Hi or low balancing and how often?

[Deleted member 9967]

I am curious about something.
I do not have the equipment to do a low balance on lifepo batteries.
Plus is scares me in that if I discharge to low, the batteries will die.
So is a top balance alone ok?
And once done.
Do I do it every time I charge the batteries?
If not, then do I disconnect it or what?
Also how often should I top balance the batteries?
I will be using 8 - 320 Ah lifepo batteries in series in case you need to know.
Thank you.

 

[GMB]

A few videos on the subject you can watch:

 

[Deleted member 9967]

Too funny. I talked with him and he recommended those videos as well.
However I did not see if he answered my questions fully though.
Thanks for the links just the same though.

 

[Pierre]

Have you built your pack ? What is the cells delta ( difference between the highest and lowest cell ) ?
Do a charge / discharge cycle , say to 20% SOC and measure cells delta again. We will then be able to see if you have a problem.

 

[Deleted member 9967]

I have not gotten it yet.
Trying to learn everything I can before making a purchase that is 20% of my annual income.
I also never said I had a problem.
Just questions about balancing is all.
But thanks for your comments anyhow.

 

[Pierre]

I have not gotten it yet.
Trying to learn everything I can before making a purchase that is 20% of my annual income.
I also never said I had a problem.
Just questions about balancing is all.
But thanks for your comments anyhow.

Good luck to you.

 

[Deleted member 9967]

LOL was hoping not for luck but for a techy person. But thanks anyways.

 

[JoeHam]

If you are adding a BMS balancer and starting out with new, well matched cells I would recommend to never top or bottom balance.

 

[Ampster]

I would also recommend to let the BMS do its balancing thing based on the cell deltas. You should buy a BMS that allows you to adjust those settings.

 

[Deleted member 9967]

If you are adding a BMS balancer and starting out with new, well matched cells I would recommend to never top or bottom balance.

Ok but everyone I have watched or read and even the manufacturer says to at least top balance.
So what is your authority is saying not to do so?

 

[Deleted member 9967]

I would also recommend to let the BMS do its balancing thing based on the cell deltas. You should buy a BMS that allows you to adjust those settings.

I am getting the BMS from the makers of the batteries and it will have bluetooth on it. So I now I can see the parameters and I belive to also adjust them as well.
Also I thought that all BMS allowed for adjusting otherwise the BMS is pretty useless is it not?

 

[Ampster]

Also I thought that all BMS allowed for adjusting otherwise the BMS is pretty useless is it not?

There are low end BMSs designed for a particular battery type. I would agree that they are useless except on an ebike.

 

[apctjb]

Ok but everyone I have watched or read and even the manufacturer says to at least top balance.
So what is your authority is saying not to do so?

No authority here but my understanding the need for top/bottom balance depends on a number of factors including your application, and there is some debate on if "manual" balancing is needed in a situation where you are starting with cells that are fairly close in voltage/charge to begin with and you are using a BMS that provides balancing. In the case where you are not going to be hammering your battery bank with frequent deep discharge cycles, you are starting with cells that are close in voltage and charge, and you don't have the equipment or experience you might be money ahead to build your bank and allow the BMS to bring the cells into balance over time.

Hopely others with real expertise will chime in.

 

[Ampster]

So what is your authority is saying not to do so?

I will let @JoeHam respond but the assumption was that one had well matched cells all at the same voltage (or State of Charge)
Since there is no garanty that the assumption will be the case, I would recommend top balancing. Voltage differences of 10 to 15 millivolts were how my cells arrived. Also voltage at that point in the curve is not a good indicator of SOC. One needs to be in the knee of the curve to get a reliable estimate of SOC. Top balancing does that.
There are two reports of cells going puffy during a top balancing effort. The jury is out about whether that was a result of operator error. If you are patient, understand how to set a power supply to not exceed a voltage, can use a multi meter and can keep a log of settings and voltages then you may have success top balancing.

 

[Deleted member 9967]

No authority here but my understanding the need for top/bottom balance depends on a number of factors including your application, and there is some debate on if "manual" balancing is needed in a situation where you are starting with cells that are fairly close in voltage/charge to begin with and you are using a BMS that provides balancing. In the case where you are not going to be hammering your battery bank with frequent deep discharge cycles, you are starting with cells that are close in voltage and charge, and you don't have the equipment or experience you might be money ahead to build your bank and allow the BMS to bring the cells into balance over time.

Hopely others with real expertise will chime in.

Actually, I am using mine for my mobility scooter and will be depleting it weekly at least probably.
And they will be new batteries that I have been told will be balanced before they send them to me. Just not fully charged, as Lifepo does not like to be stored with a full charge.

Thanks for your reply and yes that is the gist of what I have been reading myself.
I am just trying to get reassurance is all.
Thanks for your reply.

 

[Deleted member 9967]

I will let @JoeHam respond but the assumption was that one had well matched cells all at the same voltage (or State of Charge)
Since there is no garanty that the assumption will be the case, I would recommend top balancing. Voltage differences of 10 to 15 millivolts were how my cells arrived. Also voltage at that point in the curve is not a good indicator of SOC. One needs to be in the knee of the curve to get a reliable estimate of SOC. Top balancing does that.

Thank you

 

[Ampster]

..........where you are starting with cells that are fairly close in voltage/charge to begin with and you are using a BMS that provides balancing........

Just for emphasis, that is the critical assumption. I invested in an internal resistance tester to give me some insight about one aspect of how well they are matched. The other way is to run a capacity test on each cell which would take at least a day each for a total of 32 days. I don't have that kind of time so I will top balance my pack.

 

[Ampster]

Actually, I am using mine for my mobility scooter and will be depleting it weekly at least probably.

What you really need is a good BMS with an accurate SOC Guage. Those are sometimes called Coulomb counters. That reduces the likelihood of being stranded with little capacity left in your pack.

 

[Deleted member 9967]

What you really need is a good BMS with an accurate SOC Guage. Those are sometimes called Coulomb counters. That reduces the likelihood of being stranded with little capacity left in your pack.

Yes, I have been hearing about the Coulomb thing. It will become helpful when out and about to know how many Ah I have used up.

 

[Bob B]

I think most of us will initially top balance the battery and let the BMS take care of it after that. If you can monitor the pack you will be able so see if you have some problem cells that may require a more aggressive approach.

 

[JoeHam]

Ok but everyone I have watched or read and even the manufacturer says to at least top balance.
So what is your authority is saying not to do so?

My reasoning is that I cannot find any measurable benefit that is documented.

Let’s assume that there is a 10% increase in some metric although I doubt that.

There have been a few people on the forum who had a 100% loss of at least one new cell trying to top balance and that is fact.

By my simpleton analysis you are risking 100% loss for a potential 10% gain. I just don’t see the benefit.

Just my experience and observations. Here is what I did:

48v LiFePO4 build

For my application I wanted to build a semi-portable system capable of running a 1500w Inverter. Most parts are on the way so here I go. This is also a "proof of concept" thing for me and if I do a second, non-portable install down the road I could easily use the same devices powered by some...

diysolarforum.com

My new, well balanced cells have stayed that way with just passive balancing, during charge only, and I cannot imagine why I would want or need to do it differently.

N=1 here but I think it is a very reasonable choice to make.

Hope that helps.

 

[John Frum]

There have been a few people on the forum who had a 100% loss of at least one new cell trying to top balance and that is fact.

If you put $n cells in parralel
and charge with a voltage of less than or equal to 3.65 volts
and the charge amperage is within spec
What is causing the loss of a cell?

Perhaps this is a solvable problem?

 

[John Frum]

What happens to the current flow through the rest of the cells if one gets full first?
To answer my own question the other cells should draw charge from the high one until they are equal.

UPDATE: the current flow is not through the cells.

 

[JoeHam]

If you put $n cells in parralel
and charge with a voltage of less than or equal to 3.65 volts
and the charge amperage is within spec
What is causing the loss of a cell?

Perhaps this is a solvable problem?

Or perhaps the cure is worse than the disease?

My question always has been:

What quantifiable benefit is there to top balancing new, well matched cells?

 

[John Frum]

Or perhaps the cure is worse than the disease?

My question always has been:

What quantifiable benefit is there to top balancing new, well matched cells?

The cells should be matched by internal resistance.
That doesn't mean they will be matched by SOC.
Usually they are well matched by SOC but not always.
At 30mv it can take a very long time for cells to balance especially if they are only being balanced at a high SOC.
So the benefit is more capacity. You mentioned 10% it might not be that much even.