No busbars needed with two batteries?

[ArtieKendall]

I was reading this thread which pointed to this webpage which contained this statement:

"Finally, if you only have 2 batteries, then simply linking them together and taking the main feeds from diagonally opposite corners cannot be improved upon."

In other words, it claims you don't need busbars for optimal current sharing with two batteries. If true, it seems like it would actually be a disadvantage to use busbars in a two-battery system (at least for the battery connections), since they require more connections, and each connection introduces a bit more resistance. I'm setting up a two-battery system, and it would save me time and money if I didn't need the busbars.

I didn't see any comments on the two-battery statement in the thread linked above.

Can anyone confirm or deny that simple diagonal wiring is optimal for two batteries?

 

[FilterGuy]

Yes. For only two batteries, the diagonal layout is as good as it gets. However, it is important to make the two cables between the batteries as close to identical as possible.

 

[Supervstech]

And all the cables need to each be able to handle the entire load.

 

[ArtieKendall]

Yes. For only two batteries, the diagonal layout is as good as it gets. However, it is important to make the two cables between the batteries as close to identical as possible.

The batteries came with pre-made 9-inch 6 AWG cables with lugs attached. The manual says they are intended only for a battery-to-busbar connection, but since they will still only be carrying one battery's current in a diagonal configuration, I assume they are also safe for this two-battery setup. If there were more than two batteries, at least one of them would carry current from two or more batteries, and then they would not be adequate. Does that sound right?

 

[Supervstech]

Not adequate unless the total draw load never exceeds 50A...

 

[ArtieKendall]

Not adequate unless the total draw load never exceeds 50A...

My reading of the manual for my EG4 3000 indicates that the max AC output is 25 Amps (they only mention this in one small-font footnote, but there doesn't seem to be program option to change it). My input AC will come from my wall outlet that is max 20 amps. My solar input is in series, and should not exceed 11 amps. So the only thing that might exceed 50A is charging from solar to battery.

I have breakers on almost every wire in or out of the inverter, plus a T class fuse on the positive battery wire, to cover shorts.

 

[Supervstech]

My reading of the manual for my EG4 3000 indicates that the max AC output is 25 Amps (they only mention this in one small-font footnote, but there doesn't seem to be program option to change it). My input AC will come from my wall outlet that is max 20 amps. My solar input is in series, and should not exceed 11 amps. So the only thing that might exceed 50A is charging from solar to battery.

I have breakers on almost every wire in or out of the inverter, plus a T class fuse on the positive battery wire, to cover shorts.

25A 120V is 125A at 24V...

 

[ArtieKendall]

25A 120V is 125A at 24V...

Doh! I'm using 48v, but I still can't believe I made such a dumb mistake. That said, I don't see why the minimum wire would be any different (for a two-battery system) if I used busbars; I would still have the same current on a 6 AWG leg (maybe I didn't make it clear that the wire into the inverter will be 4 AWG; the 6 is only for battery-to battery). What am I not seeing?

 

[Supervstech]

Doh! I'm using 48v, but I still can't believe I made such a dumb mistake. That said, I don't see why the minimum wire would be any different (for a two-battery system) if I used busbars; I would still have the same current on a 6 AWG leg (maybe I didn't make it clear that the wire into the inverter will be 4 AWG; the 6 is only for battery-to battery). What am I not seeing?

In parallel, when a load is placed on the two batteries, if the links are small, the load can shift and pull inequally between the batteries. Using full size conductors between the batteries keeps resistance down, and lets only the battery resistance itself handle balancing.
With bussbars, equal length conductors go to a bussbar, and the load is balanced from equal draw off the bars.

 

[ArtieKendall]

In parallel, when a load is placed on the two batteries, if the links are small, the load can shift and pull inequally between the batteries. Using full size conductors between the batteries keeps resistance down, and lets only the battery resistance itself handle balancing.
With bussbars, equal length conductors go to a bussbar, and the load is balanced from equal draw off the bars.

Thanks very much for the explanation. It would still be a lot easier for my configuration to not use busbars, so I think what I'll do is buy some 2/0 for my interbattery connections. Two feet of that is only like ten bucks, much cheaper than busbars.

 

[littleharbor2]

You can use the opposite/diagonal battery connection method and still connect to a set of bus bars for adding loads and secondary charging sources without stacking lugs on the battery terminals. Don't use the other opposite/diagonal terminals for making connections

 

[ArtieKendall]

You can use the opposite/diagonal battery connection method and still connect to a set of bus bars for adding loads and secondary charging sources without stacking lugs on the battery terminals. Don't use the other opposite/diagonal terminals for making connections

I'm using an all-in-one (EG4 3000), so I don't have anything that will need a bus bar because all the charging sources are connected inside the inverter. I do have a fuse and breaker on the wire from the positive battery terminal to the inverter, but those are in series and have their own connections.

 

[littleharbor2]

I'm using an all-in-one (EG4 3000), so I don't have anything that will need a bus bar because all the charging sources are connected inside the inverter. I do have a fuse and breaker on the wire from the positive battery terminal to the inverter, but those are in series and have their own connections.

Gotcha, my reply is geared more toward a separate component system.

 

[FilterGuy]

I do have a fuse and breaker on the wire from the positive battery terminal to the inverter

Interesting.... Why both?

 

[ArtieKendall]

Interesting.... Why both?

I'm using the breaker both for current protection and as a cutoff switch (my batteries have built-in breakers, but they will be behind a locked rack door, so I want a quicker way to get to the cutoff).

The breaker is for 125 amps. I saw one of Will's videos where he said that a battery short could produce a burst of thousands of amps, which would likely arc across the breaker even if it was open, so the class T fuse is to protect against that. It's for 200 amps, so hopefully the breaker will handle most problems, and keep the fuse from melting unless there's a really bad overage.

I'm a rank beginner, so feel free to tell me if this is dumb.