Hi or low balancing and how often?

[Bob B]

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.

I have seen some of the posts you are describing.

There are a lot of different battery types used. The cells you used are top end cells and may be better matched . Others may be trying to get by with cheaper cells that may require more TLC.

When top balancing taking the cells up in stages .... and using a meter to verify your equipment voltage and being very careful at the top of the range would be the prudent.

 

[JoeHam]

The cells should be matched by internal resistance.
That doesn't mean they will be matched by SOC.

And honestly I’m not trying to be difficult but I just don’t see a quantifiable benefit.

Shouldn’t there be a study that says if you top balance new cells that arrive in good SOC match you can gain 10% in capacity or lifetime or some other metric if you top balance?

 

[John Frum]

Maybe the question is what benefit is there to top balancing

And honestly I’m not trying to be difficult but I just don’t see a quantifiable benefit.

Shouldn’t there be a study that says it you top balance new cells that arrive in good SOC match you can gain 10% in capacity or lifetime or any metric if you top balance?

If the cells are well matched in SOC and internal resistance then top balancing should not give much(if any) increase at all.

 

[John Frum]

The thing is you can't tell if the SOC matches when the cells are in the flat part of the curve.

 

[Pierre]

This whole thing about cell balancing is over rated , but what do I know

 

[Ampster]

My two cents. I can't assume my cells are well matched without doing a capacity test on each cell. That will take 32 days. I am very familiar with charging in parallel and the risk for me is 99% mitigated. The 1% is operator error but I have checks and balances in place in the form of risk management plan that includes no drugs or alcohol before 5PM.
So if it takes me as much as 5 days to top balance that is still a six to one advantage.
YMMV and you are free to choose which approach fits your indifference curve.

 

[John Frum]

I am very familiar with charging in parallel and the risk for me is 99% mitigated.

I'm not so sure I understand the risks.
If you charge one cell at 3.65votls and .2c.
I'm not sure what could go wrong other than you have a bum cell and it fails a valid use case.

Now if you have for example 16 cells in parralel and you charge at 3.65votls at .2c aggregate.
What happens if one cell gets to 3.65 volts way way way behind the others does it get charged at 3.2c amperage and bloats from the resultant stress?
If that one of the failure modes?
Are there others?

 

[Ampster]

This whole thing about cell balancing is over rated , but what do I know

I think the puffy thing is over rated. I can only speak from experience having puffed some Nissan Leaf modules by leaving a golf cart for weeks with a Vampire load. I have had a few other experiences with puffies, but I guess it all depends on where you are standing.

 

[Ampster]

What happens if one cell gets to 3.65 volts way way way behind the others does it get charged at 3.2c amperage and bloats from the resultant stress?

It can't happen in parallel that one cell gets to 3.65 volts before the others. There is a law against that but it wasn't Ohm's.

 

[John Frum]

It can't happen in parallel that one cell gets to 3.65 volts before the others. There is a law against that but it wasn't Ohm's.

Ok so far we have identified 0 valid use cases where good batteries will snuff it during top balance?

 

[Ampster]

Ok so I have so far identified 0 valid use cases where good batteries will snuff it during top balance?

I vote for operator error. Just the other day I was impatient about getting started so I hooked 8 cells in parallel to my power supply. I checked the power supply and it was set for 3.8 volts and 30 Amps. I watched them creep up slowly to 3.4 then 6 hours later to 3.5 then 4 hours later they were at 3.7. Not a puffy in sight but I did put a load on them to pull them down to 3.5.
I didn't follow my usual procedure of putting my logging powermeter on them, which might have given me a clue. Or I could have taken another minute to find the little screw on my Meanwell power supply to dial down the voltage to 3.5 volts which was my target for that phase of my 3 phase top balancing process.
Did I say I was impatient?

 

[JoeHam]

Ok so far we have identified 0 valid use cases where good batteries will snuff it during top balance?

But have we documented any quantifiable benefit?

 

[John Frum]

But have we documented any quantifiable benefit?

If the cells are out of balance then the pack capacity will be diminished.
And its proportionate to the imbalance.
And since you can't determine the the soc by voltage in the flat part of the curve many choose to top balance just to be sure.
If one cell was at 3.0 volts and the rest where at 3.45 would you then balance the cells?

 

[John Frum]

But have we documented any quantifiable benefit?

Just to be clear.
If there is no benefit then top balancing is a waste of time.
I think the point of top balancing is they are synchronised at the top.
The may diverge as they are discharged but should converge(I think) as they are charged.

 

[JoeHam]

If the cells are out of balance then the pack capacity will be diminished.
And its proportionate to the imbalance.
And since you can't determine the the soc by voltage in the flat part of the curve many choose to top balance just to be sure.
If one cell was at 3.0 volts and the rest where at 3.45 would you then balance the cells?

That’s why I keep qualifying that I am talking about new, well balanced cells.

People have killed new, well balanced cells trying to top balance and my thinking remains that it’s all risk and no reward in this particular situation.

 

[JoeHam]

Just to be clear.
If there is no benefit then top balancing is a waste of time.

Exactly.

For new, well balanced cells.

 

[John Frum]

Apologies I got caught in a loop.
Will reboot.

 

[Ampster]

People have killed new, well balanced cells trying to top balance and my thinking remains that it’s all risk and no reward in this particular situation

Yes, you are correct, in your particular situation. As for the OP she may be in a different situation. It all depends on where you are standing.
@SherylinRM did you get your question answered?

 

[Pierre]

In this whole saga with keeping batteries healthy my approach is to have a decent programmable BMS hooked up and so far my experience is that normally cells track within close tolerance on the flat part of the curve. The BMS then looks after errant runaway cells at the top and bottom. My power spectrum will thus operate between these set limits , mostly realizing 80-90% battery capacity. In most BMS’s the cell balancing current is a non-event anyway.
The problem with a non-programmable BMS is that one does not know what goes on inside the brain and one is forced to trust the manufacturer’s setting and also the cells will have to be inspected / measured every few weeks. Many people on this forum are not experimental by nature nor have the equipment / knowledge / inclination to delve into the depths of the vagaries of battery technologies.
Thus , buy wisely when you enter the realm of this ‘hobby’ .

 

[Deleted member 9967]

@SherylinRM did you get your question answered?

Yes but not here so much.
Ok so what I have sussed is this.

I am hoping to buy eight 3.2 volt 320Ah batteries and to put them in series and then top balance them as they come at only 50% full because it is not good for LiFePo4 batteries to be floated.
After charging them up fully to their safe maximum. I will then use them on my mobility scooter for a long enough to get the power down a bit.
Then I will use the bluetooth app to reset the top and bottom voltages to make sure that they do not go all the way to the top nore the bottom before being cut off.
The BMS will then act like a guard against too much or too little charge.
This will ensure a very long life for the batteries and because I am getting such large batteries I can get by with using a little less of its power.

Once every 6 months they can be charged [top balanced] fully if I use them a lot and/or if there appears any anomalies. Other than that. Once a year should be sufficient to top balance once again.
Doing this, the batteries shuld last a minimum of 15 years and even 20 years or more with a maximum loss down to 70% or more probably only 80%. And this is more than enough to take me about 50 miles round trip. So that is sufficient for my needs.

Ok so there you go then.

Thank you to everyone that participated in this discussion.

 

[Ampster]

50 miles is a long way. Based on using 5kWh to go 50 miles that is 100 Whrs per mile or 10 miles per kWh. That is a lot better than my Tesla and within the ballpark for my ebike. Hills and drag racing will reduce that.

 

[madmax]

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.

With that said, how come so many recommend daly bms? They seem to have 0 adjustability

 

[ghostwriter66]

With that said, how come so many recommend daly bms? They seem to have 0 adjustability

Becuase they are cheap and relaible ... many ppl just want a BMS to shut the battery off when the charging is done and disconnect it if it goes into a low voltage situation ... Most of the time a DALY BMS will get you pretty near the 95% charge and take you down to about the 7% -- and for many -- thats fine ...

DALY also is NOW making pretty sweet models that has bluetooth and are adjustable ... well worth the money

As for many of us that do this for a living I like the CHARGERY and the DACUS (I always spell it wrong) ... they are higher end - but can do virtually anything you want it to .. I have my Chargery actually fire up a Generator when the battery gets low -- then it sends me a message what its doing ... plus 1000 other things ...

 

[Deleted member 9967]

50 miles is a long way. Based on using 5kWh to go 50 miles that is 100 Whrs per mile or 10 miles per kWh. That is a lot better than my Tesla and within the ballpark for my ebike. Hills and drag racing will reduce that.

Well it does go 5mph actually. I want to eventually get a 10 mph or 16 kmp motor. Ironically it will use the same power to go the same distance.
It works like this. The motor would use twice the power to go twice as fast. But because it goes twice as fast, in the end it will go the same distance.
Just that I am not interested in spending 10 hours to go and come back is all LOL. 5 hours is not great either, but better than ten hours.

Now it works out like this.
24 volts times 320 Ah divided by the motor [650 watts] times the speed give me.
24 * 320 / 650 * 5 = 59 miles distance.
And yes, it is hilly here so deduct from that. But that is fine

 

[Deleted member 9967]

With that said, how come so many recommend daly bms? They seem to have 0 adjustability

Well everyone that I have seen so far you can adjust the top and bottom voltages and also even the temp cut off on some. So not sure what that is about.