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Batteries with different voltages 48v + 51.2v - SOC solution


Douw G. Gerber

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Hi there all

As mentioned in my other post re bad installers:

 

My mate was supplied with 2 x batteries (same brand) but one is a 48v and the other a 51.2v unit.  Issue is SOC calc with solar assistant emulated BMS is not working as it should and I am ssuming this could be due to different voltages of the batteries right? There is no info on th ebatteries in order to get BMS comms working and I think with them being different this would have not worked anyway.

Would we be able to fix this problem with a Victron Smart Shunt perhaps (it can also connect to SA)?  Ie get more accurate coulomb count?

Also for the battery experts here - what is the best and worst that can happen with two different volatge batteries in parrallel? No point in forkng out for a Shunt if the batteries are going to be borked in a short space of time.  Maybe just keep the 51.2v and try sell the 48v.

Cheers

Douw

Edited by Douw G. Gerber
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  • Douw G. Gerber changed the title to Batteries with different voltages 48v + 51.2v - SOC solution

I'm sorry to say guys it jot as easy as you guys make out. Just remove a cell. Firstly will the BMS of said battery operated after said cell is removed? This type of stuff is not as easy as made out. I'd suggest get the same version from the manufacturer or supplier is 15s at 48V or the 51.2v 16s version. I'd never open a battery for such a reason. 

 

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Furthermore what I could find out was that these batteries - Sungod - have their cell packs made up of 32700 cells. 

Not a simple cell removal I would think. 

He is going to request the installer either give him a 48v or 51.2v replacement unit to get them the same. 

Edited by Douw G. Gerber
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  • 3 weeks later...

Just reviving this again...

My mate is still sitting with the two batteries 1 x 51.2v (16 cell) and 1 x 48v (15 cell) - what is also happening is that one of the batteries will first run flat and then the other one starts discharging...the reverse then happens during charging...

So from what I can gather researching is that you can mix batteries of different Ah capacity as long as they are all the same nominal voltage.

But from the repsonses above it seems it is a safe to confirm that mixing nominal voltages is a bad idea and should be avoided at all costs.

 

Edited by Douw G. Gerber
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Correct, can parallel batteries with different capacities, but not with different nominal voltages. And by implication that also normally means different cell chemistries.

As a matter of interest you are saying both the 51.2 and the 48 Volt batteries are 16cell?  If they are both LiFePo4, the 48V is a 15 cell.  The two can't meet

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5 minutes ago, gimme_power said:

Correct, can parallel batteries with different capacities, but not with different nominal voltages. And by implication that also normally means different cell chemistries.

As a matter of interest you are saying both the 51.2 and the 48 Volt batteries are 16cell?  If they are both LiFePo4, the 48V is a 15 cell.  The two can't meet

No to clarify:

Both are LiFePo4 chemistry

1 x 51.2v (16 cell)

1 x 48v (15 cell)

Its just that the 48 volt has the same BMS but shows cell 16 as 0.0v - so it only has 15 cells.

 

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They can not be paralleled. If you want to do that in a way that will not damage the battery, you would need to set your inverter to charge to nominal 48V which means your 51.2V battery would only charge to a nominal 3V per cell, which is is already a very low SOC, so no can do. Also you would need to stop discharge at 2.5V per cell, which is 40V on the 51.2 battery. So you are really limiting yourself on the top and bottom side of things.

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On 2023/05/13 at 12:10 PM, Beat said:

Only packs with the same number of cells can be paralleled, otherwise disaster is programmed. My solution would be to eliminate one cell of the 51.2V pack. Keep it as a spare.

I am starting to lean very much in the direction of suggesting we just do this for him.  I doubt he is going to have any warranty any based on the fact that the guy who supplied is a schyster anyway.

Is it then simply a case of disconnecting cell 16 and removing it?

The BMS's look identical if the display is anything to go by

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2 hours ago, Douw G. Gerber said:

I am starting to lean very much in the direction of suggesting we just do this for him.  I doubt he is going to have any warranty any based on the fact that the guy who supplied is a schyster anyway.

Is it then simply a case of disconnecting cell 16 and removing it?

The BMS's look identical if the display is anything to go by

You may have to solder two pins together on the BMS chip.

Edited by Tinbum
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4 hours ago, Douw G. Gerber said:

I am starting to lean very much in the direction of suggesting we just do this for him.  I doubt he is going to have any warranty any based on the fact that the guy who supplied is a schyster anyway.

Is it then simply a case of disconnecting cell 16 and removing it?

The BMS's look identical if the display is anything to go by

Not sure why the 'supplier' won't replace it. I have heard of batteries doing the rounds with dodgy origins. Or why your friend can't sell it and get another similar battery.....

Maybe you can explain this. 

Removing one cell will require also disconnecting the balance lead and re-programming the BMS,  so that it knows what the different voltages are, what the capacity is and also how many cells there are.  Not all BMS's are capable of doing both 15 and 16 cell batteries. Some can only do one or the other.

 

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4 hours ago, gimme_power said:

Not sure why the 'supplier' won't replace it. I have heard of batteries doing the rounds with dodgy origins. Or why your friend can't sell it and get another similar battery.....

Maybe you can explain this. 

Removing one cell will require also disconnecting the balance lead and re-programming the BMS,  so that it knows what the different voltages are, what the capacity is and also how many cells there are.  Not all BMS's are capable of doing both 15 and 16 cell batteries. Some can only do one or the other.

 

Original post link in opening post explains the issues with his "supplier".

Anyway it's not as simple as it sounds it seems to remove the cell. 

I guess schoolfees for him unfortunately.

 

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It should actually be possible to connect the packs in parallel, but with some 'interesting' limitations.

If you set the max charge voltage to 3.65*15 = 54.75V, then the 16s pack will charge to 54.75/16 = 3.42V per cell - so you lose 2-3% of the maximum capacity of the 16s pack.

If you set the min discharge voltage to 2.5*16 = 40V, then the 15s pack discharges to 2.66V cell - so you lose about 1-2% of the maximum capacity of the 15s pack.

(Of course, you may want lower limits than that to preserve pack life - but that costs you exponentially more capacity.)

When fully charged, the 16s pack will carry almost the entire discharge load, and when fully discharged, the 15s pack will carry nearly the entire discharge load (and pretty much the same for charging). So each individual pack must be able to handle the entire charge/discharge load on its own (you can only use this configuration to increase energy [Wh], not power [W]).

The major disadvantage is cell balancing - the 16s pack will never enter the ideal voltage range for balancing, so if the cells aren't perfectly matched, they will soon drift far enough out of spec that the BMS will start disconnecting.

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9 minutes ago, JustinSchoeman said:

It should actually be possible to connect the packs in parallel, but with some 'interesting' limitations.

If you set the max charge voltage to 3.65*15 = 54.75V, then the 16s pack will charge to 54.75/16 = 3.42V per cell - so you lose 2-3% of the maximum capacity of the 16s pack.

If you set the min discharge voltage to 2.5*16 = 40V, then the 15s pack discharges to 2.66V cell - so you lose about 1-2% of the maximum capacity of the 15s pack.

(Of course, you may want lower limits than that to preserve pack life - but that costs you exponentially more capacity.)

When fully charged, the 16s pack will carry almost the entire discharge load, and when fully discharged, the 15s pack will carry nearly the entire discharge load (and pretty much the same for charging). So each individual pack must be able to handle the entire charge/discharge load on its own (you can only use this configuration to increase energy [Wh], not power [W]).

The major disadvantage is cell balancing - the 16s pack will never enter the ideal voltage range for balancing, so if the cells aren't perfectly matched, they will soon drift far enough out of spec that the BMS will start disconnecting.

Thank you for the comprehensive answer Justin.  What you describe is pretty much how I set up the inverter for him and what he is experiencing. But ja as you say in the long run it will give problems...

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