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Parallel battery banks with different wire lenghts

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Oakyz

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Sep 13, 2022
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Hello everyone,

I have a question regarding my current battery setup:

16x LifePo4 140A
16x LifePo4 302A

First battery bank is in the same enclosure as the inverter with quite short runs to bus bars (home made copper)
Second battery bank is in another enclosure next to the first one but with longer cable runs.
The first battery bank uses 50sqm welding cables, on the second i have used 75sqm (to compensate for the distance (aprox 1.5m on both positive and negative).

Ive attached some pictures with my current setup and want to ask some opinions if this would affect in any way the performance or current sharing between both battery banks. (Hopefully it’s clear from the pictures)

Thanks
 

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I recommend measuring the difference with a clamp-on DC ammeter. Respective BMS may give the reading also. With the 140:302 capacity I would be looking for ~ 1:2 ratio for the draw under normal load conditions. Post the results. Hopefully within 10% of the expectation.
 
time2roll said:
I recommend measuring the difference with a clamp-on DC ammeter. Respective BMS may give the reading also. With the 140:302 capacity I would be looking for ~ 1:2 ratio for the draw under normal load conditions. Post the results. Hopefully within 10% of the expectation.
Click to expand...
To be fair it’s really hard to measure due to the fact that they are lifepo4, basically both banks discharge and charge at the same current apart from when they get close to the top or bottom capacity and that’s when the smaller battery delivers less A and same on charge side, the smaller one charges faster and then the current goes into the bigger bank.
Because of the different capacity and the smaller bank not being same quality as the other one it’s hard for me to decide if the cable length makes a difference in the way they charge or discharge or it’s down to the grade of batteries and capacity.
 
The smaller battery will work harder especially if the cycles are shallow. May require extending the smaller battery cables to match the larger battery or even more to get the discharges in a ratio similar to the capacity. Or maybe it is OK as is as long as you have this knowledge to adjust expectations.
 
The extra 25sqm on longer cables doesn’t make up for the extra length?
 
Your second battery will see more resistance than the first simply because you are passing through busbar sets & longer wires, while it won't be a huge difference it will still be there. Every fuse, breaker, lug & even switch will add to the overall resistance of the circuit collectively and from "end to end" it adds up. With any Lithium Based battery system this is more noticeable because the chemistries are MilliOhm & Millivolt sensitive.

Will it be a significant enough difference to create any issues ? Not really and you will likely not notice any because your capacities are so diverse.

If you are watching both BMS' during charge / discharge you will see that they take & output proportionately relative to their capacities and this is quite normal. Now there is a bit of a GOTCHA in this, the Solar Charger Controller and Inverter/Charger will have to be configured to handle a proper EndAmps/Tailcurrent values to properly transition from Absorb to Float.

Your 140A Pack has an EndAmp/TailCurrent of 7A while the 302A Pack has an EndAmps/TailCurrent of 15.1A. What this means is that when charging the 140A (only) in Absorb and Amps taken reaches 7A that pack is fully charged & saturated to the set voltage, the 302A Pack reaches that same state @ 15.1 A, then charging flips to Float which is Constant Voltage / Variable Current UNLESS the Absorb Timer runs out and drops the system to Float. Set your EndAmps/Tailcurrent to on your SCC & Inverter/Charger to 11A to start with (this is a compromise value) and if that does not fully charge to your satisfaction you can drop to the lowest @ 7A.

For LFP, generally 10-15 minutes per 100AH of capacity is a good value for the Absorb Timer, start at the lowest and see how the packs handle it, and increase if required. Start with a 45 Minute Timer up to 70 Minutes. This is for the collective 440AH. Look at your BMS' via the app to see how many amps they are taking to evaluate if the timing is proper.

Once transitioned to float the current will continue to drop until done and then the packs will go into "storage mode" no longer taking charge other than a trickle during balancing, JK's do go into a storage mode when they are fully saturated. Then float will provide whatever demand is there by the Inverter and load demands, provided you have enough solar power coming in, if not, the batteries will provide the additional energy and once the demand is reduced Float will try to replace what was used.

I run a 1293AH Split Bank which includes 1x105AH, 2x174AH, 3x280AH battery packs with JKBMS. My gear is Midnite Solar SCC & Samlex EVO Inverter/Charger which have programmable EndAmps.

Hope it helps, Good Luck.
Steve
 
Unfortunately my inverter (Mpp Solar 11Kw) has it’s own charging logic and can’t change the absorption time or tail current, but ive been using the smaller bank for 6 months and added the second bank 1 months ago, unfortunately about the same time the weather started to go south so didn’t had enough solar to do few conclusive tests (don’t really fancy drawing 20kw from the grid to play with the batteries at the moment).
 
Steve_S said:
Your second battery will see more resistance than the first simply because you are passing through busbar sets & longer wires, while it won't be a huge difference it will still be there. Every fuse, breaker, lug & even switch will add to the overall resistance of the circuit collectively and from "end to end" it adds up. With any Lithium Based battery system this is more noticeable because the chemistries are MilliOhm & Millivolt sensitive.

Will it be a significant enough difference to create any issues ? Not really and you will likely not notice any because your capacities are so diverse.

If you are watching both BMS' during charge / discharge you will see that they take & output proportionately relative to their capacities and this is quite normal. Now there is a bit of a GOTCHA in this, the Solar Charger Controller and Inverter/Charger will have to be configured to handle a proper EndAmps/Tailcurrent values to properly transition from Absorb to Float.

Your 140A Pack has an EndAmp/TailCurrent of 7A while the 302A Pack has an EndAmps/TailCurrent of 15.1A. What this means is that when charging the 140A (only) in Absorb and Amps taken reaches 7A that pack is fully charged & saturated to the set voltage, the 302A Pack reaches that same state @ 15.1 A, then charging flips to Float which is Constant Voltage / Variable Current UNLESS the Absorb Timer runs out and drops the system to Float. Set your EndAmps/Tailcurrent to on your SCC & Inverter/Charger to 11A to start with (this is a compromise value) and if that does not fully charge to your satisfaction you can drop to the lowest @ 7A.

For LFP, generally 10-15 minutes per 100AH of capacity is a good value for the Absorb Timer, start at the lowest and see how the packs handle it, and increase if required. Start with a 45 Minute Timer up to 70 Minutes. This is for the collective 440AH. Look at your BMS' via the app to see how many amps they are taking to evaluate if the timing is proper.

Once transitioned to float the current will continue to drop until done and then the packs will go into "storage mode" no longer taking charge other than a trickle during balancing, JK's do go into a storage mode when they are fully saturated. Then float will provide whatever demand is there by the Inverter and load demands, provided you have enough solar power coming in, if not, the batteries will provide the additional energy and once the demand is reduced Float will try to replace what was used.

I run a 1293AH Split Bank which includes 1x105AH, 2x174AH, 3x280AH battery packs with JKBMS. My gear is Midnite Solar SCC & Samlex EVO Inverter/Charger which have programmable EndAmps.

Hope it helps, Good Luck.
Steve
Click to expand...
Do you have all the wires from batteries to busbar the same lenghts?
 
In My Build: The wires are all the same length from battery to busbars, even the sense wires within the packs, the mains wires (+) & (-) etc as well.
 
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