Battery for RV system?

tr0y · January 2012

Wow my little thread got lots of attention, Thanks All !

Now if I could throw in a question, not sure if I should do it here or start a new thread.

I asked this because I have added some BP solar panels to the top of my RV and was trying to figure out what would work best as a battery bank. It is a small RV about 100 Sqft of living space, a 110V 5 CUft bar fridge and a microwave, small TV and a car audio system for sound.

What I have added are:

Four (4)

bp- 135W
Voltage at Pmax (Vmp) 17.4V
Current at Pmax (Imp) 7.7A
Warranted minimum Pmax 128.2W
Short-circuit current (Isc) 8.4A
Open-circuit voltage (Voc) 22.1

One (1) Xantrex C30 Charge Controller

One TrippLite 1512 RV MSW inverter charger

Two (2)

SunXtender 405 Amp Hour AGM Batteries

We also carry a Honda 2K Generator

I also have four more of the exact same batteries that were over delivered by the vendor when I bought batteries for the house, when I told them about the mistake they just said keep the batteries due to shipping cost. My lucky Day !

We spend most of our time in the west either on the coast or the mountains and get a lot of sun.

I was wondering if I would be foolish to put all the batteries in the RV and have that big bank ? The charger can deliver seventy or so amps and when we are not using the trailer where it is parked has grid power that we can plug into.

Your thoughts ?

Mods feel free to move this if you think this is the wrong place.

Thanks

T

Cariboocoot · January 2012

Re: Battery for RV system?

Yes, we'll put this in a new thread because it's quite a different question.

Now there's a few different ways of looking at this. Since it's an RV you don't have to rely on the PV's for "full charging". One thing to keep in mind: the panels could "max out" that C30 controller, which is only good for 30 Amps.

If you wanted to, the four 135W panels could fully charge about 350 Amp hours @ 12 Volts. That would be near the 10% charge rate.

For a "low" or "maintenance" charge you could go closer to 690 Amp hours. That would be around 5%.

If you had one 405 Amp hour 12 Volt battery that would fit nicely in the middle. Two would be over the max, and you'd be relying on the gen or shore power more. All the batteries you have together would be way over the limit, and you'd soon have dead batteries. Even your 70 Amp shore charger couldn't manage it.

You could take an extra charged battery along just in case, but I wouldn't go overboard on wiring them in.

mike95490 · January 2012

Re: Battery for RV system?

Be sure to give the spare batteries a charge once a month, even AGM batteries have some self-discharge, and WILL be ruined if ignored.

The C30 controller, while very good, will blow a fuse at 35amps or thereabouts. You have too much panel for it.

tr0y · January 2012

Re: Battery for RV system?

Thanks all, could someone from the specs provided, give me the way that you would calculate watts, amps, voltage or is this simple addition that I am thinking to much about ? Also does anyone think moving to an moot charge controller would be worth the time, effort and money ? If so suggestions ?

Since I did forget this in the first post, the system is 12v and the batteries are 6v.

Thanks

Cariboocoot · January 2012

Re: Battery for RV system?

What does it say in my posting signature?

Notice how if you multiply Vmp * Imp you get something close to the panel's rated power: 17.4 * 7.7 = 133.98 (an optimistic "135 Watts"). Don't count on seeing this all the time. Usually the panel will average around 77% of its nameplate rating in terms of actual available power. So 4 * 135 Watts = 540 * 0.77 = 415.8 Watts. Divide this by the system nominal Voltage (not good to go below that) and you get an expected maximum current of 34.65. Could be higher with just the right conditions, most of the time will be lower. Still well able to max a 30 Amp controller.

The SunExtenders @ 6 Volts means you put two in series to get 405 Amp hours @ 12 Volts.

That array size is just "on the border" of where MPPT starts to make sense. You'd need at least a Morningstar 45 MPPT http://www.solar-electric.com/motr45ampmps.html Is it worth $420+? Probably not in this application. Panel angle on RV's tend to be less than ideal at any time, wiring length isn't much of an issue (no V-drop trouble), and you probably don't camp in the cold much. That means the MPPT advantage would be minimal here, in my opinion.

scrubjaysnest · January 2012

Re: Battery for RV system?

Cariboocoot wrote: »

What does it say in my posting signature?

Notice how if you multiply Vmp * Imp you get something close to the panel's rated power: 17.4 * 7.7 = 133.98 (an optimistic "135 Watts"). Don't count on seeing this all the time. Usually the panel will average around 77% of its nameplate rating in terms of actual available power. So 4 * 135 Watts = 540 * 0.77 = 415.8 Watts. Divide this by the system nominal Voltage (not good to go below that) and you get an expected maximum current of 34.65. Could be higher with just the right conditions, most of the time will be lower. Still well able to max a 30 Amp controller.

The SunExtenders @ 6 Volts means you put two in series to get 405 Amp hours @ 12 Volts.

That array size is just "on the border" of where MPPT starts to make sense. You'd need at least a Morningstar 45 MPPT http://www.solar-electric.com/motr45ampmps.html Is it worth $420+? Probably not in this application. Panel angle on RV's tend to be less than ideal at any time, wiring length isn't much of an issue (no V-drop trouble), and you probably don't camp in the cold much. That means the MPPT advantage would be minimal here, in my opinion.

Since wiring typically used in RV solar is 12 awg a 10% volt drop is rather significant. This is based on the calculator from this site. The biggest problem with RV solar systems is not charging the batteries fully because the wire size is to small. I agree with everything else said.

tr0y · January 2012

Re: Battery for RV system?

Lucky for me I did a remodel on the RV interior and when I had it apart I knew solar was coming so I wired for it. 6awg from panels to combiner, 4awg from combiner to charge controller, and 0/2 awg welding wire to and from all batteries with custom built copper cable ends, soldered and crimped into place.

BB. · January 2012

Re: Battery for RV system?

tr0y,

Luck had little to do with it... Thinking/planning ahead by you did the deed.:cool:

Much of the work I did in engineering was just working out the problems in advance.

-Bill

mike95490 · January 2012

Re: Battery for RV system?

tr0y wrote: »

..... custom built copper cable ends, soldered and crimped into place.

Keep an eye on the connections, where the solder wicks up inside the wire, it makes it "solid" and the wires are prone to break right at the spot where the solder stops, even if there is heat shrink over the spot. Just a heads up to watch for that.

Battery for RV system?

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