Partially discharge Lifepo4 with fully charged sister packs? (1 Viewer)

[Carpmart]

So I have connected up a third LifePo4 in parallel with my existing two larger LifePo4 packs. The two larger are 204A each and the smaller 105A.

The smaller battery will in a few weeks be used to power my electric outboard trolling motor, as well as providing some extra capacity in the Motorhome power storage.

This will mean that the SOC both from a % of Amps and voltage, will be out of step with the two larger cells when I reintroduce it to the parallel set up.

Now, I can envisage a short Amp transfer spike when I first reconnect the smaller battery, but then I believe it will, with my VERY limited knowledge, quickly get up to a similar voltage to the other two larger batteries and once that voltage is equalised, will not draw anymore significant Amps.

99% of my charging is done via Solar and B2B and so when one of these charging source is introduced, post reconnection and the short Amp transfer spike, I then think that the small battery will just receive a disproportional share of the charging Amps until a happy balance is achieved.

Do people agree? Am I missing anything serious?

 

[Ditcha]

You need to fully charge all the batteries before connecting them in parallel Harvey

 

[Carpmart]

You need to fully charge all the batteries before connecting them in parallel Harvey

Why?

 

[Ditcha]

Why?

Because you won't know what state of charge they are at the only way you will is when they are all fully charged.

 

[Carpmart]

Because you won't know what state of charge they are at the only way you will is when they are all fully charged.

Yeah but why does it matter? As soon as the initial voltage stabilisation happens, yes the will be a brief amp spike, the smaller battery will just sit there at a lower state of charge until it gets some charge from the solar or B2B.

With more than one battery, I’m sure we have all seen battery drifting apart in terms of SOC until they get fully charged up again across the whole bank. They all show similar voltages. I don’t understand why having two batteries at 100% and one at say 50% is a problem? They will get back in sync when fully charged?

I’m just applying my own logic Dave, I’ve no idea if I’m right or not?

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[Ditcha]

When you join the 50% battery to the 100% battery there will be a inrush of amps from the 100% battery to the 50% battery it's not the volts you need to worry about it's the amps that will transfer from one to the other and the massive spark when connecting them together.

 

[Carpmart]

When you join the 50% battery to the 100% battery there will be a inrush of amps from the 100% battery to the 50% battery it's not the volts you need to worry about it's the amps that will transfer from one to the other and the massive spark when connecting them together.

100% agree. However in a short space of time, the volts climb on the 50% battery and will equal out, then no more amps will flow. Then the small battery should sit there until it gets charged. I accept it may not fully balance until fully charged.

 

[Ditcha]

100% agree. However in a short space of time, the volts climb on the 50% battery and will equal out, then no more amps will flow. Then the small battery should sit there until it gets charged. I accept it may not fully balance until fully charged.

The volts will only climb in relation to the state of charge and how would you suppress the spark from the initial connection?

 

[Guigsy]

How quickly do you expect that couple of dozen Ah of energy to equalise? What will be the current draw during that period?

 

[Bessy765]

I would put something like this between the smaller battery positive and the larger ones.

https://www.amazon.co.uk/Bolatus-1-2-Both-Off-Waterproof-Disconnect-Motorcycle/dp/B09GG7XRGH/?tag=mhf04-21

As you state you may remove this battery from time to time for other uses there will always be a spark when connecting or disconnecting. This way the contact can be made or broken safely.

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[Carpmart]

The volts will only climb in relation to the state of charge and how would you suppress the spark from the initial connection?

I thought the volts would equalise quickly? The higher SOC batteries would drop and the smaller part charged battery would climb quickly?

How quickly do you expect that couple of dozen Ah of energy to equalise? What will be the current draw during that period?

No idea, but my theory was, as soon as the volts equalise out, there will be no current flow whatsoever?

Ive no practical experience with this whatsoever?

I’m also assuming that the BMS will kick in and protect the smaller battery if more that 1.5-2C of amps spike it?

 

[Carpmart]

I would put something like this between the smaller battery positive and the larger ones.

https://www.amazon.co.uk/Bolatus-1-2-Both-Off-Waterproof-Disconnect-Motorcycle/dp/B09GG7XRGH/?tag=mhf04-21

As you state you may remove this battery from time to time for other uses there will always be a spark when connecting or disconnecting. This way the contact can be made or broken safely.

Thanks. Good idea!

 

[bigtwin]

Now, I can envisage a short Amp transfer spike when I first reconnect the smaller battery, but then I believe it will, with my VERY limited knowledge, quickly get up to a similar voltage to the other two larger batteries and once that voltage is equalised, will not draw anymore significant Amps.

Nope! Current (Amps) I =V/R

Am I missing anything serious?

The fact that I = V/R

Yeah but why does it matter?

Because I = V/R

the smaller battery will just sit there at a lower state of charge until it gets some charge from the solar or B2B.

Nope, it will be charged by the higher SoC battery.

I don’t understand why having two batteries at 100% and one at say 50% is a problem?

Because the resistance (R) is a function of the SoC.

I’m just applying my own logic Dave, I’ve no idea if I’m right or not?

I don’t know how to break this to you, but you’re not.

As Ditcha says:

it's not the volts you need to worry about it's the amps that will transfer from one to the other and the massive spark when connecting them together.

However in a short space of time, the volts climb on the 50% battery and will equal out, then no more amps will flow.

The transfer of energy to balance the SoC charge in your two batteries, lets say 50Ah, in your ‘short space of time’ will require a fairly significant flow of current.

Then the small battery should sit there until it gets charged.

No, it will suck a high current from the connected charge source (i.e. your 100% SoC battery)!

The higher SOC batteries would drop and the smaller part charged battery would climb quickly?

Yes, with significant current flow between them.

my theory was, as soon as the volts equalise out, there will be no current flow whatsoever?

Nope, I = V/R.

Ive no practical experience with this whatsoever?

You make a good job of disguising that fact.

I’m also assuming that the BMS will kick in and protect the smaller battery

One would hope that at least one of the two BMS’s would kick in (to protect the 100% SoC battery on high discharge current and/or to protect the low % SoC battery on high charge current) but it would be better just to follow standard advice by ensuring that both batteries are at 100% SoC before attempting to parallel them.

Ian

 

[Gromett]

As others have said, FULLY charge ALL batteries before connecting them in parallel.
The inrush current can be extremely high with even a very low voltage different.
I = V/R

If R is the resistence of the connecting cables then it will likely be very low (yes I know technically there is the internal resistance of the battery as well).

This low resistance will mean the current will likely be VERY high.

My guess would be. The current will be very high causing your BMS to go into over current protection mode and shut the battery down. Then reset, then shutdown rinse and repeat.
I don't think this would be good for the battery in question.

 

[Carpmart]

Been googling…

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[Carpmart]

Thanks for all the all the replies so far!

Returning to the crux of my problem of getting the smaller battery back into the parallel bank of three whilst not in a position to fully charge all three to 100% before connecting them together… any alternate ideas?

I guess if I get the trolling battery back to a similar state of charge to the other two, that will be good enough to then connect back in..? I’m likely not going to have access to a mains charger, so with just solar, any ideas on a setup and or way of addressing this?

 

[Ditcha]

Thanks for all the all the replies so far!

Returning to the crux of my problem of getting the smaller battery back into the parallel bank of three whilst not in a position to fully charge all three to 100% before connecting them together… any alternate ideas?

I guess if I get the trolling battery back to a similar state of charge to the other two, that will be good enough to then connect back in..? I’m likely not going to have access to a mains charger, so with just solar, any ideas on a setup and or way of addressing this?

Yeah buy another battery

Could you not use your solar to charge the smaller battery after the other bank is full ?
If they are not that far apart you might be able to turn the charge and discharge off on the BMS then join them and once joined turn the charge and discharge back on just a thought but not sure if this would work or not ?

 

[Terry]

Ready to be shot down in flames If the above switch / isolator is used I think you could safely connect the battery without sparks then once connected you could turn it on then it would act like a pair of jump leads via your cabling ? If not would a simple pair of jump leads bring your smaller battery back up to the same charge level.
I may be thinking/ talking nonsense or would doing the above damage one or all 3 batteries?

 

[MichaelT]

I added a 2nd fogstar battery to add to my ks one, the ks is 200a and was 100% or there abouts the 280a fogstar was 82%, didn't get any sparks and solar bought both to 100%.

After reading this maybe if the batteries Carpmart has are fogstar you could turn the discharge off on the full battery?

 

[Raul]

So I have connected up a third LifePo4 in parallel with my existing two larger LifePo4 packs. The two larger are 204A each and the smaller 105A.

The smaller battery will in a few weeks be used to power my electric outboard trolling motor, as well as providing some extra capacity in the Motorhome power storage.

This will mean that the SOC both from a % of Amps and voltage, will be out of step with the two larger cells when I reintroduce it to the parallel set up.

Now, I can envisage a short Amp transfer spike when I first reconnect the smaller battery, but then I believe it will, with my VERY limited knowledge, quickly get up to a similar voltage to the other two larger batteries and once that voltage is equalised, will not draw anymore significant Amps.

99% of my charging is done via Solar and B2B and so when one of these charging source is introduced, post reconnection and the short Amp transfer spike, I then think that the small battery will just receive a disproportional share of the charging Amps until a happy balance is achieved.

Do people agree? Am I missing anything serious?

I have done this now few times, purely to charge another battery at 30-40%soc to 100% and parallel on a bigger fully charged battery. It behaves exactly as you described, and once the voltage is balanced the empty battery will take most of the charge.
Pylontech modules work same way up to 1/4 soc paralleled with 3/4 soc. any higher difference the bms will shut one down anyways.
You will hear all dramas of what will happen, but in reality no dramas. I have done it.
When I added my third battery on the van, the two 200ah in parallel where at 100% soc. The new fresh built 300ah was at about 30-35%soc. not a spark. Yes it did take about 30a from the 400ah to start with, but, it slowed down within seconds, and continued at 5-7a later, until voltage equalised.

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[Guigsy]

I have done this now few times, purely to charge another battery at 30-40%soc to 100% and parallel on a bigger fully charged battery. It behaves exactly as you described, and once the voltage is balanced the empty battery will take most of the charge.
Pylontech modules work same way up to 1/4 soc paralleled with 3/4 soc. any higher difference the bms will shut one down anyways.
You will hear all dramas of what will happen, but in reality no dramas. I have done it.
When I added my third battery on the van, the two 200ah in parallel where at 100% soc. The new fresh built 300ah was at about 30-35%soc. not a spark. Yes it did take about 30a from the 400ah to start with, but, it slowed down within seconds, and continued at 5-7a later, until voltage equalised.

It's that peak inrush that's the issue. You don't want to cook the BMS or the cabling. If it's 30A, then that's manageable. It's just a bit hard to estimate what it might be. I'd just make the batteries all roughly fully charged before connecting to reduce the risk.

 

[Carpmart]

I added a 2nd fogstar battery to add to my ks one, the ks is 200a and was 100% or there abouts the 280a fogstar was 82%, didn't get any sparks and solar bought both to 100%.

After reading this maybe if the batteries Carpmart has are fogstar you could turn the discharge off on the full battery?

I’ve two 204A KS and then the 105A Fogster.

I guess I could turn of the charge and discharge function on the Fogster before connecting and when it’s securely connected switch it back on…

 

[Carpmart]

I have done this now few times, purely to charge another battery at 30-40%soc to 100% and parallel on a bigger fully charged battery. It behaves exactly as you described, and once the voltage is balanced the empty battery will take most of the charge.
Pylontech modules work same way up to 1/4 soc paralleled with 3/4 soc. any higher difference the bms will shut one down anyways.
You will hear all dramas of what will happen, but in reality no dramas. I have done it.
When I added my third battery on the van, the two 200ah in parallel where at 100% soc. The new fresh built 300ah was at about 30-35%soc. not a spark. Yes it did take about 30a from the 400ah to start with, but, it slowed down within seconds, and continued at 5-7a later, until voltage equalised.

Thanks for this… it’s reassuring to hear from someone with real world experience!

 

[CAB96]

Invertor and mains charger? To charge it up first?

 

[Raul]

It's that peak inrush that's the issue. You don't want to cook the BMS or the cabling. If it's 30A, then that's manageable. It's just a bit hard to estimate what it might be. I'd just make the batteries all roughly fully charged before connecting to reduce the risk.

Why is it a issue? Your cables are sized to carry full bms capability, and the bms has settings to limit current. It wont cook anything, it will disconnect if is outside parameters. The only thing to look out for, if you make the connection by hand, don't faff about with it, just one strong contact. If you switch the bms off, make the connection then turn on, or have a DC switch in between.
Honestly this is over hyped.
No diferent when you jump start a lead battery, except with lithium you have a nice safety box before it: the bms.

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[Ditcha]

I’ve two 204A KS and then the 105A Fogster.

I guess I could turn of the charge and discharge function on the Fogster before connecting and when it’s securely connected switch it back on…

Just make sure you connect the battery the right way round

 

[Carpmart]

Invertor and mains charger? To charge it up first?

It’s clearer now that there will be no catastrophic event if I do what I suggested I would do in my first post, so no real necessity to get the small battery SOC up before reconnecting to the parallel bank.

 

[Carpmart]

A good healthy discussion. Thanks for all the replies! Raul & MichaelT it’s always good to hear from someone who has done what I was planning with no dramas.

It’s interesting to hear people advice always. It’s also interesting to hear how that advice differs.

I think the battery manufacturers put out consumer information which helps them to control behaviour for the average user, rather than real truths. That means everyone believes there will be a mahoosive disaster as that’s what average consumers have been trained to think!

So, I have decided to follow my original approach but with some added precaution of avoiding a spark with an isolation switch (thanks Bessy765) and I may turn off the charge/discharge on the Fogstar battery until all connected and the isolation switch is thrown. Double belt and braces.

Thanks all!

 

[Guigsy]

Why is it a issue? Your cables are sized to carry full bms capability, and the bms has settings to limit current. It wont cook anything, it will disconnect if is outside parameters. The only thing to look out for, if you make the connection by hand, don't faff about with it, just one strong contact. If you switch the bms off, make the connection then turn on, or have a DC switch in between.
Honestly this is over hyped.
No diferent when you jump start a lead battery, except with lithium you have a nice safety box before it: the bms.

30A is fine. But what if it's 300A?

I've fried jump leads before. Not a good experience. And I'm still not completely sure why it happened. And lead acid and lithium batteries have very different internal resistance profiles. I'm not sure you'd get the same sag you do on lead acid.

 

[Carpmart]

30A is fine. But what if it's 300A?

I've fried jump leads before. Not a good experience. And I'm still not completely sure why it happened. And lead acid and lithium batteries have very different internal resistance profiles. I'm not sure you'd get the same sag you do on lead acid.

In my experience, there are jump leads and there are JUMP LEADS!

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