[Another newbie question] Two batteries in parallel - How to properly hook up?

As the title says I am a bit confused what is the correct way how to hook them up without a rack?
I think it will work both ways but, I think I read somewhere, that the A option will kill the top battery faster?

What is the correct way, A or B?

Thanks!

B

Could someone explain why this is the case? It seems counterintuitive, so I'm wondering what detail I'm missing.

Edit: I suppose the reason is that, if there is a significant voltage drop across the connecting wires, the voltage at the common terminals will be lower than the voltage on the bottom battery terminals (or higher if charging). I suppose, that could have an effect in the long run, but has anyone actually quantified it?

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webbbn said:

Could someone explain why this is the case? It seems counterintuitive, so I'm wondering what detail I'm missing.

Edit: I suppose the reason is that, if there is a significant voltage drop across the connecting wires, the voltage at the common terminals will be lower than the voltage on the bottom battery terminals (or higher if charging). I suppose, that could have an effect in the long run, but has anyone actually quantified it?

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The resistance in the jumper wires between the batteries. Flows less current.
Option B makes the resistance equivalent for both batteries.

I would say it is more the connections than the wires, but that is more theoretical than practical.

In A, the current for the top battery goes through 8 transitions/connections and the bottom goes through 16. In B, they both go through 12.

timselectric said:

The resistance in the jumper wires between the batteries. Flows less current.
Option B makes the resistance equivalent for both batteries.

Click to expand...

Thank you, and yes. I think that is essentially what I said, but in terms of voltage.

The voltage seen on the cell terminal determines how much current they're going to contribute to the overall load, based on their voltage. If both batteries are equal voltage to start, and there's a voltage drop between the second and first battery (assume current is being drawn out), the first battery will contribute more current. Assuming a constant current, that will only last until the voltage on the first battery drops low enough to equalize the voltage drop over the wire so that the current drawn from both batteries is equivalent. Once that state is reached, there should be no difference, other that a (hopefully) negligible amount of power lost though the higher resistance.

In most parallel arrangements that would be pretty fast, but, since we have large batteries that don't drop much voltage over the linear region of the SOC, that could be significant.

When you reverse voltage to charge, that all goes in reverse, so the affect on the top battery is essentially double, I believe. It's going to charge faster than the bottom battery until there's equilibrium.

So, it seems like this is most significant when there are many transitions between charge discharge cycles, but I'm still convinced that it matters much for a setup that primarily charges during the day and discharges at night.

All that said, I think the cell voltages likely change much more due to internal resistance than the voltage drop across a connection, so how much does internal resistance matter? I've also heard that, between the voltage extremes (in the middle of the SOC), these batteries will outlive their usefulness in most situations, so there's that as well.

All that said, if all else is equal, I agree B is the better choice overall.

Please correct me if I made an incorrect assumption, and thank you for indulging my stream of consciousness.

Shimmy said:

I would say it is more the connections than the wires, but that is more theoretical than practical.

In A, the current for the top battery goes through 8 transitions/connections and the bottom goes through 16. In B, they both go through 12.

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That is a good point, and I agree that connection resistance likely dwarfs a few inches of wire length, assuming you're using appropriate wire.

Aaand...B.

webbbn said:

That is a good point, and I agree that connection resistance likely dwarfs a few inches of wire length, assuming you're using appropriate wire.

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I am planning on using the connecting wire that comes with the battery.

I think I'm changing my vote to neither A or B. From what I can see, the manual recommends to use a busbar, and I can't see anywhere a recommendation to what amounts to a serial-parallel connection. Why not just wire both batteries to a 3-terminal busbar, and wire that to the inverter?

You may be able to get by doing this for a pair of batteries, but, if you expand in the future, you're going to need to use a busbar anyway.

webbbn said:

I think I'm changing my vote to neither A or B. From what I can see, the manual recommends to use a busbar, and I can't see anywhere a recommendation to what amounts to a serial-parallel connection. Why not just wire both batteries to a 3-terminal busbar, and wire that to the inverter?

You may be able to get by doing this for a pair of batteries, but, if you expand in the future, you're going to need to use a busbar anyway.

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This is portable (in emergency use) unit mounted on a dolly. I do not expect to expand past 2 batteries. That is also a reason why I don't want to use busbars - as they would be exposed and therefore extremely dangerous (the batteries are not mounted in an enclosure).

B

conus00 said:

This is portable (in emergency use) unit mounted on a dolly. I do not expect to expand past 2 batteries. That is also a reason why I don't want to use busbars - as they would be exposed and therefore extremely dangerous (the batteries are not mounted in an enclosure).

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Fair enough, and I appreciate the safety aspect. I believe you could make busbars safe, but I doubt that using them would matter much in your case.