Best way to balance charge and load hookups

8n-bob · February 2015

I searched and did not find what I am looking for so I'll just ask knowing that it probably has been answered before.

6 golf cart batt's, series/paralleled for 12V 660 A/Hr
2 235 watt PV's through a MPPT 45 amp charge controller
1 100 watt PV through a MPPT 15 amp charge controller The Charge controllers are in parallel
2000 watt full sine wave inverter

What I am looking for is how to hook up the charge and load to the battery bank.
I have seen the balancing for 4 batt's but not for 6.

Thanks
BobO.

BB. · February 2015

Your 6 GC batteries are actually 2 in series (each jumper cable should be the same for all three sets). Each is now a 12 volt module.

You just have 3 parallel strings to connect together.

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

I like Method 3 in the above link the best... Note each of the cables from Battery to Common Point should be the same length. I like this because you can put a fuse on each series string (to protect wiring against shorts). And you can use a DC Current Clamp Meter on each "leg" so you can measure the charge/discharge current for each string--Make sure all is working well. If you have a bad connection/cell/battery/etc... You will quickly see it with the DC Current Clamp Meter. Also, you can measure the voltage of each cell/battery (high or low voltage on one cell/battery indicates further investigation may be needed to figure out why the one or two batteries don't match the rest).

Lastly, each cable does not need to "match" the length of its mirror image... The idea is each series string of batteries has the same "total" wire length--And the same total resistance for each series string. This helps prevent current miss-match between strings (higher resistance, lower current flow in that string).

-Bill

8n-bob · February 2015

Hey Bill:

Thanks for the reply.
I understand that I have three parallel strings to put together. I thought that my saying that the 6 batts were series/parallel in 12volt explained that. :-)
I guess that makes the most sense about resistance and lengths. Since all are the same length on the positive side and that all on the negitive side are the same size the resistance in each parallel circuit will be the same. I guess I could not see the forest through the trees.

A follow up question for me is

Do I hook up the charge controller to the same place as the load? I have seen so many different takes on crossing the load with the charge. Like #2 on the diagram with the charge controller hooked up opposite the load. I hope you are understanding what I mean.

Thanks again for the reply.
BobO.

BB. · February 2015

Hi Bob,

When you said:

8n-bob wrote: »

I have seen the balancing for 4 batt's but not for 6.

I was not sure if you saw that 2x 6 volts batteries in series = 1x battery in the drawings.

No problem--I just try to be really clear in my answers--And sometimes they can be redundant (or answering questions not asked

).

Yes, in some of the battery wiring diagrams (#2), there are two "equal cable length points"... So you can put the loads on one, charger on the other. Or just put both loads and charger(s) on the same point (and the other is unused). For a correctly wired battery bank, really does not make a difference.

Personally, I do not like #4 -- Way too complicated. I like #3 because you can measure / fuse each string--Although, you can also fuse/measure on the cable that connects the two 6 volt batteries together in your case.

-Bill

8n-bob · February 2015

Sounds good. I may have enough to wire that way with out purchasing any additional stuff. I'll just make up some lugs and just insulate the positive side real well.

I never thought of fusing the "series" tie between 2 6 volts. a good place to put them just to keep them out of the way. Probably something like a 100 amp fuse would be just about right.

I see what I wrote and that is a big oops. I should have said 4 batts not 3 My Bad. I was leaning toward number 4 but with only "3" I did not know how to balance since I thought you "had" to keep the charge and load not on the "same" terminal. Lesson learned.

Thanks again
BobO.

jonr · February 2015

I recommend thinking about the following quote and why it is true. Hint: there is more to balance than just equal cable lengths.

A cross-tied connection may be added to parallel series strings in a battery bank. These connections may assist in balance of charge and improve battery performance. Series connections are made between each battery in a string as well as connections from positives to positives and negatives to negatives in adjacent strings.

http://rollsbattery.com/public/docs/user_manual/Rolls_Battery_Manual.pdf

BB. · February 2015

I don't really like it... If the battery bank is running correctly, there should be zero current flowing though the cross ties.

if there is a problem with a bad connection or bad cell--It can "hid a problem" until you have done damage... I.e., you have two paralleled cells, and one goes dead. The second cell carries current (and voltage) for the rest of the battery bank--When the battery bank goes to 50% state of charge--The one "good battery"/cell, is now taken to Zero and probably permanently damaged. At the point that the remaining good cell (or good connection to good cell) finally "tanks"--The battery/cell is damaged.

if you don't parallel connect--If there is a bad connection, the one parallel string is disconnected and you will notice a fairly steep decline in battery bank voltage (1/2 capacity). Going through with a volt meter (and charging/discharging current), you will quickly find the open connection/bad cell.

If you go through with a DC current clamp meter--You have to not only check string current, you have to check each cross link to make sure there is no current flowing.

I just don't like redundant connections--There are just too many ways for things to fail and "blind" the owner to the failure(s). Also makes it very difficult to "fuse" in a sane manner without having a ton of "extra fuses" because of the multiple possible fault current paths (dropped tools, shorted cells, open series connection, etc.).

I know that paralleling cells then a single series connection is common for LiFePO4 cells--My belief is that companies do this so they only need one Voltage monitor per group of cells. Vs a full set of VMs for each parallel string. Saves hardware costs and a mess of rat's nest wiring with all of the per cell sense leads.

-Bill

jonr · February 2015

If the battery bank is running correctly, there should be zero current flowing though the cross ties.

It would be more correct to say "if all of the batteries are perfectly matched/balanced/equal internal resistance (and they never are), there will be zero current flowing though the cross ties.".

Strings (parallel, series or combinations) will always have downsides. As Roll's says, cross ties may improve balance at the cost of additional failure modes.

IMO, measuring individual batteries and organizing (which battery goes where) the strings (and/or adjusting cables lengths) for best match would provide better balance without the above downsides. I have no idea how much such better balance would be worth in terms of $ saved vs the minimal effort.

Best way to balance charge and load hookups

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