How would you wire six 12V 110Ah batteries for a 24V system?

softdown · August 2016

A friend has recently purchased six 110Ah 12V batteries. I want to help him maximize this string.....on a 24V system. With batteries, it becomes important to do it right.

mike95490 · August 2016

Your friend is off to a bad start. That can only end up as 3 parallel strings of 2 series batts. Best advice is wire on the diagonal

Image: https://us.v-cdn.net/6024911/uploads/editor/dp/5295cnmysk6b.jpg

BB. · August 2016

Here is the Smartgauge web page on parallel battery connections:

http://www.smartgauge.co.uk/batt_con.html

With three or more parallel strings--I would suggest that they put a circuit breaker/fuse per parallel connection.

Next time--I would suggest 2x 6 volt @ ~220 AH x 2 parallel strings (if available/cost effective).

-Bill

jonr · August 2016

The smartgauge site makes the huge error of assuming that the internal resistance of each battery is equal. Use a clamp on ammeter, measure the balance and adjust it with cabling and/or battery position.

softdown · August 2016

BB. said:

Here is the Smartgauge web page on parallel battery connections:

http://www.smartgauge.co.uk/batt_con.html

With three or more parallel strings--I would suggest that they put a circuit breaker/fuse per parallel connection.

Next time--I would suggest 2x 6 volt @ ~220 AH x 2 parallel strings next time (if available/cost effective).

-Bill

Nice link...thanks

They are making plenty of mistakes. I'm not exactly sure what to do. I don't think .01% of the population is qualified to do their own solar without a lot of help.

Marc Kurth · August 2016

jonr said:

The smartgauge site makes the huge error of assuming that the internal resistance of each battery is equal. Use a clamp on ammeter, measure the balance and adjust it with cabling and/or battery position.

@jonr:

I tend to see that individual battery resistance can be a moving target and will change over time. Concorde says that the "nominal" readings can vary by 35% between individual batteries. I am a believer in checking individual batteries in parallel, and rotating positions or equalizing as needed.

Same thing on a standard single 48v string with 2v cells, you can quickly see imbalances by checking cell voltage/current while charging and/or under load. It becomes painfully obvious if a particular cell is developing a problem.

jonr · August 2016

> "nominal" readings can vary by 35% between individual batteries.

@Marc: And this 35% may exceed the entire path issue that smartguage is talking about. "Wire it like this and you are all set" is bad advice. I agree with you, check balance (parallel and serial) periodically, not just once.

I think we are headed towards lithium battery banks that include everything to manage themselves.

Marc Kurth · August 2016

Yep, battery technology is due for any update - and it will happen! Just a matter of when.......

In the mean time, applying a battery management system to existing battery banks could really go a long way to improve performance. We all see it as standard and necessary for lithium, but never think twice about it for lead acid batteries including gel, FLA and AGM.

I have a feeling that the next really big step won't be lithium based, but lithium formulations will provide incremental improvements for now.

Marc

softdown · August 2016

Wonder if it would be best to draw from the middle of three 24V banks....or equally from the perimeter banks. Mathematically....I don't see too much difference.

mike95490 · August 2016

jonr said:

The smartgauge site makes the huge error of assuming that the internal resistance of each battery is equal. Use a clamp on ammeter, measure the balance and adjust it with cabling and/or battery position.

Assuming they are equal, is the best choice. Let me know what the clip on meter costs, that can tell the difference of a few mA when the total current is 15A.
The telcom dude at AT&T can do this, but 90% of solar installers won't be able to perform this sort of test. Then you need the milliohm meter to test each battery connection .........

jonr · August 2016

$40 and we are talking talking about amps, not mA of difference.

Marc Kurth · August 2016

@mike95490

I may be missing your point, but most of us deal with larger numbers than that on 12v to 48v battery banks. Is a 15 amp load what AT&T techs deal with? I have no idea.

But I do know that most of us see larger numbers going both directions - to and from the battery bank. We can certainly see large differences between strings and individual cells (when using 2v batteries) with normal current flow - and that is good information. Then add voltage measurement comparisons while charging and discharging, and we have tools to help understand the balance of their batteries

I am not challenging your comment, so help me understand your point.

Are you saying that we can't see any problems without precision instrumentation?
Or that you feel that it doesn't matter anyway?

Marc

mike95490 · August 2016

I feel that without good instruments, you cannot tell which string is the slacker. The cheapest DC clip-on meter I've seen is the $60 (when on sale) Sears meter. If you have a new bank with same lot factory batteries, there won't be much difference, and you will need a very fancy meter to see what the imbalance is.
If you have 6 parallel junkyard batteries, I expect the $60 Sears meter to be able to show the difference, if it's more that 3 or 4 amps. Most of the clip on meters are not much more sensitive than that on their 40A range. Heck, even the classic line powered HP DC ampmeter had different heads for different ranges, and a degausser that had to be used for each use, and it was only about 4% accurate. So, I don't know of any affordable DC clip on ampmeter that is precise enough to reliably point out the 25A string in a batch of 20A strings. I'd love to know of the precise $40 one you use, that would be a game changer.
Then there is the hassle of fixing it, when you find the string hogging the power, or the slacker, how do you fix it? Start swapping batteries and maybe it gets better?

vtmaps · August 2016

mike95490 said:
Then there is the hassle of fixing it, when you find the string hogging the power, or the slacker, how do you fix it? Start swapping batteries and maybe it gets better?

This point is not often made, but is central to the discussion of parallel batteries.

When you find one battery is different from all the other batteries in the bank, there are two possibilities.... the battery itself is intrinsically different, or there is an external cause such as poor wiring or non-uniform temperature in the battery box.

If the battery is intrinsically different, you can't fix it. Rotating the batteries doesn't fix anything... If a soft tire on my car is a problem, then rotating my tires is not the solution.

--vtMaps

Marc Kurth · August 2016

I agree! It is never a good idea to use a flat tire, or a problematic battery

With typical series-parallel battery banks, batteries in one part of the bank impact the others. If one string is carrying less load, then the others are carrying more. If the reason is not a battery out of spec, then moving batteries actually can help to "spread the damage" when dealing with an imbalance caused by installation variables.

Of course it doesn't "fix" the problem, but it can extend the useful life of a battery bank that was installed with less than optimum layout/conditions. I sometimes get called in to consult after the system starts performing poorly, instead before it was purchased and installed.

Generally, I am referring to very small differences that can have a cumulative effect over several years. I commonly see 4-6 parallel strings running for several years without touching them - because of a good installation.

Is it best to use a single string? Yes, of course. In fact, I strongly prefer using 2v batteries instead of 6v batteries for 48v systems that are critical.

My point is that in the field, I very often see slightly uneven resistance in multi-parallel battery banks. I do not mean to insult anyone, but experience has taught me that the type of customer who installed a big bunch of 12v batteries in series-parallel is not likely to do a super job of creating equal resistance..........

Respectfully,
Marc

How would you wire six 12V 110Ah batteries for a 24V system?

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