Battery wiring configuration questions...

Milopez

I have a question about whether I have my battery bank wired correctly.

My system is comprised of 4 x 165 watt BP panels (24 volts) a Xantrex C40 charger controller, and 8- 100 AH batteries powering a Xantrex inverter and a 50 amp DC 24 to 12 stepdown transformer.

* I have the 8 - 100 AH 12 volt batteries wired in series to form 4 sets of 24 volt "batteries".

* The positive to negative connections for each pair of batteries is with 4AWG wire.

* I then have 2/0 connectors that go from 1 positive terminal to the next as well as similar 2/0 connectors that go from 1 negative terminal to the next.

* Finally, I have a single 2/0 wire that goes from 1 of the positive terminals of one of the batteries and a single 2/0 wire that goes from the negative terminal of the other half of that same pair of batteries. to the D/C disconnect box and then to the inverter.

* I just noticed that I have the 2/0 wires going from the batteries to the disconnect box, but then have a 250 amp circuit breaker in the D/C disconnect.


1. The first question is whether this is the best way to configure the connections or should I have done something different? For example, should I have placed the two 2/0 lines at diagonally opposite corners of the battery bank rather than on the single pair of 12 volt batteries in the first set?

2. Is there a problem with the match (or mismatch) between the 2/0 wires and the 250 amp circuit breaker?

3. The batteries are 6 years old and at some point I am planning on adding 2 additional panels to go from 640 to 960 watts. At some point, when the batteries need to be replaced, I may also go from the 8 100 AH batteries to 12 100 AH batteries. Given both of these options, would I need to change any of the wiring that I currently describe above? I figure that given the likely expansion, if there's anything I need to correct with my current configuration, I'd hope to do it in a way that will also allow for this future expansion.

audredger

Re: Battery wiring configuration questions...

Best way.... The best is to have only one bank. Why only 100 Ah? Four of these http://store.solar-electric.com/6cs-25ps.html would give you 820 Ah @ 24 v in one bank. In a multiple battery bank system; each bank should be fused seperatly, all the leads from every bank should be the same length and same gauge and should run to a common point ( read bus bar ) where the load ( inverter) and charger ( controler ) connect. Note: the fuse only the posative side.

The # 4 wires are a bit small IMO, How long are the 2/0 wires. Our host provides a link to an exelent calculator http://beta.circuitwizard.bluesea.com/

BB.

Re: Battery wiring configuration questions...

You can use tables like in the NEC, ones for boats, or like this one, to estimate maximum current capacity of a wire/cable (done right, is a fairly complex task that includes ambient temperatures, type if insulation, in conduit, type of conduit, fill factors, wire gauge, etc.).

For your 4awg wire, the reasonable maximum current is probably somewhere between 60 and 135 amps.

So, when you have parallel strings of batteries--you should have a breaker/fuse in each string to limit short circuits in the string, and then tie all of them to a common point/bus bar... Then any cable that exits the positive bus should be protected, for example, with its own 190-283 amp fuse/breaker appropriate for that OO gauge of cable.

The idea is to protect against any reasonable single short.... Remember, you can have shorts in the inverter, wiring to the inverter, tool dropped on battery, or even an internal battery short... The fuse/breaker needs to protect the wiring so that it will not start a fire (note: fuses/breakers do not protect inverters or batteries, etc. from over current--just wiring and the attached "boxes" from catching/starting fires).

Note too, that most fuses and breakers should be run at 1/1.25 or 0.8x the required current for the load... For example:

If you have a 100 amp load, you should have a 125 amp fuse breaker.

If you have a 100 amp breaker, your maximum continuous load should be 80 amps.

The other question about proper wiring configuration... Seems like it should be easy, but it does get complex quickly. The idea is that from every battery to the common bus point, the total length/gauge of cable be the same among all possible paths... If you have some batteries very close to the bus bar with heavy wiring, and others farther from the bus bar and/or with lighter wiring--the "electrically close" batteries will take most of the charging/discharging current and share the loads poorly with the other batteries in your bank. A thread discussing the details of wiring parallel battery strings:

Wiring for 16 6 volt batteries

From the above thread, this website is a good read on proper battery paralleling techniques:

Smart Gauge

And here is another thread talking about some of the other wiring issues (such as crimp connections):

Re doing the Battery Box

Take this with a grain of salt--Me personally, I would recommend larger batteries and fewer parallel battery strings (even using 4 volt or 2 volt cells to keep battery weight manageable -- If required for your battery box/shed). I try to discourage people connecting more than three parallel strings of batteries if they can avoid it... Saves costs (fewer breakers/fuses per string, less cabling) and headaches (fewer batteries to water, easier to find weak cells/batteries, etc.). Also--it becomes more difficult to properly share current over a large numbers of parallel strings.

Of course, there are lots of people out there with more than three parallel strings of batteries in their battery bank--And they are perfectly happy.

You planned up grading to 960 watts of solar panels for 12 x 100 AH batteries:

• 960 watts * 1/12 volts * 1/ (12 x 100AH) = 0.067 = 6.7%

Is a bit on the low side (5%-13% of Bank AH rating)--But OK, if you loads are not too high. Just watch the battery state of charge.

-Bill

Milopez

Re: Battery wiring configuration questions...

This is very helpful feedback.

First, although I had help from my father in law who is a retired electrician, it looks like we had the batteries hooked up incorrectly according to the useful Smart Gauge website. I'm a bit embarrassed to say that we had it set up like the 1st example, so I immediately switched it to the set up in the second example.

Regarding the 4AWG connections between each set of 12 volt pairs of batteries and the fuses, I'm still a little confused. The system provides power for my detached home office and I only run a few things. The most power demanding items are several sets of halogen track lights, but when I use them I typically only use a set of 6 lights which draws 120 watts or 10 amps (I have another post trying to figure out a whole separate set of issues with those lights!) and during the summer a small window air conditioner that draws just 5 amps. Other than that, I only tend to use a couple of transformers to power my laptop and computer peripherals (docking station, speakers) as well as a rechargeable phone. Thus, I've estimated the typical draw to be only 25-45 amps at most, but more often closer to the 15-20 amp range.

Given these estimates, I'm not sure what that means for (1) the 4 AWG connectors between each battery pair and (2) the recommended size of fuses that I should put on the positive end of each of the 4 battery "pairs"? Also, what are the best types of fuse options, including ones that could be connected to the terminal posts of the batteries and wouldn't require additional connections (If I keep the configuration as it and don't switch to having the batteries all terminate at a common bus bar?).

Also, the battery bank is located to one side of the main box that contains my inverter, 12volt transformer, etc. If the 2/0 main battery cables (4-5 ft in length) go to a 250 amp DC disconnect circuit breaker, is that a sufficient protection between the battery bank and everything else? (That's the way it was configured when I purchased everything, but I thought I'd double check).

Lastly, thanks for the input on the possible increase in panels and batteries. Based on your figures, it sounds like if I do increase the panel output from 640 to 960 watts that I may be better off either keeping just the equivalent to the 8 100AH 12 volt batteries or consider only going up to the equivalent of 10 versus the 12 since 12 would be closer to the lower end compared to the panel output. I'm not as worried about replacing the batteries for now, but do need to increase the number of panels since I am planning on working down in my office more often and for longer periods of time each day.

BB.

Re: Battery wiring configuration questions...

The wiring and protection (fuses/breakers) need to be sized to the smallest wire in the current path. So, with your low currents the 4 awg wire is not an issue. Just fuse the string according the maximum safe current for the 4 awg wire.

You can also fuse for the maximum planned current * 1.25 NEC safety factor (never run fuses/breakers/wires at 100% of their rating--fuses/breakers will false trip at those levels of current).

Wiring in fuses and breakers is always a pain.

This is about the cleanest fuse mount I have ever seen:

Steve961 wrote: »

Here is the simplest thing that I have found. It is a terminal fuse block from Blue Sea Systems that attaches directly to the battery post. Fuses are available from 30 to 300 amps.

http://www.blueheronmarine.com/Detail.bok?no=5552

Otherwise, you can check out our host's website for fuse/breaker options.

High current fuse blocks
Midnite Solar Protection Devices
Outback Protection Devices

-Bill