2 lithium 120ah issue

[cookie]

I've got 2 x 120ah lithiums, both charge at different rates , 1 charges much quicker than the other, also 1 discharges more quickly.

Wired in parallel so would have thought both would drain and charge at similar rates.

 

[AdrianChen]

How is the wiring actually done?

This may help:

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

 

[cyberyacht]

An interesting illustration although the vast majority of us would probably only have two batteries. Like the OP, I have the same disparity in my batteries performance. One of them is a bit newer and, I suspect, has a slightly different BMS as the former will show discharges below 1A but the newer one does not.

 

[Carpmart]

Assuming wiring is correct and identical? IR and thus performance will vary across cells in each battery and across the pair giving the disparity. How different are they?

 

[Peregrinator]

Disconnect both batteries, fully charge each one, leave for 24 hours, then check to see their Amperage, you need to get the Amps to match each other, or very close, plus or minus 1 or 2 Amps, again leave for 24 hours, then check them again, if stable, reinstall.

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[Carpmart]

Disconnect both batteries, fully charge each one, leave for 24 hours, then check to see their Amperage, you need to get the Amps to match each other, or very close, plus or minus 1 or 2 Amps, again leave for 24 hours, then check them again, if stable, reinstall.

Why? What will this achieve?

I have a pair of LifePo4’s permanently connected in parallel and a third half the capacity of the other two which I connect in parallel when it’s often 30-50% SOC compared to the pair permanently connected… works absolutely fine and they largely balance out with each other after a few charges from the solar (the third battery takes the lions share of the solar charging at first) and complete parity across all three is reached after a full 100% SOC is achieved across all three packs.

 

[LostInSpace]

What is Amperage? You don't get Amps from a disconnected battery.

This needs some explanation.

 

[autorouter]

Wired in parallel so would have thought both would drain and charge at similar rates.

You can regard a battery theoretically as having zero internal resistance, and you can also regard the connecting wires as having zero resistance. In that case, it doesn't matter how you connect loads and chargers to batteries, provided they are in parallel.

However that is not exactly true. Batteries have a very small internal resistance, and wires have a small resistance too. If you want the batteries to be charged and discharged equally, then you have to arrange the wiring so that the charging voltages at the actual battery terminals are exactly equal, taking into account any small voltage drops in the wires.

The smaller the battery internal resistance, the more the slight resistances in the connecting wires affect the voltages. So lithium batteries are more affected by this problem than lead-acid batteries. The easiest way to ensure the resistances of the connecting paths are exactly equal is to arrange for the wire lengths from positive and negative common points to be exactly equal.

That is the thinking behind those arrangements in the AdrianChen's SmartGauge link. The total wire length between the two common points is exactly the same for each battery.

For the voltage drops to be equal, care must be taken that the wire crimps and joints are done carefully and tightened accurately, to avoid introducing any extra small resistances.

 

[Carpmart]

You can regard a battery theoretically as having zero internal resistance, and you can also regard the connecting wires as having zero resistance. In that case, it doesn't matter how you connect loads and chargers to batteries, provided they are in parallel.

However that is not exactly true. Batteries have a very small internal resistance, and wires have a small resistance too. If you want the batteries to be charged and discharged equally, then you have to arrange the wiring so that the charging voltages at the actual battery terminals are exactly equal, taking into account any small voltage drops in the wires.

The smaller the battery internal resistance, the more the slight resistances in the connecting wires affect the voltages. So lithium batteries are more affected by this problem than lead-acid batteries. The easiest way to ensure the resistances of the connecting paths are exactly equal is to arrange for the wire lengths from positive and negative common points to be exactly equal.

That is the thinking behind those arrangements in the AdrianChen's SmartGauge link. The total wire length between the two common points is exactly the same for each battery.

For the voltage drops to be equal, care must be taken that the wire crimps and joints are done carefully and tightened accurately, to avoid introducing any extra small resistances.

Taking it even further, a battery as we refer to it is actually multiple cells which each can have different IR’s… in practice it doesn’t matter for our application.

The OP is suggesting big variations in SOC across his pair of batteries which suggests some other issue.

 

[Langtoftlad]

Are we assuming both batteries are the same brand?

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[Jimbohorlicks]

I've got 2 x 120ah lithiums, both charge at different rates , 1 charges much quicker than the other, also 1 discharges more quickly.

Wired in parallel so would have thought both would drain and charge at similar rates.

Best wired to bus bars with same length cables.
If you can show how yours are wired in parallel we can offer further advise.

 

[autorouter]

What is Amperage? You don't get Amps from a disconnected battery.

This needs some explanation.

It is true that to charge batteries equally, the flow rate (amps) into each battery should be equal.

If you confuse electric charge flow rate (amps) with amount of electric charge (amp-hours), then you might end up thinking that the amounts of electric charge (amp-hours) in both batteries should be equal. And in fact they probably will be equal when fully charged. But as Carpmart implies, that won't help during the charging process.

The amount of electric charge stored in a battery is sometimes erroneously referred to as the 'amperage' of a battery. I'm not sure that the 'correct' term 'amp-hourage' would actually be an improvement on that, so I try to avoid mentioning it unless absolutely necessary.

 

[CAB96]

Why does it matter?

So, one gets used a bit more than the other (registers more 'cycles'). They'll still outlast the vehicle.

 

[cookie]

Both batteries are ks energy 120ah.

Wires are all equal length and size

As an example at end of day 1 battery is soc 88% other is 95% , also 95% battery seems to charge first znd more quickly

 

[Guigsy]

Lithium has an extremely flat voltage-charge curve. I wonder if even small differences in voltage between the two batteries might explain this behaviour?

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[autorouter]

Both batteries are ks energy 120ah.

Wires are all equal length and size

As an example at end of day 1 battery is soc 88% other is 95% , also 95% battery seems to charge first znd more quickly

You could try a longer wire on the 95% battery. Or even swap the '95%' wire for the next size down, to see if it makes any difference.

 

[bigtwin]

As an example at end of day 1 battery is soc 88% other is 95% , also 95% battery seems to charge first znd more quickly

Do they both reset to 100% at the end of a full charge cycle? If not, that’s the most likely reason that you are being led to believe that they are behaving differently when, in fact, they’re not.

If they are both resetting to 100% then it will be, as others have said, be as a result of resistance variations in the charge path.

Ian

 

[Guigsy]

How about swapping the batteries over to see if the behaviour is related to the wiring, or the batteries themselves?

 

[bigtwin]

How about swapping the batteries over to see if the behaviour is related to the wiring, or the batteries themselves?

Good shout.

Ian

 

[AdrianChen]

Do they both reset to 100% at the end of a full charge cycle? If not, that’s the most likely reason that you are being led to believe that they are behaving differently when, in fact, they’re not.

If they are both resetting to 100% then it will be, as others have said, be as a result of resistance variations in the charge path.

Ian

Good point, if they don't both reset it may also be that one does not fully cell balance, which would slowly affect capacity untill it does....

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[Carpmart]

Both batteries are ks energy 120ah.

Wires are all equal length and size

As an example at end of day 1 battery is soc 88% other is 95% , also 95% battery seems to charge first znd more quickly

My pair of 204A KS show they are out of sync on the app and indeed they often are by 0.01v - 0.03v across the packs. The SOC shows as much as 5% difference. There is nothing wrong, it’s just an old Bluetooth app and has poor accuracy! Don’t panic!

 

[Sewdhull]

2 batteries in parallel, but not identical...

1 will charge faster than the other and discharge faster in absolute terms as the voltage will be the same at all times on the 2 batteries but the IR and capacity will be different. Because the voltage remains the same on the 2 batteries they should remain healthy so long as they are not too dissimilar. Bulk charging will charge them quite differently as the charging will be controlled differentially according to the IR of the battery. Once you get to higher SOC the batteries should come closer until they end up 100% or whatever you charge them to.
The only thing to do to know, is to charge them separately and see how they charge.

 

[Peregrinator]

Why? What will this achieve?

I have a pair of LifePo4’s permanently connected in parallel and a third half the capacity of the other two which I connect in parallel when it’s often 30-50% SOC compared to the pair permanently connected… works absolutely fine and they largely balance out with each other after a few charges from the solar (the third battery takes the lions share of the solar charging at first) and complete parity across all three is reached after a full 100% SOC is achieved across all three packs.

Depends on the Batteries you have, and some don't have BMS in their Battery, and some BMS are sh**, and don't help, reason to first charge each one.

 

[Peregrinator]

What is Amperage? You don't get Amps from a disconnected battery.

This needs some explanation.

If you don't know how to test Amperage, plenty of u-tube videos explaining, but if that is too much, just check your voltage after 24Hrs.

 

[Jimbohorlicks]

Have you got a +ve coming off one battery and -ve off the other or both off the one battery?

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[Carpmart]

Depends on the Batteries you have, and some don't have BMS in their Battery, and some BMS are sh**, and don't help, reason to first charge each one.

News to me… which brands don't have a BMS then? Is that an optional extra?

 

[Carpmart]

Have you got a +ve coming off one battery and -ve off the other or both off the one battery?

Afternoon Jim… now the OP has explained further, my 99% suspicion is he’s looking at the KS app SOC and comparing… at best it’s moderately accurate, but only occasionally!

 

[Jimbohorlicks]

Afternoon Jim… now the OP has explained further, my 99% suspicion is he’s looking at the KS app SOC and comparing… at best it’s moderately accurate, but only occasionally!

Hi Harvey.
There was a post somewhere with a video test of 3 batteries wired up in parallel.all the permutations. The batteries at the ends lost charge first and also the battery that the loads were connected to.( ie pos and neg off one battery) really informative.
Even with just 2 batteries there are 3 ways to wire them up in parallel and to ensure they all charged up the same or discharge the same they really need connecting to a bus bar or 2nd best a pos off one and neg off the other.

 

[Sewdhull]

It is true that to charge batteries equally, the flow rate (amps) into each battery should be equal.

If you confuse electric charge flow rate (amps) with amount of electric charge (amp-hours), then you might end up thinking that the amounts of electric charge (amp-hours) in both batteries should be equal. And in fact they probably will be equal when fully charged. But as Carpmart implies, that won't help during the charging process.

The amount of electric charge stored in a battery is sometimes erroneously referred to as the 'amperage' of a battery. I'm not sure that the 'correct' term 'amp-hourage' would actually be an improvement on that, so I try to avoid mentioning it unless absolutely necessary.

Charge is Amp Hours

The unit of charge, the Coloumb, is an Amp for a second. So Ah, Amp hour is a measurement of Charge. It's there in your second para.

 

[cookie]

A lot to look at with all these answers.
Away in spain now but when we get home batteries are being moved inside under seating wheres its warmer and a b to b being fitted so a chance to make good all the wiring.

Just as an extra point, these batterys where fitted to our old adria last year and similar difference in charging and discharging existed then , just not quiet as much as now in this autotrail.

Thanks for assisting

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