What limits a 12V parallel bank in size?

chrisski

Photon Sorcerer
How long is your run of 2/0 wire between the battery bank and the inverter?

I'm estimating the run will be 10' total positive and negative. 4' total from the inverter to the bus bar, and the other 6' from the busbar to battery. This is a conservative estimate and I hope it will be shorter when I go out later this week to cut and crimp the cables.

Something else you mentioned was the runs between the batteries and the inverter need to be equal on the positive and negative side. I was not planning on doing that. Because the way that my battery sits near my power board, the positive side is much closer than the negative side, so the positive and negative was a different length by 2'. Everything I've looked for said after the current leaves the batteries, you cut down on resistance by making the wire as short as possible, and that if the positive is longer than the negative, that's OK.

For the parallel series connection, great care was made when I cut the cables to make those the same length so all the batteries get the same juice. The picture I attached, the powerboard would be on the right of the picture with the Positive bus bar at the front and the negative bus bar at the back.

EDIT: I do not plan on coming close to the 2000 watt limit on the inverter, but would like to stay no more than 1300-1500 watts for five minutes, four to six times a day. No air conditioning, no hair dryers, but a couple of cups of coffee in a single brew electric cup and a couple of bags of microwave popcorn. Other than that, a ceiling fan, 100 watts I think and a 300 watt entertainment system would be the big loads.

In the future, I want to take this up to 24 volts and 600 AH battery bank to use the microwave and hair dryer more frequently, but that will really depend on how things go the first few times I take it out.
 

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GXMnow

Photon Sorcerer
The positive and negative can be different lengths, but you want the positives from each battery to the buss bar to be the same from each battery. Also do that for where the negatives all join as well. This will help ensure that the batteries evenly share the current. It looks like you did do a pretty good job of that, so you should be fine. I don't have the link handy, but I think it was Blue Sea that has a very good wire size calculator that takes into account the length, current, and voltage to calculate the percentage of loss on the cables. 2% is ok, 5% is getting into trouble. I am using 2/0 to go about 8 feet total for 150 amps max (typically under 80 amps) at 51 volts (48 volt system nominal).
 

chrisski

Photon Sorcerer
Thanks. Makes sense what you’re saying now.

When I did the wire size calculator, I got 1%, but I will do it again before I cut and crimp. I can’t say for sure I Calculated 200 amps, so I do need to do this again. Thinking about it, I probably used 50 amps, the max my charge controller is rated for. I have since made a spreadsheet I’m keeping all those calculations.

The only place I’ll consider a 5% loss is from a 12 VDC fuse box to four or five charging ports I’ll be installing in the RV. With that, I’m having a little trouble deciding what wire to put in. There’s some 14 Gauge wiring I will run from the 12 VDC fuse box to the charging port where I expect the out and back runs can total 50 feet. I’m still doing the calculations for that part and there’s some primary stranded wire that is rated to 600 volts, but that is much pricier than some 60 volt wire automotive wire I could use. What is amazing me at this point is that 14 gauge my calculations show I need to meet the 5% loss at those lengths, I know that is thicker wire than is powering any of the DC devices which are drawing more amperage than this 3 amp charger will which is in the same part of the RV.
 

chrisski

Photon Sorcerer
2/0 is pretty safe for the current, as in, it won't burn up, but if you run 200 amps a lot, a little heavier wire might be in order, especially if the length is over 10 feet. 12 volts is not forgiving on voltage drop at all when it comes to length though.

I reran the numbers for the 200 amps with 2/0, and purchased some 4/0 wire to install. This meant another tool to cut the wire; a hacksaw crossed my mind, but I won't do that on this project.

I am unclear about how long it will take my 200 amp ANL fuse to blow, and I did see it could tolerate 200 amps for 10 minutes and perhaps 400 amps would get it to blow in a millisecond. So, thicker wire it is.

I'm glad you posted that.
 

eXodus

Solar Addict
Your 200 amp fuse will not blow, these things are slow. If you short it, will make a crazy spark.

Cables are cheap these days - but you are really going big here an AWG 0 could carry your use case - 200A - will get warm, but would still work. 0/2 is already giving you enough margin of safety.

If you would be running this continuous - different story, but that cable will not have an issue before your batteries are depleted.

Can't you get the inverter closer to the batteries?
 

Zil

Solar Addict
My 2000w inverter runs a 1000w microwave and it be very happy with 4 feet positive and same length negative 2/0 cable. The maximum running amperes is 150 at 12 volt. The surge is not a problem. Samlex recommends 2x heating watts of microwave, so 2000W inverter. They also recommend 2/0 cable for up to 10 feet. 4/0 is serious over size.
 
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