Please help - One side of battery parallel not drawing current...

[DoctorCaptain]

Hi,

I'm having an unexpected issue with my battery connections. I have 4 x 115Ah 12v house batteries. Two from each are in series to create 2 x 115Ah 24v batteries.

When I check the current draw only one set of the 24v batteries in parallel seem to be providing current.

I expected current to draw from both at equal rates. Is what I'm seeing normal or is it likely one of my batteries is faulty?

I have attached a drawing to show the wiring. I've included an original set up and then a later more standard set up.

Originally I had each 24v battery wired to a joining terminal then to a switch. With the idea of having all wiring connections equal. I then reverted to the normal positive to positive set up.

A troubleshooting step would be to switch the batteries around on the connections and if the results reverse it's likely a battery issue but I'm still not sure that would be definitive.

Also, I assume it is more likely to be one faulty battery rather than both 12v units. And I feel it's unlikely to be the wiring because I've used all correctly size and installed components and done that twice with the same results.

Would appreciate if someone could help.

[Avionics_Joe]

What is the battery chemistry? If LiFPo4 it could be a different state of charge. Did you balance the batteries first?

[wholybee]

Do like the lower drawing, except take the positive from the top battery. Such that positive comes from one battery, and negative from the other.

When you connect them, both batteries should be at the same state of charge, which most easily is full. Make sure all the connections are clean. Don't assume they are, use a wire brush or sandpaper on them. If the batteries are different ages, different states of charge, or some are otherwise in a lessor condition, then the current draw will not be equal. Nothing you can really do to change that.

How different is the current draw, and how are you measuring it? How do all the batteries test when checked one at a time out of the circuit? What type of batteries are these?

[DoctorCaptain]

Batteries are sealed lead acid.
They are now set up as per the lower drawing.
All 4 are bought new together 24 months ago.
All connections are good, brand new wires, connectors etc everywhere.
Batteries all had 12v nominal voltage when tested individually. I.e. they all had the same voltage.
The current draw is all from one side and zero from the other side.
I'm using a clamp on multi meter to test the current.

Another idea would be to test each 24v set individually to see if one is faulty.

I'm guessing from the responses so far what I'm seeing isn't normal and I'll need to troubleshoot further...

[Sailer4life]

Hi,

Water always chose the shortest way from A to B ..... And so do electrical current. Also the batteries should have the same lot numbers, and have been used in the same battery Bank making them almost identical.
On your drawing the Lower two batteries have shorter connections ( less resistance than the Upper two batteries ) hence the Water example..
Make two identical positive cable connections, same diameter and length, between the + poles and the switch as well as to identical negative cables to the - side of the batteries ( same cable diameter and length ) and the current will see the same resistance, from each group of batteries to the switch and share the delivered current.

[DoctorCaptain]

Originally Posted by Sailer4life

Hi,

Water always chose the shortest way from A to B ..... And so do electrical current. Also the batteries should have the same lot numbers, and have been used in the same battery Bank making them almost identical.
On your drawing the Lower two batteries have shorter connections ( less resistance than the Upper two batteries ) hence the Water example..
Make two identical positive cable connections, same diameter and length, between the + poles and the switch as well as to identical negative cables to the - side of the batteries ( same cable diameter and length ) and the current will see the same resistance, from each group of batteries to the switch and share the delivered current.

All batteries are exactly the same bought and used at the the same time.

They were originally set up as per the first drawing with the same wire lengths.

I had the same problem then.

[s/v Jedi]

You need to be able to isolate the batteries. Use an OFF-A-B-BOTH switch.

[DoctorCaptain]

Originally Posted by s/v Jedi

You need to be able to isolate the batteries. Use an OFF-A-B-BOTH switch.

Will that not start to get messy if I have 3 - 4 batteries?

I take your point though, now I'm going to have to manually isolate each battery to fault check.

[HeinSdL]

It's not a big intervention to simply disconnect the working side and see what happens.

[s/v Jedi]

Originally Posted by DoctorCaptain

Will that not start to get messy if I have 3 - 4 batteries?

I take your point though, now I'm going to have to manually isolate each battery to fault check.

If you plan more than 2 batteries, use simple on/off switches for each battery that connect to a shared busbar. I use that even though I only have two batteries for redundancy reasons (switches can go bad too).

Imagine a situation where a faulty battery is drawing down other parallel batteries without blowing a fuse. You want to be able to quickly isolate batteries.

[deblen]

https://marinehowto.com/category/electrical/

[DoctorCaptain]

Originally Posted by s/v Jedi

If you plan more than 2 batteries, use simple on/off switches for each battery that connect to a shared busbar. I use that even though I only have two batteries for redundancy reasons (switches can go bad too).

Imagine a situation where a faulty battery is drawing down other parallel batteries without blowing a fuse. You want to be able to quickly isolate batteries.

That's a fair point and I like the simplicity. Thanks 👍

[cram]

Have you checked the electrolyte level? Specific gravity? When were they last equalized? If you disconnect everything, what are the individual battery voltages?

I'd take the neg off one bank,then the pos off another bank. Is there a fuse before the bus bar?

[Wolfe10]

Yup, loads from positive of "pair #1". Negative from negative of "pair #2"


And, a little concerned that an odd number of batteries was mentioned. Batteries need to be installed ONLY in pairs of 12 VDC to give you 24 VDC.

[donradcliffe]

Simply measure the voltage across each 12v battery. One set should measure about the same voltage, the set without output should measure quite differently.