Storing an RV with 400ah Battery System

[chrisski]

What would you recommend with the battery bank in an RV when it is not being used for weeks or months at a time? Its AZ, so outside day temps would range form 45° to 120° F, with no shore power available. There would be times where the RV is in storage and I would not see the RV for two months at a time. The batteries would most likely be AGM batteries.

Some of my thoughts:

1) Remove the battery bank. I really want to avoid this.
2) Disconnecting the batteries. I did not have good luck with Interstate Deep Cycle SRM 27 batteries alone in the trailer for months at a time.
3) Leave all three panels of the array connected and charging the batteries?
4) Leave one Array Connected to charge controller and batteries.

[Sunking]

None of the above, and all are bad options for storage. Ideally the two best options in order of preference are:

1. Heat and cycling are the two killers of batteries. Charge Controller Algorithms are extremely harsh and hard on batteries because they use 3 stages that intentionally overcharge the battery. Every day your battery gets slammed with an over charge in the heat. Remove the batteries to a cool dry place and put them on a AC Float Charger.

2. If the batteries must remain in the RV, disconnect them and connect to an AC Float Charger.

Using solar to store batteries is a very bad idea.

[Mike90250]

If you don't have AC power, just use one of your panels to charge the batteries while in storage. You have turned off all the loads, right ?

What size are your panels ? You would likely need about 80-100w of panel to maintain 400ah of batteries.

[chrisski]

If you don't have AC power, just use one of your panels to charge the batteries while in storage. You have turned off all the loads, right ?

What size are your panels ? You would likely need about 80-100w of panel to maintain 400ah of batteries.

All Loads would be off while the RV is in storage, except perhaps the battery shunt for the voltage monitor.

I do have a 100 watt panel I could add to the roof just to float charge it. Right now, the 12v 100w panels for the main system will be wired three in parallel for 36 volts, so if I simply tripped a breaker, there'd be 300 watts going into the charger.

Also, for this float charge, do you recommend a specific charging device, or just use my MPPT controller and set it for a float charge, if that setting is even available? By looking in my Victron manual, seems the only thing I can shut off is the Equalize mode, otherwise, the charge controller will use the bulk, absorption, or float mode.

[Mike90250]

> Right now, the 12v 100w panels for the main system will be wired three in parallel for 36 volts,

This is very unusual. generally 12V panels are rated at about 18Vmp. I think you have some terminologys mixed up. With solar and batteries, there is no room for miscommunications, unless you like dead batteries or fire.

When you have more than 2 panels in parallel, it's required to have a Combiner with overcurrent protection for each string of panels.

[chrisski]

> Right now, the 12v 100w panels for the main system will be wired three in parallel for 36 volts,

This is very unusual. generally 12V panels are rated at about 18Vmp. I think you have some terminologys mixed up. With solar and batteries, there is no room for miscommunications, unless you like dead batteries or fire.

When you have more than 2 panels in parallel, it's required to have a Combiner with overcurrent protection for each string of panels.

I meant three panels in series, with two of those in parallel. I think that is 3S2P. For overcurrent protection, both strings of three panels will have a circuit breaker prior to the combiner, with an additional circuit breaker prior to the charge controller.

[Sunking]

Also, for this float charge, do you recommend a specific charging device, or just use my MPPT controller and set it for a float charge, if that setting is even available? .

If you have a choice, an AC powered float charger.

If all you have is solar, you might be able to make your Controller a Float Charger with the settings. First consult the battery manufacture specs for the proper Float Voltage. Should be around 13.6 volts. Now Set:

Bulk = Absorb = Float = 13.6 volts.

Here is the issue with solar and your equipment. You will mini-cycle your battery every day. More so if you have cloudy days. Your Controller and Monitors all use battery power 24 hours a day, and does not turn off when the Sunsets for the night. Couple that with the heat in AZ is not ideal storage conditions. Kept properly and your battery last a couple more seasons. Pay your money and take your chances.

FWIW I will tell you something you want to hear. That is what you came here for right? You do not have to reconfigure anything except the Controller Settings. Leave the panels alone. If you can set your Controller to be a Float Charger is all that matters. Even if you have 1000 watts of panels, the battery will never demand more than 10's of watts in the morning. There is no where for power to go. Only time you might see any significant current is if you experienced a several cloudy days.

[Mike90250]

> I meant three panels in series, with two of those in parallel. I think that is 3S2P.

So you have six PV panels, wired as 3S2P ? (3 panels in series, and then another 3 panels in series, wired in parallel with the first group of 3 ? )

[chrisski]

> I meant three panels in series, with two of those in parallel. I think that is 3S2P.

So you have six PV panels, wired as 3S2P ? (3 panels in series, and then another 3 panels in series, wired in parallel with the first group of 3 ? )

That is what I plan on doing.

Right now I have panels, brackets, wiring, controller, battery monitor, and a few other odds and ends in the boxes in my house. I’m waiting to be installed when it is a bit cooler, around October, when I will be able to spend the entire day on the roof of the RV. I’m saving the battery purchase until later, around September. The Battery purchase will include battery wires, battery box, and some sort of mounting system.

So between now and then, I may tweak the actual setup. Based off voltage loss and the run from the combiner to the batteries, Because I’m limited to No bigger than a 6 AWG wire into the Charge Controller, I just don’t see any other way to rig the panels other than 3S2P. Voltage losses seem to become too much. I had wanted to run 6 panels in parallel at first, but voltage loss was too much, and 2S3P was better, but a little high, and 3S2P was acceptable.

I’m taking months on this set up so I get it right the first time.

[Sunking]

I had wanted to run 6 panels in parallel at first, but voltage loss was too much, and 2S3P was better, but a little high, and 3S2P was acceptable..

No such thing as too high of a voltage. Higher voltage is always better and less expensive. Only limitation is selecting the correct controller that allows you to use as high of a voltage as possible. At 3S2P in a RV should not require any panel wiring to be any larger than 12 AWG. Feeder after combiner might have to be a little larger say 10 AWG, but the distance will be less than 1 to 2 feet. RV do not have long runs of wire.

[chrisski]

Mentioning a 1 or 2 foot run from the combiner to the charge controller gives me something to think about.

For the RV, I have the combiner on the top of the roof, with perhaps an 8 - 14 foot run from the combiner to the charge controller. Now that I’m thinking about it, could be better to just route the two arrays through the roof and place the combiner in the “Battery Bay” and then I have that one to two foot run you talked about. I plan on drilling a hole through the roof and floor, and running the wire along the wall through metal electrical conduit. With this, I may be able to now run the two array wires behind the kitchen cabinets without putting an unsightly electrical conduit along the bedroom wall.

The fifth wheel is 35’ long with perhaps 25’ of that where I’m looking to place the panels.

I have a midnight solar combiner I was going to leave out of this project and make my own circuit boxes and combiner, but this will let me use it also, That Midnight Solar combiner would be better than anything I’d make, but I could not lay that manufactured combiner horizontally without fear of it leaking, but in the “Battery Bay,” won’t be a problem.