Adding batteries and layout

[Tulex]

I've got 3 304ah batteries with Seplos V2 BMSs (passive balancing) below my inverter. They run individually to a busbar, then using both breakers in my Sol-Ark 15k to connect the inverter to the busbar. The 3 batteries are connect via comms, and the master battery connected to the inverter. These batteries are side by side, the cables are fairly close to the same length going to the busbar.

I want to add 3 more batteries, most likely made from 280ah cells. But, they have to be a few more feet away, and will have to be stacked, going almost from ceiling to floor. I know I have to stay with the V2 BMS to connect them all together. My main question is, can I run these 3 batteries to a busbar at the batteries, then connect that busbar to the existing busbar? I mean, I know that will work, but is that the best way?

 

[sunshine_eggo]

My main question is, can I run these 3 batteries to a busbar at the batteries, then connect that busbar to the existing busbar? I mean, I know that will work, but is that the best way?

The main rule is that the electrical resistance between each battery and the Sol-ark need to be the same. The closer you can get, the better. If you can't manage it, and the currents are still within limits in operation, that meets the good-enough criteria.

I see the weak point as the connection between the bus bar set resulting in higher resistance for the second group.

G1 = 1st group of 3.
B1P and B1N = bus bars for G1

Inverter currently connected to B1P and B1N.

G2 = 2nd group of 3.
B2P and B2N = bus bars for G2

B1P connected to B2P
B2N connected to B2N

Inverter positive connected to B1P and negative to B2N - this would be analogous to cross-connecting two batteries/banks in parallel. Crudely Sketched:

 

[rhino]

Asking because I really want to know but I do not see why the resistance between all the batteries need to be the same with LiFePO4. Sure, one set of batteries will work harder but what difference does that make in real world? The other batteries with more resistance will still provide some power and cycle life of these batteries is so long it won't really make a difference especially when you start having quite a few batteries in parallel.

 

[timselectric]

It's a non issue on the battery side of the bus bars.
In my opinion.

 

[Tulex]

How does battery flow actually work? With AC, you can have a power strip with 6 things plugged into it going to an outlet, and a single plug in the other outlet receptacle. All will draw the power needed. If they are all 60 watt bulbs, they will all use 60 watts. But you have unlimited supply.

With DC, you may be sending say only 300 watts out. If I have 3 batteries attached to a bus bar, and one additional line going to 3 other batteries, is it the batteries resistance to taking a charge that controls the flow? If the batteries are all at the same state of charge, will it send 50 watts to each of the 3 batteries and 150 watts to the cluster of 3, other than the difference in resistance caused by the wire?

 

[timselectric]

is it the batteries resistance to taking a charge that controls the flow?

Yes
Resistance of battery, wiring, and connections.

 

[sunshine_eggo]

Yes
Resistance of battery, wiring, and connections.

And these little bastards are what get you... on a DIY 16S battery, you have 32 additional connections you need to make sure are properly torqued.

On non-DIY batteries, there are internal connections you can't control that may influence resistance.

 

[DougfromdaUP]

How does battery flow actually work? With AC, you can have a power strip with 6 things plugged into it going to an outlet, and a single plug in the other outlet receptacle. All will draw the power needed. If they are all 60 watt bulbs, they will all use 60 watts. But you have unlimited supply.

With DC, you may be sending say only 300 watts out. If I have 3 batteries attached to a bus bar, and one additional line going to 3 other batteries, is it the batteries resistance to taking a charge that controls the flow? If the batteries are all at the same state of charge, will it send 50 watts to each of the 3 batteries and 150 watts to the cluster of 3, other than the difference in resistance caused by the wire?

And don't forget the overpotential voltage. It's way larger than the voltage drop internal resistance and the resistance of connections and cables causes unless you have done something really, really bad.

And it tends to equalize the current flow to and from batteries. A battery sourcing more current will have a higher overpotential voltage, tending to limit the current.

See @RCinFLA 's many posts for an in depth explanation.

Keeping cables the same length isn't nearly as important as with other chemistries. Of course making good connections is.

 

[sunshine_eggo]

And don't forget the overpotential voltage. It's way larger than the voltage drop internal resistance and the resistance of connections and cables causes unless you have done something really, really bad.

And it tends to equalize the current flow to and from batteries. A battery sourcing more current will have a higher overpotential voltage, tending to limit the current.

See @RCinFLA 's many posts for an in depth explanation.

Keeping cables the same length isn't nearly as important as with other chemistries. Of course making good connections is.

@RCinFLA = the electron whisperer