Combine two battery banks, 120' apart, with a pair of cables?

[kjmclark]

My farm has off-grid solar and wind. I have a 12 marine deep cycle battery bank at the barn, and a four battery bank at the hoophouse. Both systems are 12V, FLA. They're about 120' apart. The barn system has about 750watts of solar (in summer, 650 in winter), the hoophouse has 100watts of solar (200 in winter) and an 800w wind turbine. The barn system has two mppt controllers, the hoophouse has a small mppt controller (for the solar) and a dump load controller (for the wind). The barn system has lights and a 1hp water pump, as well as some other AC and DC loads. The hoophouse does a lot in the winter - extra lighting, some heating. Both systems run separate electric fences spring/summer/winter.

I'm thinking about connecting the two battery banks. I'd do it with a pair of 6 or 8awg cables in a buried conduit. Does that make any sense at all? I realize that not much energy will flow that way. 2A on a pair of 6awg cables that long should have a 1.7% voltage drop. So, a 13.5V charge voltage coming out of my controllers would be down to 13.3V on the other side.

I can't decide if there would be problems beyond a bit of loss in doing that. The bigger reason to do it is that the wind turbine produces a *lot* of power sometimes, and all the solar on the barn produces a lot of power fairly often. This seems like the only way (short of lugging batteries around myself) to balance the power from those sources.

[Mike90250]

The bane of low voltage systems - distance. You can try it and see what happens, but it's tough to predict if it will work well or not. For sure use a 15A DC circuit breaker .

[chrisski]

I'm thinking about connecting the two battery banks. I'd do it with a pair of 6 or 8awg cables in a buried conduit. Does that make any sense at all? I realize that not much energy will flow that way. 2A on a pair of 6awg cables that long should have a 1.7% voltage drop. So, a 13.5V charge voltage coming out of my controllers would be down to 13.3V on the other side.

I’m not seeing what is going to be done here. If the two battery banks are connected, to me that means these banks are now in parallel or series, so I don’t see a way to limit current flow to 2 amps either for charging or any of the higher wattage loads you have.

I’m sure you don’t mean paralleling the banks, with 8 gauge wire because there’s potential for a whole lot more than 2 amps going though the wire.

[kjmclark]

That's a good point. I was thinking in terms of charging, where in most cases both sets of panels will be feeding current in, and the batteries will largely be at the same level. But with any load, it's going to pull from everywhere at once. So, I'll need some current limiting circuitry on both sides. Like Mike said, a 15A DC circuit breaker, with a 10A limiting circuit or something. This page http://wiresizecalculator.net/wiresizechart.htm says 8awg UF in the ground has a maximum of about 40A. So I might even risk a 20A breaker, depending on the details of the cable. Either way, I'd want the current limit circuitry to be below the breaker by some percentage.

[Mike90250]

Stick with the 15A breaker. If something goes really wrong, you have all that resistance in the wire, and 15A "should" pop, but larger may not.
For the initial hook up, do it with both banks full and at the same voltage level, otherwise you will have an imbalance and way too much current flow..
After the initial connection, both banks should stay connected - all the time, so they don't get unbalanced and you would have to worry about getting them
both back to the same voltage before re-connecting.

There is no current limiting circuit, only the resistance in the long cables. I'm trusting since you posted resistance and calculated amps, that part is correct.

[bcroe]

My farm has off-grid solar and wind. I have a 12 marine deep cycle battery bank at the barn, and a four battery bank at the hoophouse. Both systems are 12V, FLA. They're about 120' apart. The barn system has about 750watts of solar (in summer, 650 in winter), the hoophouse has 100watts of solar (200 in winter) and an 800w wind turbine. The barn system has two mppt controllers, the hoophouse has a small mppt controller (for the solar) and a dump load controller (for the wind). The barn system has lights and a 1hp water pump, as well as some other AC and DC loads. The hoophouse does a lot in the winter - extra lighting, some heating. Both systems run separate electric fences spring/summer/winter.

I'm thinking about connecting the two battery banks. I'd do it with a pair of 6 or 8awg cables in a buried conduit. Does that make any sense at all? I realize that not much energy will flow that way. 2A on a pair of 6awg cables that long should have a 1.7% voltage drop. So, a 13.5V charge voltage coming out of my controllers would be down to 13.3V on the other side.

I also have my doubts of making such an arrangement effective. But I will say this.
Figure the money you would spend on copper, then spend that amount on a far
larger gauge aluminum pair. The aluminum needs to be 2 gauges larger to equal
copper conductance, but you might increase performance by 5 for your money. My
last install was 4/0 aluminum. And use direct burial cable, not conduit for this huge
stuff. good luck, Bruce Roe