Remote battery bank question (pics)

Crash · December 2015

Hello to all. Long time visitor but first time poster.

I am configuring a trailer, that I tow behind my motorhome, to act as a solar power plant of sorts. I already have 315 watts of solar installed on the motorhome, and although it works pretty well, I will be needing more power as we will be full timing in the future.

I currently have two 440ah battery banks. One is in my motorhome and has been performing well. The other is in my trailer. All 8 of the 6v batteries were purchased 4 months ago, and have been well maintained. I now need to connect the two banks of batteries, as I have added another 630 watts of solar panels to my trailer. However, I do not want to use heavy gauge cable to connect the two battery banks as I will be towing and disconnecting the trailer often. I also do not want to use another charge controller in the trailer.

All loads are attached to the motorhome, and I obviously want to utilize the second battery bank to increase my storage capability.

The reason for the graphic of the water tanks at the top of my drawing is to illustrate how I perceive what I want to do should work. I want to two banks to equalize, but this doesn’t have to happen in any short period of time. I also have no need to draw power directly from the bank in the trailer, other than to slowly equalize with the bank in the motorhome. In other words, like the two water tanks, the banks will equalize over time as water is used from the one in the motorhome. But there are most likely problems which I have not considered.

I am not a purist, and not concerned with efficiencies so much as I am safety. I can accept a 10% degradation in performance, but not 30%.

I would appreciate any input that anyone has to some of my questions.

1) Can this be done effectively?

2) Can I use maybe 10ga wire to connect the two banks? ( I will have a plug of some sort.)

3) If I use such a small cable, will I have problems when the inverter draws heavy amps?

4) Is there a simple way of reducing the amount of amps that flows through the proposed wire to prevent the inverter from over loading it? (Of course, there will be appropriately sized fuses to protect the circuit.)

5) Are there any other ways to achieve what I am trying to do here (besides 2/0 cable)?

Thanks for your help!

westbranch · December 2015

Are you amenable to sending 120V AC to your Mhome from the trailer? You could power part of the MH loads from the trailer source and the others from the MH source... balancing battery banks DoD is difficult to do. Even as you have drawn each bank will deplete based on its own characteristics... Also the DC wiring will be costly.
ADD: ps, same applies with charging, you wold get better results with separate charge controllers.

Crash · December 2015

I do not want to use an inverter in the trailer to send 120 volts to the motorhome. My inverter, and all other systems, are already installed in the motorhome, so I would rather not make any changes there. All I want to do is connect my two battery banks using a smaller cable than would be considered in a home type installation where a large cable could be used.

I should also mention that the two battery banks are only about 8 feet apart, as one is in the back of the motorhome, and the other is in the front of the trailer. The actual path of the cable would be less than 20 feet.

Crash · December 2015

westbrance, "balancing battery banks DoD is difficult to do.". Sorry, I am not familiar with what "DoD" means. Can you explain?

Mountain Don · December 2015

I think down deep you know that using 10 ga wire for that interconnection between the two battery banks is not a good idea, however you are hoping to find affirmation that there will be no problem with the proposal. The minute you apply a large load via the RV inverter the bank in the trailer is going to be lagging behind in discharge. Or the 10 ga wire will overheat and that could be fun. Then there is the opposite issue when the solar will be pumping amps into the two banks.

If you must interconnect the two banks at least use some large cables and make the connection using some appropriately sized Anderson Power Poles. The PP180's accept up to 3/0 wire. Yes it will be costly but at least it would give a chance of working.

Your water tank drawing should make it clear that the trailer tank/battery willlag behind when the flow exceeds the size of the pipe/wire. I think if the loads were always small it might stand a chance but if you microwave a dinner that creates an imbalance. Folks who parallel connect too many parallel battery strings usually eventually find they have some battery imbalance that slowly kills abattery or two, or more.

Anyhow, you asked and that is what I see

westbranch · December 2015

DoD ( Degree of Discharge) is the inverse of SoC(State of Charge) and usually stated as a %... Mountain Don said the same thing I was saying, you need BIG wire (expensive) as a minimum... The Best configuration for any bank is to have 1 string of cells. Complications arise with adding another string, and that is assuming they are side by each... you proposal is adding another level of issues...

Crash · December 2015

Thank you both. I have no concern for the cost of 2/0 or 3/0 cable or connectors. I have all my banks currently wired with 2/0. My concern is the flexibility and connections including some type of plug between the motorhome and trailer. Perhaps someone could recommend some type of connector for 2/0?

And maybe my concerns about the constant flexing is not justified at all?

I certainly was aware of the lag between the charge/discharge between the two banks, indicated by the small tube between the water tanks in my drawing. My question was how big of a concern is that lag? I assume that both banks will eventually equalize.

The other concern was when requiring large loads, like 150 amps, when using the microwave. I think we have answered that question in that it will overload any 8 or 10 gauge running between the two banks.

So now I am left with looking for some large type of plugs (male and female) that will accept 2/0 to 3/0 cable. I just hope the cable will take some constant flexing.

Thanks, I'll start searching and use large cables.

Mountain Don · December 2015

Welding cable is very flexible, use 3/0 along with the Anderson power poles in the largest size. We use the power poles for connections with portable* vehicle winches.

* Not small, but large 9000 # capacity winches mounted to "portable" plates that plug into a vehicle trailer hitch receiver F & R. At times we draw more than the rated 350 amps through them.

Crash · December 2015

Understood. Welding cable is what I have now,although 2/0. Those Anderson power poles look like they might do the trick nicely! Thanks Don.

Iceni John · December 2015

What you're wanting to do is not much different than what I've done in my bus. I have essentially two separate PV systems running in parallel, but their combined power feeds my house loads. Each bank of four PV panels has its own CC that feeds its own bank of four golfcart batteries, and then each bank of batteries goes through its own 250A Schottky diode to prevent one bank back-feeding into the other; only after the diodes is their power combined and sent to the loads. This way I have complete redundancy in case something fails, important for potential full-timing off the grid.

If I were doing what you're doing, I would simply duplicate the trailer's and the RV's systems, with each one self-contained and independent. To transfer power from the trailer to the RV will probably need 4/0 to handle any large loads with minimum voltage losses. 4/0 isn't expensive - I pay no more than $4 a foot for Carolprene welding cable, and tinned 4/0 lugs are less than $2 at NAWS, not much in the big scheme of things. I suspect that quick-connects are made for 4/0, because we have a Clark electric tug at work with big Q-Cs on what looks like 4/0 cable from its huge battery pack.

John

Johann · December 2015

Maybe a connector like this? 350 amps
http://store.hgmforkliftparts.com/battery_connectors/an-202.aspx

Mountain Don · December 2015

That is the biggest Anderson Power Pole

Remote battery bank question (pics)

Comments

Categories