First read this https://diysolarforum.com/threads/calculation-of-parallel-string-battery-currents.37937/ until at least post #61.
This post will show the theoretical calculated battery currents in parallel strings of 5, 6, 7 and 8 batteries with load currents of 100 amps times the number of batteries in the string. That is, load current of 500 amps for a string of 5 batteries, 600 amps for a string of 6, etc. For smaller load currents simply scale down the calculated battery currents in proportion.
The calculations are made assuming battery internal resistance of 1 milliohm, link resistance of 50 microohms. It is also assumed that the batteries are nominally identical--same manufacturer, same model numbers, same date of manufacture, etc.
The Image shows a string of 6, but what follows applies to any string size.
A well known method of connection is the diagonal connection which purports to give (somewhat) balanced currents. What is usually meant by "diagonal connection" is shown by the red and black cable in the image. I call this "total diagonal". Moving the connection of the load from the very corner of the diagonal to a place on the busbar that is more toward the middle of the string can improve the balance. Consider the distance on the busbar between the negative terminal of battery 1 and the negative terminal of battery 2 to be "unit distance". Then when I say connect the load to a point .60 of the way from battery 1 to battery 2 I mean 60% of the unit distance from the upper right corner of the string toward battery 2. In the chart I use the descriptive phrase "diagonal minus .60" for that situation.
More about this here: https://diysolarforum.com/threads/c...ing-battery-currents.37937/page-2#post-488325
The description "Diagonal minus 1.34" means go past the negative terminal of battery 2 and then 34% of the way to battery 3, and similarly for the other descriptions.
As examples, in the image the blue connections are "Diagonal minus 1", green connections are "Diagonal minus 1.5", orange connections are "Diagonal minus 2".
This chart shows the results of the mathematical calculations of the theoretical currents.
Ideally the current in each battery should be exactly 100 amps, but we see that the calculated theoretical currents for the total diagonal connections are far from balanced, and the amount of unbalance becomes worse as the string has more batteries.
There is no simple connection of this kind for which the theoretical balance is perfect as is the case with strings of 3 or 4 batteries. But for each string size of 5 or more there is a connection of the load for which the amount of unbalance is minimized. The currents for this connection are shown between the characters < >, and the connection description is shown in the first row of the chart.
The worst case shown is the 8 battery string. Look how bad the balance is for the total diagonal connection--and how much better it is for a simple change in how the load connects to the batteries!
This post will show the theoretical calculated battery currents in parallel strings of 5, 6, 7 and 8 batteries with load currents of 100 amps times the number of batteries in the string. That is, load current of 500 amps for a string of 5 batteries, 600 amps for a string of 6, etc. For smaller load currents simply scale down the calculated battery currents in proportion.
The calculations are made assuming battery internal resistance of 1 milliohm, link resistance of 50 microohms. It is also assumed that the batteries are nominally identical--same manufacturer, same model numbers, same date of manufacture, etc.
The Image shows a string of 6, but what follows applies to any string size.
A well known method of connection is the diagonal connection which purports to give (somewhat) balanced currents. What is usually meant by "diagonal connection" is shown by the red and black cable in the image. I call this "total diagonal". Moving the connection of the load from the very corner of the diagonal to a place on the busbar that is more toward the middle of the string can improve the balance. Consider the distance on the busbar between the negative terminal of battery 1 and the negative terminal of battery 2 to be "unit distance". Then when I say connect the load to a point .60 of the way from battery 1 to battery 2 I mean 60% of the unit distance from the upper right corner of the string toward battery 2. In the chart I use the descriptive phrase "diagonal minus .60" for that situation.
More about this here: https://diysolarforum.com/threads/c...ing-battery-currents.37937/page-2#post-488325
The description "Diagonal minus 1.34" means go past the negative terminal of battery 2 and then 34% of the way to battery 3, and similarly for the other descriptions.
As examples, in the image the blue connections are "Diagonal minus 1", green connections are "Diagonal minus 1.5", orange connections are "Diagonal minus 2".
This chart shows the results of the mathematical calculations of the theoretical currents.
Ideally the current in each battery should be exactly 100 amps, but we see that the calculated theoretical currents for the total diagonal connections are far from balanced, and the amount of unbalance becomes worse as the string has more batteries.
There is no simple connection of this kind for which the theoretical balance is perfect as is the case with strings of 3 or 4 batteries. But for each string size of 5 or more there is a connection of the load for which the amount of unbalance is minimized. The currents for this connection are shown between the characters < >, and the connection description is shown in the first row of the chart.
The worst case shown is the 8 battery string. Look how bad the balance is for the total diagonal connection--and how much better it is for a simple change in how the load connects to the batteries!
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