Calculation of parallel string battery currents

[OffGridForGood]

Get the configuration as balanced as you can and call it good. Don't let perfect be the enemy of good.

Good advice,
honestly I am mostly interested in what I can learn on this topic, not too concerned (not at current loading at least) about how my rack in performing. Lets put it this way: I am interested to learn what is actually happening with the loads between these batteries, how it could be improved and if it is as simple as moving the main bus feed cable over a few inches, well why not? LOL.

 

[Zwy]

Maybe if the two different Ah sets of batteries were in independent banks?

It's not helpful to say this, but I wouldn't be putting batteries with such disparate Ah capacities together. But the batteries will absorb as many amps as they can for their rating.

I have 280Ah and 304Ah (which test min 310Ah) all tied together on the busbars. I used the 4 battery diagram @Solarod posted where the positive cable is halfway between 1 & 2 and negative is halfway between 3 & 4 and it works fine, batteries stay in balance. I do see the temp is slightly lower on bottom banks but doesn't seem to have any effect.

 

[OffGridForGood]

For the last two weeks I have been playing with the five batteries to see what changes would improve the setup. I installed a short bus to allow connecting the two 100Ah racks to this new bus bar, then from the centre of the new bus I ran a 1AWG wire to the main bus (one pos and one neg) and let this set up run. Then I moved the main pos and main neg from being centred on the Main bus, to the configuration Solaroid shows for the four battery set up - since the two 100Ah batteries acting like a single 200Ah rack, now I can use the four battery set up. This seems to work much better.

 

[OffGridForGood]

Got some new numbers - under a steady load, now the two 100Ah rack batteries share one connection to the main bus (act like a single 200A rack) while the other three racks are 280Ah each. The main connections to the inverters are set as Solarod indicated (about half way between rack 1-2 on Pos, and between 3-4 for Neg)
2-100Ah racks sharing connection to bus 12.3A
Rack 3 13.7A
Rack 4 13.2A
Rack 5 14.1A
I feel this is (as HRTKD said) good enough, no need for perfection.
Although, now I wonder about a variable resister on each connection to Pos...to dial in...okay never mind good enough.

 

[offgridfarmgineer]

Hi @Solarod ,

Just came across this thread. Are you still convinced that wiring 6 EG4s in in an EG4 rack w their busbars should be configured like you show in post #57 (looks good to me)? I have two racks (12 batteries total) I'm wiring and plan to use 4/0 to go to current connected busbars from the rack busbars, and then switch to 2/0 to each of my two 6500 inverters.

Not sure if I'm using the 6 AWG leads already provided to connect each battery to the busbar or not but should be fine. I can't pull more than 13 kW split phase out of them anyway. If I assume 52 V, then that is 250 A over 12 batteries or only about 21A each ASSUMING all evenly contributing.

 

[offgridfarmgineer]

Anyone else have thoughts on my post. Appreciate the excellent info here.

 

[OffGridForGood]

Anyone else have thoughts on my post. Appreciate the excellent info here.

I haven't seen Solarod posting since maybe March,
I assume your 12 100Ah packs are in two racks of 6 each? and if you have the racks, these will have bus bars for connection of each pack.
From his earlier comments, there will be no 'ideal' way to connect your main 4/0 feeders, close best option may be between 1/2 on pos and between 5/6 on neg (or vise versa) but this would be better with a four pack set up. There will be some imbalance with six batteries.
I would just try it and then monitor the packs for a while, after some cycling up and down the SOC range. See how it does.
I suppose if it doesn't work well, you can change your 12 batteries to 3 sets of 4.
In my own set up, where I have the ackward use of 2 100Ah packs plus 3 280 Ah packs, connecting the two 100's up as a pair (as if a 200 Ah pack) has worked out pretty well, and now that the bus bars have only four connections, I was able to shift the main pos and main neg to the 1/3 2/3 locations Solarod noted for a four battery set up on bus bars and this has been far better than where I started.

 

[Nobodybusiness]

I've done some calculations on a 5 battery parallel string and here are my results.

I'm using a value of 1 milliohm for the battery IR, and .1 milliohm for the link resistances which I think is more representative of what one gets with busbars rather than cable links. The load current is 100 amps. I'll reference this image:

The red and black load connections are the standard diagonal connection, and that connection gives me the following theoretical battery currents in amps:

Example 26
23.5
18.2
16.6
18.2
23.5

Some have suggested moving the load connection toward the middle of the string. If the black load connection is moved to the point on the busbar adjacent to the negative terminal of battery 2, and the red load connection is moved to the point on the busbar adjacent to positive terminal of battery 4, we get these theoretical currents in amps:

Example 27
18.3
21.8
19.8
21.8
18.3

This is a substantial improvement. In some of the earlier hookups it was possible to get a perfect balance by connecting the load to the right place on the busbars between the last and next to last batteries in the string. Of course, I have tried to do that for this 5 battery string by varying the connection point. It isn't possible to get perfect balance, but it is possible to get improvement. I had to consider how to measure the improvement, and what seems reasonable is to get the least variation in the battery currents. I had my calculations include the standard deviation of the battery currents, and tried all the connections between the end batteries and the next-to-the-end batteries in 5% increments. For this 5 battery string, the connection which is 66% of the way from the last battery to the next battery inward gives the minimum variance in the battery currents. This would be a connection like the yellow/blue connection shown in the image above, at the 66% point. Here is the result of the 66% connection theoretical battery currents in amps:

Example 28
20.0
20.6
18.7
20.6
20.0

I haven't given up on the possibility of a perfect balance for this string, but no joy so far.

I guess this as good as we can get on the SOK racks.

Will try this at some point and see how it goes.

I can tell you your calculations are correct just based upon what I see with battery draw on the other different scenarios listed.

 

[offgridfarmgineer]

I haven't seen Solarod posting since maybe March,
I assume your 12 100Ah packs are in two racks of 6 each? and if you have the racks, these will have bus bars for connection of each pack.
From his earlier comments, there will be no 'ideal' way to connect your main 4/0 feeders, close best option may be between 1/2 on pos and between 5/6 on neg (or vise versa) but this would be better with a four pack set up. There will be some imbalance with six batteries.
I would just try it and then monitor the packs for a while, after some cycling up and down the SOC range. See how it does.
I suppose if it doesn't work well, you can change your 12 batteries to 3 sets of 4.
In my own set up, where I have the ackward use of 2 100Ah packs plus 3 280 Ah packs, connecting the two 100's up as a pair (as if a 200 Ah pack) has worked out pretty well, and now that the bus bars have only four connections, I was able to shift the main pos and main neg to the 1/3 2/3 locations Solarod noted for a four battery set up on bus bars and this has been far better than where I started.

Thanks for replying, yes two racks of 6. Looking back at post 57 he does cover banks of 6 and I plan to do it that way by drilling my own holes in the busbars for precise balancing. Would be interesting to see @Will Prowse do a geeky video on this subject. Tricky part may be controlling variables for a good test.