rarely a perfect solution, but I think the above would work for my anticipated usage. Thanks in advance!


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Cargo van to RV conversion - electrical

    Welcome to the forum.

    Okay you left out one critical factor which you may not be aware of: the charge controller.
    If you try to use a 250 Watt panel with an inexpensive PWM type controller all you'd get is the current: typical a panel that size has a Vmp around 30 and an Imp around 8. So you'd be passing 8 Amps to the batteries. For 200 Amp hours that's a peak charge rate of <5% which is below minimum.

    Now for the magic of MPPT. More expensive controller, but it can down-convert the higher panel Voltage into more charge current. You would get about 16 Amps, and the charge rate just doubled to an acceptable 8%.

    You are right to expect less-than-optimum power from panels flat on the roof. If you were to go with the three 100 Watt units instead you'd be near 10% on peak charge rate which is even better.

    Don't think about 'X' Amps for 'Y' hours = 'XY' Amp hours; it doesn't work like that. The charging isn't linear. Maximum current is available when insolation is highest and the battery is lowest. The real world doesn't neatly line those two things up either (best light midday, lowest battery early morning). That's why we look for peak charge current, hope the weather holds, and realize there is some efficiency loss in everything.

    I think I'd go for as much panel as will fit, preferably around 400 Watts and an MPPT controller of about 30 Amp capacity (MidNite Kid).

    You could Bulk charge directly from the vehicle's alternator. You would not want to go vehicle battery -> inverter -> charger -> house battery. That is quite inefficient. The solar can manage the finish charging if the alternator give it a good head start.
  • citysnapscitysnaps Registered Users Posts: 8
    Re: Cargo van to RV conversion - electrical

    Thanks a ton Cariboocoot for having a look at what I proposed and commenting - appreciate it!

    I was looking at the MidNite Kid with WBJR and shunt for battery monitoring - your recommendation seals the deal. I'm still a little apprehensive about hooking up to the vehicle's alternator. The 200 AH of AGM battery will gladly accept > 3C charge and the alternator can supply (under the right conditions and RPM) 220 amps, assuming I can adequately wire to supply that amount of current. But for how long under those conditions? I would hate to lose my alternator stuck in the middle of nowhere due to failure.

    I realize alternator charge current will gradually decrease as the battery voltage increases from charging. Seems it would be nice to set a limit on charge current to protect the alternator. That was the idea, as inefficient as it would be, of using an inverter running off the alternator driving the battery charger, which can be programmed for a maximum output current - say, 50 amps or some other number.

    Or, am I worrying too much about alternator failure from routinely supplying such large currents, i.e. in the real world, some other mechanism will prevent that from happening?

    On the pv side, I'll see how much panel I can get on the roof to increase output.

    Thanks again...
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Cargo van to RV conversion - electrical

    You're worrying too much.

    Everything together isn't going to draw 200 Amps. That would be about 2400 Watts.
    Alternators, like most other charge sources, are regulated by Voltage; as the Voltage comes up the current goes down and even at minimum system Voltage it is unlikely the alternator is going to be at its maximum. In theory it was built to provide that much so it should be able to handle it.

    You won't leave the house battery connected all the time. It's a good idea to have a dual battery system. Some of them are electronic, some are mechanical (relay cuts in/out with engine running or other criteria). It seems the preference is for mechanical as the electronic ones cut some Voltage and are perhaps more prone to failure. But lots of RVers do this, so it can be done.

    Oh and have a fuse or breaker on the line from the vehicle to the house battery of course, because no alternator can stand a dead short on its output.
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