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nigelb100 asked

Batteries discharging unevenly

Hello,

I have made myself a mobile power pack with 4 x Victron Smart LiFePo4 batteries, (12.8V 200Ah), wired in series, powered from a Multiplus II 48V inverter with BMS installed.

I have the Victron app so can monitor each battery using Bluetooth.

My question is: when in inverter mode (mains power off, load powered from the batteries) should I expect all batteries to discharge evenly?

At least one battery discharges faster that the others (not always the same one), the BMS then picks up under-voltage and shuts down the system. However, when I look at each battery they are at varying states of charge. Is this normal in a series system? Presumably I am not getting the full capacity of the system?

Thanks for the help

Nigel

Lithium Battery
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Hi @nigelb100,

couple of questions.

What makes you so sure that one battery discharges faster than the others? How do you see state of charge for each individual battery? What is your BMS? Did you follow the manual on how to setup the batteries in series and how to initially charge them up? Do you have a system diagram or images of your installation?

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5 Answers
Alexandra avatar image
Alexandra answered ·

@nigelb100

Your wiring has to be balanced for the same discharge or perfect current sharing from all batteries. Almost impossible with a 48v bank made from 4 x 12v batteries.

The end ones will discharge and charge faster - OHMS law. See the book wiring unlimited for more information.

Did you do an initial full charge of each individual one before install? What happens when they have not had a good initial charge is you get cell imbalances. and battery imbalances. With a 48v system you should also look at getting battery balancers. (also covered in the book)

What dimension are your DC links between each battery? I find people tend to under-size and it makes bank balancing worse.

You could try hold them for over a day at 100% to see if they do balance out a bit, then try your discharge again. But really each one should be individually charged.

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nigelb100 answered ·

Thanks for the answers.

Yes, batteries were charged individually before installation as per instructions using a Victron charger prior to putting them into the power pack so I don't' see that as the issue.

Stefanie - the BMS is a Victron VE.Bus BMS hooked up to the inverter.

It is a simple set-up: four batteries wired in series. The positive of the inverter is connected to battery one and the negative of the shunt is connected to battery four. The other side of the shunt then goes to the negative of the inverter.

I can see the SoC of each battery using the Victron Connect App. I have had the inverter plugged into the power overnight to see if it balances out. Coming in this morning what I can see now is:

BMS red light is on.

The issue seems to be with battery three. Looking at the Victron App this battery is much too hot and is overcharged (probably why the BMS red light is on).

Readings for each battery as follows:-

Battery one (positive to inverter) - "Ready for use". 13.21V/17 degree C/Balanced. Each cell roughly 3.30V

Battery two - "Ready for use". 13.21V/18 degree C/Balanced. Again each cell roughly 3.30V

Battery three - "Ready for use". "Charging disabled". 14.34V/32 degree C/Balanced/ Each cell roughly 3.57V

Battery four - "Ready for use". 13.20V/19 degree C/ Balanced/ Each cell roughly 3.30V.


I have a spare battery and this was previously installed in battery position three with the same issues.

I subsequently charged this battery using a Victron charger and it behaved correctly.

Alexandra - I had a battery balancer in the system in the past but I took it out as I thought with the BMS & the battery BTV system it wasn't needed.

Size of the cables might be something worth looking at....I think they are 50mm

Thanks again both..

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@nigelb100,

I don't have much to add. @Alexandra has already explained it very well. 4 batteries in series hardly discharge evenly.

I'm still not sure how you read out SoC for each individual battery. The only device that gives a good estimate of the state of charge is a shunt, like a BMV or SmartShunt. The VictronConnect app connected to the batteries says nothing about the SoC of each battery.

Battery three and VE.Bus BMS: The problem here is that the BMS has stopped the MultiPlus to charge, probably due to high cell voltage (>3.75V) at some point or high temperature (before your measurement). This is indicated by the red LED. The charge disconnect output goes low, which should stop the MultiPlus from charging.

Consult your SmartLithium manual and follow the troubleshooting guide in Section 6.1 (specifically “How to recover an unbalanced battery”). Since this is the second battery you had trouble with and in the same position, double check your wiring.

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nigelb100 avatar image nigelb100 Stefanie (Victron Energy Staff) ♦♦ commented ·
Thanks Stefanie. My mistake about the SoC. As you say the app doesn't tell the full story...
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Paul B avatar image
Paul B answered ·

Well In would think that the batteries are NOT balanced and may have NOT been balanced at the start, all cells have to be at 3.65v to be balanced. . all batteries should be the same voltage.

when connected in series the same amount of amps is taken from ALL batteries, at ALL times

to fix this you have a few options

1. leave the system on bulk charge for as LONG as it takes for the other 3 batteries to raise to the same voltage as battery 3 (make sure that NO cell goes over 3.65 volts ) if you choose this option slowly increase the charge voltage to 56 volts and set the bluk and float to the same voltage until balancing is finished - once there slowly raise it to 58.4 to get all cells to 3.65v in all batteries. then return to normal charge level of max 56volts bulk and 54 float or the normal lithium setting and ( NOTE they will get hot - well the cell balancers inside will so having a fan blowing through the cooling slots would be helpfull) its not the batteries getting hot its the cell balancers. Doing this option depending on how unbalanced they are - this could take a LONG time even 1 month or longer without doing any discharging..

2. take all batteries OFF line and charge one at a time so that ALL cells are at 3.5 volts or 14. volts in the balancing process some cells may rise to a higher voltage, (do not go over 3.65v) in each and then all batteries you must do the same - then and only then reconnect them in series and bring the system back online ( this would be my preferred option.)

of note bring all cells to 3.5 volts - then once they are all at 3.5v on each battery - slowly raise the battery voltage until all cells are at 3.65 volts 14.6volts (again do not let any cell go over 3.65 volts)

once you do this the battery will then be in balance.


there are many ways to do this balancing but the main area is do not go over 3.65 volts on any one cell.

of note the cell starts being balanced at 3.5 volts and then the cell balances inside will get warm to hot.



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nigelb100 answered ·

Thankyou very much Paul....I'll try it out

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nigelb100 answered ·

I set the battery charger so it charged each cell on my spare battery to 3.65V and it has now settled back down to 3 cells @ 3.63V and one @ 3.64V (14.53V). I guess this is optimal now so I just need to do the other three.

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Paul B avatar image Paul B commented ·

yes but you should not be swap them in and out in preferance - if you do then bring the others up so that say the battery number 3 is also showing 3.65 on all cells , then take the LOWEST voltage battery out and insert your spare. then repeat untill all 4 end up at 3.65


Its important that the batteries are in a KNOWN state Ie FULL


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