Need a little help on parallel systems of different voltages.

Spence_McCallie · November 2017

I am installing a system to run a well pump and charge my RV.

The well pump requires 48VDC, and the RV is 12VDC.

Here's my plan:
3 x 270W (24V) panels in series to produce the charge current for the well. This makes 93VDC+/- at max charge. These would go to a 30A MPPT charge controller feeding a 48V battery bank (4 x 12V, 75AH). I don't need the well that often (once a day for 20 minutes), so I'm not worried about deeply discharging the batteries.

1 x 270W (24V) panel to charge the RV batteries (2 x 12V parallel for a total of 150AH) through a 30A MPPT controller (much smaller).

Ultimately, my question is can the two PV arrays and the charge controllers share a DC common bus, or do these need to be independent?

I know that the charge controllers cannot share a DC common with the batteries, but can the panels and CC's?

Spence_McCallie · November 2017

Diagram...

Image: https://us.v-cdn.net/6024911/uploads/editor/ta/g18orxai29z2.jpg

Estragon · November 2017

No, each controller wants its own array.

myocardia · November 2017

You cannot share anything whatsoever...except the chassis ground...between the two systems. No wiring, no batteries, no panels, no panel wiring, no charge controllers or wiring, only the chassis ground. BTW, make sure that you have a good, large, bare metal chassis ground for each and every bank of batteries. In other words, either a very large one for the 48V bank (the best way to do it), or 4 smaller ones, one for each battery in the 48V bank, along with a 5th for the dual 12V battery bank.

For 75aH batteries, I would recommend a single 2 gauge for the dual 12V bank, and at least a single 1/0 gauge, also known as single aught, or zero gauge, for the 48V bank. Oh, and when you sand away the paint from the chassis for the chassis ground, buy and apply a couple of 99 cent from AutoZone packages of dielectric grease, unless you want to resand them every few months.

edit: Actually, because of the higher voltage and lower amperage, 2 gauge would also be perfect for the 48V bank to share.

Spence_McCallie · November 2017

myocardia said:

You cannot share anything whatsoever...except the chassis ground...between the two systems. No wiring, no batteries, no panels, no panel wiring, no charge controllers or wiring, only the chassis ground.

So I'm guessing I need just a couple of pass-through terminal blocks to keep the DC negative isolated for each array, huh?

myocardia said:

BTW, make sure that you have a good, large, bare metal chassis ground for each and every bank of batteries. In other words, either a very large one for the 48V bank (the best way to do it), or 4 smaller ones, one for each battery in the 48V bank, along with a 5th for the dual 12V battery bank.

I'm going to use a 10' x 5/8" ground rod for the entire 48V system. The panels will be tied to the disconnect box, tied to the charge controller box, tied to the battery box, tied to the well. Picture a 14AWG ground grid. The batteries will be enclosed in a MIdnite NEMA 3X enclosure.

The 12V batteries are a little tougher since they are on a trailer. The chassis ground is the negative... any ideas?

myocardia said:

Oh, and when you sand away the paint from the chassis for the chassis ground, buy and apply a couple of 99 cent from AutoZone packages of dielectric grease, unless you want to resand them every few months.

I typically use a drill and tap set to thread the ground studs into the backplanes and/or bottoms of my enclosures. Then I grease them. It has served me well.

myocardia said:

For 75aH batteries, I would recommend a single 2 gauge for the dual 12V bank, and at least a single 1/0 gauge, also known as single aught, or zero gauge, for the 48V bank.

edit: Actually, because of the higher voltage and lower amperage, 2 gauge would also be perfect for the 48V bank to share.

My intent is to just charge the batteries for the 12V system (<50' from the CC). I would install the cable on the bus bar next to the battery bank. The power distribution is handled on the trailer by a 8AWG wire that was factory installed (with a 50A fuse). In this case, the 10AWG should be fine, right?

I completely agree with you on the 2AWG for the 48V system. I was going to install a 125A fuse between the positive bus bar and the battery bank. That way I can install an inverter later.

I'll have to post some photos when I get the gear in.

myocardia · November 2017

Okay, you need to stop and reconsider. What you are wanting to do, having both the panel and the charge controller anywhere in the neighborhood of 50 feet away from the batteries would require you running about 6 separate runs of 4/0 gauge copper wire. That may be a bit of an exaggeration, but honestly not too extremely*. The charge controller goes absolutely as close as possible to the battery/batteries as possible. Six feet is pretty much the maximum (for a 12V nominal system), and that is not 6 linear feet, that is 6 wire feet, from the batteries. Now, running the ~30V from your solar panel 50ish feet to the charge controller is no problem, when charging a 12V system with an MPPT CC. Any other way is very, very highly not recommended. BTW, any reason why you aren't wanting the solar panel going with you, when you go camping?

Also, I somehow missed that you are wanting to run a 75aH 48V battery, to run a well pump. Yes, I see very clearly that you mentioned it in your first post, but I somehow didn't catch it. Unless it's a low volume/low amperage pump, it may very well not even be enough battery power to get the pump "spun" up. Pumps require absolutely, positively huge amounts of amperage on startup. Oh, one more thing. Why are you wanting to use two 12V batteries in your RV, instead of two 6V batteries? You can buy 215 aH 6V golf cart batteries for $100 each, and then you have zero parallel connections, and they are actual deep cycle batteries. Any of the cheaper 12V "deep cycle" batteries aren't actual deep cycle batteries. You'll know it isn't a deep cycle, if the specs for the battery make any mention whatsoever of either "marine", or cranking amps. No true deep cycle batteries even mention cranking amps.

*For example, it may "only" require 4 or 5 runs of 4/0, instead of 6.

Spence_McCallie · November 2017

Yes, the well pump is small. The maximum it can draw is 8.4A ever. The motor is current limited at that amperage.

Yes, I have a small panel that goes with me when I travel, but this one will remain fixed on my property in the mountains. I have an extra panel for my array so that I can charge my batteries.

I have had awesome results with my PowerSonic AGM 12v batteries. Yes, they are normally used in industrial backups, but I like them. No, they aren't cheap and they do say amp-hours capacity, not cranking amps.

Estragon · November 2017

You can run 10ga the 50' to the trailer, the ampacity of the wire is better than the 20a max from the controller. The problem that may arise is the controller will see the output voltage at its output terminals, but the voltage at the batteries will be lower. How much lower will vary with the current, but will likely be enough that the charging algorithm won't operate as intended.

The best way to overcome this would be to run some light voltage sense wires (if the controller has provision for same). Because the sense wires carry little/no current, they can read close to actual voltage at the batteries.

Spence_McCallie · November 2017

Image: https://us.v-cdn.net/6024911/uploads/editor/zg/kyy253jk053q.jpg

Complete system redesigned. Let me know what you think.
1. PV systems are now isolated except for grounds.
2. Nothing touches battery terms except batteries.
3. Distances: Disco is next to solar rack +/- 10' wire length, Disco - Control Cab +/- 30' wire length, Control cab is mounted above Batt cab, Pump controller is +/- 25' from Control cab.
4. Ground wires are 14AWG.
5. I am not screwing with the RV circuit at this time since I will probably be getting a new one next year. At that time I will consider moving the controller into the RV.
6. Pump Cable will be as designed by the manufacturer.

myocardia · November 2017

Okay, you said in your first post that you were wanting to charge your RV batteries, so that was how I was responding. You are bringing back in the RV completely, or almost completely, charged batteries that you just want to keep float charged. That is a completely different story, and 10 gauge will be be perfect for that. Batteries that are already charged use/take almost zero amperage, compared to what they request/require when they are discharged. Good luck, and happy camping.

Need a little help on parallel systems of different voltages.

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