Battery Bank Confusion

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Lostranger
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Battery Bank Confusion

Post by Lostranger »

I need to hear from someone who actually knows about wiring battery banks. I've read too much contradictory info on the internet, and I need the straight skinny.

It's taken almost two years, but I now have most of our solar system in place. We've been living off-grid since last February and loving it. I have four Samsung 255w solar panels rated at 24v. I have them wired series/parallel to send 48v to my Morningstar 60 amp MPPT controller. So far, so good.

In my Gillig low floor conversion, "underfloor" space is at a premium. I chose AGM batteries so that offgassing is less of a problem. I currently have six group-27 12v batteries arranged in two rows of three batteries each. They are wired for 24v with each adjoining pair in series and that pair paralleled to the next and then the third. I had planned to add two more batteries to this arrangement for a total of eight. Each battery is rated at 92 amp hours or a planned total of 368 amp hours with 8 batteries in 24v mode.

The problem is that now I'm far enough along to realize that I actually have room for 12 batteries. Three rows of four each. What I read about wiring battery banks confuses me. Some sites say that using that many batteries is wrong and that the ones in the middle will not properly charge properly. Is that true? Is there some way to wire 12, 12v batteries for 24v that will allow them to charge and discharge properly, or would the additional batteries be a waste of money. We've not yet bought our primary load which will be a Sundanzer chest-type refrigerator running on 24v, but I can tell that we need more battery capacity to keep it happy. It seems silly to not fill my available space with more batteries, but it also seems silly to buy expensive batteries that will not be used efficiently.

Part of my confusion is the concept of "strings". Is my current bank of six batteries considered three strings? Some of the sites I read say to never wire more than three strings, but they do not adequately define what constitutes a string.

I'm in the process of upgrading my wiring to 2/0 fine-strand copper.

Please help. Sharkey, are you still on this channel?
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stuartcnz
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Re: Battery Bank Confusion

Post by stuartcnz »

I see no one has replied yet. We lost a lot of knowledge when Sharkey went walkabout.
Here is the page on his system. http://web.archive.org/web/201211020437 ... /solar.htm Don't know if it has anything for your situation, but possibly worth a read anyway.
roach711
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Re: Battery Bank Confusion

Post by roach711 »

Lets say you've got 6 12v batteries connected together in parallel, in a row with the positive and negative leads coming off the battery on the far right of the string. Due to cable voltage drop the right hand battery will be charged/discharged the most and the battery on the far left will be charged/discharged the least. This will cause the right hand battery to be over stressed and the left hand battery to be under charged with the others in the string somewhere in between. Bottom line, your battery bank will fail prematurely. If you had the positive lead coming off the right hand battery and the negative lead coming off the left hand battery the end batteries would be over stressed and the center batteries under charged.

A better way is to have positive and negative bus bars with all batteries connected directly to them with equal length cables. Your alternator, solar, and converter all attach to the bus bars and the negative cable to the frame attaches to the negative bus bar. Now all batteries get the same charge/discharge cycles and the whole battery bank lasts a lot longer.

With your 24 volt system you basically make several 24 volt, two battery banks and cable them into the bus bars so all battery pairs get equal charging and discharging. You'll buy more cable up front but will ultimately save on battery replacements later. Two rows of six batteries would be pretty straight forward to cable but you'll have to get creative to get three rows of four cabled up. Just be sure all cables between battery pairs and from the batteries to the bars are equal length.

Ideally, for best battery life you start out with batteries from the same production lot and treat them exactly the same until you get one or more battery failures, then you replace them all at once and start over. Ideally.
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Lostranger
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Re: Battery Bank Confusion

Post by Lostranger »

Thanks, Roach, for your reply. Sharkey took the time to send me a long and detailed email. I confess, though, that I'm still confused. I see people using the word "string" in apparently contradictory ways. I had finally come to the conclusion that a "string'' is the smallest subset of a battery bank which, when connected in series, equals system voltage. In my situation of using 12v batteries and a 24v system, a "string" would consist of two batteries. Sharkey says to never use more than 3 "strings", and that two is better. If that's the case, and if my understanding of "string" is correct, then I'm stuck with a maximum of 6 batteries. That's the number I already have, and with the size batteries I'm using, it's not enough.

Now roach711 tells me that six 12v batteries connected in parallel is one string. That arrangement does me no good since I need 24v. This is not the first time I've read the suggestion to use "bus bars", but I've never seen any details of how this arrangement would work. Is a "bus bar" an exposed solid copper bar that runs the length of the battery bank? If so, what is the ideal width and thickness? How should it be mounted? How does one protect the positive side from accidental shorts? How can I pull off the bank robbery that would be necessary to pay for that much copper?

Sharkey and roach711 agree that all batteries should be purchased at the same time from the same manufacture lot. I can see the advantage, but in our case, that was not an option. I read about others adding to their battery bank at later times. Rob Gray comes to mind. Maybe they're not telling me that their entire bank died two weeks after the addition. I rather think that the new batteries soon come to mirror the state of health of the older batteries. In my case, that is an acceptable outcome since my existing batteries are only six months old and still in excellent condition. They are never deep cycled and they are fully charged every day.

Given the space that I have available and the fact that I do not want to use flooded batteries, I've about decided to start over with four 8D AGM batteries. That would give me 490 amp hours at 24v. I can use the six group 27s in other ways. For instance, I'm planning to add a smaller 12v system to the bus powered by one panel just to start the generator and operate a few 12 accessories such as our cable modem and broadband router. That would keep me from having to use the 24v to 12v converter I bought. It appears to be horribly inefficient. Two of the group 27s would be good in this application, and I could use the other four in various vehicles.

On the other hand, I hate to drop more than two grand on four new batteries when we're spending money hand-over-fist on the conversion, and we haven't yet bought our refrigerator. Yep, I'm still confused.

I'm aware that positive and negative leads to the inverter need to come from opposite corners of the bank.

If Sharkey does not object, I'll post his response to me in this thread. I'll wait until I hear from him. In the meanwhile, can someone point me to a definitive definition of the term "battery string"?

Thanks for all input.

Jim in NC
roach711
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Re: Battery Bank Confusion

Post by roach711 »

I'm not sure what constitutes string of batteries either. My understanding is that a bus bar could be as simple as a common attachment point for all battery cables. My system is much smaller than yours (two 6v trojans) but I made a bus bar out of heavy aluminum stock. If you went this route be sure to use dielectric grease on all connections to avoid corrosion. I'm no battery expert, but I would think you could attach as many 24v battery pairs as you want as long as the voltage drop issue was addressed.
Stealth Camper
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Re: Battery Bank Confusion

Post by Stealth Camper »

String is one of those unfortunate terms that can mean several things. It is too nebulous by far. You can have serial connected 'strings' and parallel and combinations of the two...I have heard them all referred to as 'strings'.

"Bus bar" is another one of those things that will make you crazy - there is no one size/shape, but the size must be determined for the application. I made a device to charge/discharge batteries that used bars that were (IIRC) about 1/2" x 3" x 6' long. The tricky part was connecting from the bar to the lead terminal on the battery. Top mount bolt on connectors would seem to be convenient - just run them across from battery to battery, but if it goes over the 'caps', cannot view inside. AGM - should be less of a problem...doesn't seem like access is an issue - haven't used one before, but looks like should never open..?

I guess if I were doing this now, I would determine the wire size needed, probably go one or two gauges higher, then take the cross sectional area of that wire and use the bus bar closest to that number that would 'fit' the terminals used. Stay with copper!! Roach, the aluminum is more than 50% more resistive - combined with the corrosion possibilities, copper replacement asap would be good - save some heat, move some more electrons more easily, etc. - 2.82 aluminum versus 1.68 copper.

Here are some example bus bars....
https://www.google.com/search?q=bus+bar ... 40&bih=708



This guy has some good information - particularly related to Dodge Ram diesels - but generic enough to help with connecting. There is a wire size section...I like a higher temperature wire (105 deg C or more) and more strands - like welding lead cable.

http://custombatterycables.com/applicat ... ge_ram.htm
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Re: Battery Bank Confusion

Post by roach711 »

Stealth Camper wrote: Stay with copper!! Roach, the aluminum is more than 50% more resistive - combined with the corrosion possibilities, copper replacement asap would be good - save some heat, move some more electrons more easily, etc. - 2.82 aluminum versus 1.68 copper.


Yeah, I know about the advantages of copper, but my max draw is pretty low (10 amps with everything on at once) and my bar is good for 5 times that. I added it in mostly to have a convenient attachment point for all those ground wires (I have a fiberglass body bus).

To save on copper, why not use a relatively short piece and attach two or three cables to each bolt?
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Re: Battery Bank Confusion

Post by Stealth Camper »

roach711 wrote:
Stealth Camper wrote: Stay with copper!! Roach, the aluminum is more than 50% more resistive - combined with the corrosion possibilities, copper replacement asap would be good - save some heat, move some more electrons more easily, etc. - 2.82 aluminum versus 1.68 copper.


Yeah, I know about the advantages of copper, but my max draw is pretty low (10 amps with everything on at once) and my bar is good for 5 times that. I added it in mostly to have a convenient attachment point for all those ground wires (I have a fiberglass body bus).

To save on copper, why not use a relatively short piece and attach two or three cables to each bolt?
To last - yes, you would like to have all connections happen in one place. There are issues with that related to current densities at the "choke point" where it all comes together, so many times, may be forced to spread it out some. Your 10 amps is not much current - aluminum is fine as an electrical conductor at that point, depending on the bolts used - I would not be using #8 screws - but if they are big enough, should be no problem at all. I would be using larger ring lugs like for 10 gauge wire, even though smaller would work - it's all about the current and I have a well developed sense of paranoia for electricity in general. I pretty much never use anything smaller than 12 awg for any kind of power use, even though there are many places where smaller is fine. (My vacuum cleaner uses 16 awg in its power cord - makes me crazy!)

I don't like the possible corrosion issues...copper doesn't seem to pit and erode as much to me...
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