How fast will multiple LifePo4 batteries in parallel charge from 30amp shore power?

[Samsonite801]

I'm looking at the image posted by @Samsonite801 (post #11) and notice the 'Diagonally' and 'Busbars' setups have a very short cable (green) to one side of the battery bank and a much longer cable (white) to the other side. That can't be good, can it?

Just to reiterate, once the multiple paths from all the packs join into one (single) cable at the tapping point of attachment, that is the point where length differences between + and - don't matter. It's where they split into many, where the path lengths matter to get equal resistances between packs (if you care about SoC staying more balanced throughout overall SoC cycle)...

 

[sunshine_eggo]

Electrically it works fine, as long as all of the connections are good of course.

It would be better to connect them like this. There is less resistance / voltage drop with shorter cables, especially at high current loads.

View attachment 103648

Nope. That's bad. Even with beefy bus bars, batteries will disproportionately share current with the closest handling the most and the farthest handling the least.

 

[B-ManFX4]

Nope. That's bad. Even with beefy bus bars, batteries will disproportionately share current with the closest handling the most and the farthest handling the least.

Maybe on paper but not in the real world. The difference of the internal resistance of the batteries will have a much greater effect on the charge / discharge profile of each battery than the .00001 Ω difference in resistance between the connection points of a quality busbar.

When I built my battery bank I did extensive testing before I ever installed them in the RV. I measured the discharge current of each battery while pulling a steady 75 amps from each pair (I am wired for 24 VDC). I measured the voltage drops across the inter-cell wiring, across the Anderson connectors I'm using for connecting the batteries to the busbars, across the fuses, across the Victron battery switches and across the cables I made to series the batteries. I used a Fluke 8808A so I KNOW what the voltages were at every junction / measurement point. Interesting enough, the Victron battery switches demonstrated the biggest voltage drop and variances in the entire circuit. I found their resistance to be lower and much more repeatable after cycling them back and forth a dozen or so times (not while under a heavy load).

For my "big" test I charged the battery bank to 100% SOC. I then disconnected shore power and ran both AC units in the RV wide open. I ran them until the BMS in the batteries shut them down. Theoretical total battery capacity is 608 AH (two home-built 24 V 304 AH EVE batteries in parallel) - the Victron shunt reported 610 AH of usage when the BMS finally shut down. The units ran for a little over 5 hours before shutdown occurred. During the testing I measured temperatures at the various connection points with an infra-red thermometer - again the Victron switches were the worst offenders and that is what you would expect - the highest resistance would generate the most heat. Even still nothing in my circuit was elevated more than 15° F above ambient - I'm calling that good.

Once the batteries were recharged and had settled I connected to the BMS of each battery - the cells in each battery were all within .003 V of each other.

To sum all of that up, there are way more points of varying resistance that will affect the circuit performance in a more significant way than the cable length of the busbar feed. Shorter cables = lower resistance and lower cost.

That is real world, not theoretical.