Battery Bank Wire Size

Thanks for all the input

After installing the new 2/0 cross over wires for my four 12 volt battery bank I performed a load test with batteries only using the the Microwave.

AND

I saw no improvement on voltage drop.

I do however think that the upgrade from 2awg to 2/0 is a better fit for the 4/0 main 5' Inverter to battery cable.

So far I have to say my 500 watt 2000 watt Inverter solar system has performed as I had hoped. I may add two more 100 watt panels in the future with a bigger charge controller.

Plus I will be changing the batteries next year.

I'm glad I up-sized the 10ga panel wires to 8ga when I did the install.

A 85 Ah battery sure sounds like a RV/Marine so called deep cycle battery. 4 6V GC2 batteries will provide about 440 Ah. I suspect your batteries are part of the problem.

Are your 4 parallel batteries wired in a balanced configuration? If not then some batteres supply more amps than others and take more charge amps than others.

1L243 wrote:
I built my battery bank four 12 volt deep cycle batteries with 340 amp hours.

Any thoughts.

Assuming you are using 4 FLA "group 27" RV/marine batteries which would be about 79-80 Ahrs each.

The voltage sag you are seeing with that kind of amperage and time is normal with those batteries and should be expected and should be well within the operating parameter of the inverter without shut down.

RV/marine batteries are a hybrid deep discharge/staring battery and as such they do neither function well. They do not do deep discharge well nor do they work all that great as a starting battery.

The battery voltage should and will "bounce back" some once the heavy load is removed and that is normal and to be expected.

You are asking a lot for the small amount of capacity and the type of batteries you have. Your dealing with smaller, thinner plates in RV/marine batteries. Thinner plates in theory should give you a high amperage draw in a very short burst like for a starter, but for your specific use it is far longer than what a starter runs and they will fall flat on you.

For greater capacity in FLA, 6V GC2 batteries would be a better choice but with loads over 100A for 2-5 minutes at a clip, 2 pairs may be needed.

Alternately one pair of 6V GC2 batteries in AGM form due to lower internal resistance loss might actually work better for what you want. Although the AGM versions typically cost $10-$20 each more and will have a few less Ahr capacity. I don't typically push AGM, they do have a few shortcomings and habits I don't like, but one big plus for them is the voltage drop under high amperage loads will be a bit less than FLA.

I don't think the issue is your interconnects but rather the battery choice.. Much better batteries out there than RV/Marine combo batteries.

1L243 wrote:
I don't think Lithium batteries will be in my budget in the near future BUT I do plan to go with four 6 volt batteries next year which will get me to 440 amp hours.

4 six volt batteries will have 1/2 the number of cells as 4 twelve volt batteries. This will increase voltage drop under load, as each cell has to work twice as hard.

Do make sure the battery bank is wired in a balanced manner.

My next bank will be SiO2 batteries. I can squeeze in 7 100 amp-hour batteries. They can do 9C for 5 seconds, and 4C continuously.

I have one that I'm torture testing on my generator. It spins the starter far faster than the AGM it replaced.

1L243 wrote:
I built my battery bank four 12 volt deep cycle batteries with 340 amp hours. At the time I had not added the 500 watts of solar and a 2000 watt inverter that I now have. vel before the event.


Any thoughts.

Batteries are rated at a draw of 25 amp-hours each. So with 4--expect not much voltage drop until reaching 100 amps.

Inverting eats up 12% of the energy, so at 88 amps voltage drop may start showing up.

My microwave draws 166 amps. At the moment the battery bank is 556 amp-hours (but is 4 years old). I get pretty much the same results as you. 11.4 volts under load.

1L243 wrote:
I built my battery bank four 12 volt deep cycle batteries with 340 amp hours. At the time I had not added the 500 watts of solar and a 2000 watt inverter that I now have.

At the time I used 2 AWG cable between the the batteries which are wired in Parallel. The 2 AWG was a upgrade from the 4 AWG battery cables that it came with.

I'm using 4/0 cable from the inverter to the batteries. I occasionally have a high demand on the battery bank by running the microwave or a drip coffee maker. These events only last for 2 to 5 minutes at a time. When this occurs the battery voltage will drop suddenly as low as 11.8 volts due to the high demand. As soon as the demand is over the voltage recovers almost immediately to it's pre voltage level before the event.

I decided to beef up the battery cables that link the four batteries together with 2/0 cable. I could not find anything that really set the parameters for battery cable size.

I have not had a chance to try it yet but I am curious to see by
increasing the battery cable size to 2/0 if the voltage drop during a high amperage discharge will improve.

Any thoughts.

basicly you have to look at the maximum amprage tht cn be drawn on that line and the distance over what it has to carry that amprage and look up the size on a chart. so a 2000 watt inverter that has say a 3000 watt surge capacity can have a maximum of 250 amps plus a tiny bit for the efficency but that will only ever be momentairly at this level most of the time it will run much less. so at 250 amps on stranded wire for less than 4 feet distance you need 4ga, 4 to 10 feet 2ga, 10 to 19 feet 1/0 ga and 19 to 22 feet 2/0 ga. I like to go one or two sizes larger as a saftey vs. cost factor .

for the battery conection wire I usaly go with the inverter sizing as thats the largest draw that will go through them. the larger you go the less voltage drop but thats only to a point. the only time it will help is if your cables were sized on the small side to start with, but there also is a point of just not being worth it, you could upgrade the cable from a 2/0 to a 4/0 and spend twice the money and only get a 0.001V increase if the 2/0 was already more than good enough, so there also you have to look at charts and there are some that show voltage drop vs wire size also. you might get people thorwing wire sizes at you or just saying do everything in 4/0. the reality is 4/0 isn't cheep and you don't need it for most situations. I used it for the 2 feet between my inverter and bateries like you did but I wouldn't have used it if I had to go 10 feet as I probably wouldn't have been able to aford to do a long run of it, and it gets much harder to work with for pulling and such, but even at 10 feet it is overkill so thats not an issue. but having said all that, if you have it, why not. some people just seam to be able to come across larger wire sizes for free or a screeming deal, if your one of them all the power to you.

just remember building a larger battery bank doesnt change the load at all, just lets you run it longer so it won't affect your wire size requirments adding two more 6V batteries. what would affect it is if you decided to put in a 4000 watt inverter or any apliance that will draw more than 250 amps dc.

Steve

Depending on how your batteries are wired, each battery supplies 1/4 of the amps delivered to your inverter.

1L243 wrote:
BUT I do plan to go with four 6 volt batteries next year which will get me to 440 amp hours.

That may help. As you may have heard me say before, I recommend going to a higher voltage with large inverters. Those amperage draws at 12v are staggering.

Interesting discussion. The WindyNation wire chart I’ve used doesn’t go to those numbers.

“I don't think Lithium batteries will be in my budget in the near future…”. Don’t be so sure. Prices are dropping while wets are increasing.

This is fairly typical for lead acid batteries and not an indication of a problem.

If you've ever ran down the battery trying to start a troublesome engine (to the point it won't turn over), you will often find that if you let it sit for a few minutes, it will be able to turn over the engine.

As far as sizing cables, look up a voltage drop table.
- 2000w draw at 12v will be drawing about 170amps. Then take the length of the cables from the battery to the inverter.
- The solar panels at 500w can be done similarly.

Then you just have to decide how much voltage drop you are willing to accept. If the cables don't get crazy thick, I would shoot for 3% voltage drop or less but 5% would typically be acceptable.

Ideally, having a short distance from batteries to inverter is helpful. The solar panels are more difficult as they are likely on the roof but even there, don't bring the cables down at the rear of the unit if the battery bank is at the front as it will result in a lot of length which increases voltage drop.

I'm pretty sure I got the Inverter to Battery Bank covered a 5 foot run of 4/0 cable from the Inverter.

I will get it finished up tomorrow and see if there is any change but I suspect your right about the Sag.

I don't think Lithium batteries will be in my budget in the near future BUT I do plan to go with four 6 volt batteries next year which will get me to 440 amp hours.