Battery Isolators... when are they necessary?

DXHum · Dec 8, 2022

I have a battery backup system that charges via the utility company. It contains 3 Led-Acid deep cycle batteries connected in parallel.

My friend insisted that I need battery isolators to allow for even charging across all three batteries. But I haven't seen any setups that have isolators where the charging source is solar panels.

Are battery isolators required for a battery bank connected in in parallel when the charging source is solar panels?

octal_ip · Dec 8, 2022

An isolator is usually in place to allow for easy maintenance and to meet regulations. I have no idea why someone would use them for the purpose your friend described.

DXHum · Dec 8, 2022

octal_ip said:
An isolator is usually in place to allow for easy maintenance and to meet regulations. I have no idea why someone would use them for the purpose your friend described.

The reason I am asking is because one of the three L-A batteries swelled up. And I was told it overcharged as a result of voltage passing through it to the other batteries.

octal_ip · Dec 8, 2022

DXHum said:
The reason I am asking is because one of the three L-A batteries swelled up. And I was told it overcharged as a result of voltage passing through it to the other batteries.

This would only happen if they were in series. Yours are in parallel.
If the battery develops an internal failure like a short, it would be able to pull current from the other batteries in addition to the charger, but an isolator wouldn't have helped prevent this. It'd just let you easily disconnect the battery once you'd identified the fault.

MisterSandals · Dec 8, 2022

DXHum said:
The reason I am asking is because one of the three L-A batteries swelled up. And I was told it overcharged as a result of voltage passing through it to the other batteries.

Sounds like they may be wired in an imbalanced manner. Like this?

Screen Shot 2022-12-08 at 2.16.00 PM.png

Or more balanced (there are better) like this?

Screen Shot 2022-12-08 at 2.16.26 PM.png

Delphinus · Dec 8, 2022

DXHum said:
I have a battery backup system that charges via the utility company. It contains 3 Led-Acid deep cycle batteries connected in parallel.

My friend insisted that I need battery isolators to allow for even charging across all three batteries. But I haven't seen any setups that have isolators where the charging source is solar panels.

Are battery isolators required for a battery bank connected in in parallel when the charging source is solar panels?

Doesn't make sense for even charge across batteries unless you use it to fully charge each one at the time, an balancer/equaliser will do it better.
You should have a fuse for each battery, I use mine to protect and maintenance you shouldn't open them under high load but when it's low it's OK, in my case before I close the circuit I shunt the fuse terminals for some seconds in case there is a difference of voltage to avoid blow the fuse.

DXHum · Dec 8, 2022

MisterSandals said:
Sounds like they may be wired in an imbalanced manner. Like this?
View attachment 123703

Or more balanced (there are better) like this?
View attachment 123704

Aren't these two diagrams the same... electrically speaking?

octal_ip · Dec 8, 2022

DXHum said:
Aren't these two diagrams the same... electrically speaking?

You might think so, but the resistance of the wires and terminal connections at high current make a non-negligible difference across the batteries. When under load, the wiring in first diagram pulls more current from the 4th cell due to lower resistance. This leads to an imbalance in discharge/charge rates.

MisterSandals · Dec 8, 2022

DXHum said:
Aren't these two diagrams the same... electrically speaking?

Go forth and drink from the fire hose of knowledge... (the first 4 or 5 posts should be enough)

Calculation of parallel string battery currents

Hi, everybody! I'm a retired EE. I didn't want my EE skills to fade away after retirment, so I exercise them when I can. Whenever I see a non-trivial circuit analysis problem on one of various forums I frequent, I challenge myself to solve it. Recently I watched Will's video about current...

diysolarforum.com

DXHum · Dec 9, 2022

octal_ip said:
You might think so, but the resistance of the wires and terminal connections at high current make a non-negligible difference across the batteries. When under load, the wiring in first diagram pulls more current from the 4th cell due to lower resistance. This leads to an imbalance in discharge/charge rates.

This makes complete sense to me. So you balance the battery bank by adding/removing resistance through wire length?

MisterSandals · Dec 9, 2022

DXHum said:
So you balance the battery bank by adding/removing resistance through wire length?

I don't think anybody "wants" to vary the resistance, its common to have all the wires the same length and make it easiest.

Note that with bus bars, the wires from bus bar to batteries are identical.

Screen Shot 2022-12-09 at 9.38.36 AM.png

Discussion of the diagonal connection is on post #14 (this is likely how i would wire it initially).

Calculation of parallel string battery currents

Hi, everybody! I'm a retired EE. I didn't want my EE skills to fade away after retirment, so I exercise them when I can. Whenever I see a non-trivial circuit analysis problem on one of various forums I frequent, I challenge myself to solve it. Recently I watched Will's video about current...

diysolarforum.com

Screen Shot 2022-12-09 at 9.39.55 AM.png

Battery Isolators... when are they necessary?

DXHum

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octal_ip

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DXHum

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octal_ip

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MisterSandals

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Delphinus

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DXHum

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octal_ip

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Calculation of parallel string battery currents

DXHum

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MisterSandals

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Calculation of parallel string battery currents

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