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How much faster is top balancing cells individually?

kolek

kolek

village idiot
Joined
Sep 29, 2021
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I believe @Steve_S said somewhere it's faster charging individual cells than groups of cells.

Meaning, for example, it's more than 4X faster charging a single cell than a group of 4 cells.

Is this true, and how much faster is it, roughly?

Also, related question, I'm using 18GA wire for charging, is this thick enough, and how much faster would using thicker wire be? I'm only charging at around 4 amps with the 18GA wire.
 
Hi Kolek,

Charging each battery individually will be slower because you'd have to switch each time from one battery to the other, you can't do that when you are sleeping. As shavermcspud pointed out, you will be transferring the same amount of energy in total. Now unless the batteries are seriously out of sync, or you are not interested in testing the capacity, top balancing is not strictly needed as the BMS will balance the pack over time anyway. And you can even manually balance them as well when they are in the pack, but I would recommend against that as it requires you to switch the wires every time the cell you are topping up is "full".

18 Gauge is fine assuming it is copper wire, check if it is getting warm by hovering your hand over it.

I hope this helps.
 
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Only benefit to charging individually is if you want to log all of the data for each individual battery.. capacity, curves, etc.

Otherwise just parallel all of them and charge it up for a week+

kolek said:
Also, related question, I'm using 18GA wire for charging, is this thick enough, and how much faster would using thicker wire be? I'm only charging at around 4 amps with the 18GA wire.
Click to expand...
no if you are using only 4 amps that wire size is fine and won't make a difference

depending on the outer coating of the wire it may get too hot for it though

kolek said:
Meaning, for example, it's more than 4X faster charging a single cell than a group of 4 cells.
Click to expand...
Yea that is true.. because 1 cell = 1 cell and 4 cells = 4 cells
thus it is 4x faster to charge 1/4th of something
I think you got confused in what you read
 
No guys, I think it's more than 4X faster to charge a single cell vs 4 cells because... battery impedence, Ohm's law, Moore's law, the 3rd law of thermodynamics, Spock, the singularity, and biggest reason of all, pretty sure that @Steve_S says so.

But don't quote me on that.
 
My advice - ask your cell manufacturer (not reseller), how they balance and match their cells and follow what they say.
 
interesting topic - i always thought we 'top balanced' to make sure all the cells were taken to their capacity together and were therefore balanced against and with each other ... would this be accurate if cells were brought to their 'top' capacity individually?

for my battery, i 'top balanced' one lot of 8 cells, then the next lot of eight cells and finally linked them all together and did it again (obv took a lot less time as they were all pretty much already sorted) - eight months down the line, they are all pretty much in sync !!
 
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MisterB1959 said:
interesting topic - i always thought we 'top balanced' to make sure all the cells were taken to their capacity together and were therefore balanced against and with each other ... would this be accurate if cells were brought to their 'top' capacity individually?
Click to expand...
Of course it is an accurate method. I’d say over 99% of batteries have their cells balanced by individually charging to a known setpoint.
 
MisterB1959 said:
interesting topic - i always thought we 'top balanced' to make sure all the cells were taken to their capacity together and were therefore balanced against and with each other ... would this be accurate if cells were brought to their 'top' capacity individually?
Click to expand...
yes... the reason for top balancing is making sure the SOC of the cells is the same..
There is 1 way to do that properly , given the flat curve of LifePO4 ..
Top balancing -> fill all cells to 100%

it doesn't matter if one does that individually or as a group.. the point always is getting the SOC to the same level as voltage is not a good measurement to determine SOC on lifePO4
 
I have been invoked for comment...
I top charge cells to 3.650V and do them in batches of 4 cells at 1 time. This is because I am working with 280AH cells at this time.
I was doing LF105 cells with 8 cells in one go.
I use a TekPower TP1540E Bench Power Supply
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When the First Top Charge is done to activate the electrolytes, take them up to 3.650V and hold there till the Amps taken drops to least the Endamp value. Endamps = ( Cell AH X 0.05 ) or 280AH X 0.05 = 14A. With Top Charge you can go lower down and cut off @ 50% of Endamps Value which would be 7A for 280AH cells.
NOTE: The Power-supply to Battery cells wire is critical, I use 8-AWG Silicone Fine Wire with a Terminal Lug (no fricken clips!)

Upon charge completion, the LFP cells WILL settle as expected from the chemistry. They should settle to just above 3.400V within 2 hours +/-.
If doing in batches, like I do, I do each set, then when the entire set is done, I assemble it all together in Parallel and do one last top charge of the whole pack and then proceed to build the pack.


quick-voltage-chart-lfp-jpg.150247
 
Here's how I charge them for the first time... I can figure them in a 4S configuration and hit them with pretty much anything that can deliver close to the right voltage and high current

It would not hurt to attach a BMS at this time although it's not completely necessary

During this process I monitor each individual cell

But again a BMS could easily handle this by letting it hit individual cell high voltage disconnect

Once you reach high voltage disconnect on a cell simply stop charging (obviously if you're doing this manually you would need to stop a little earlier)

Now you should be somewhere around 95% charged or better for the entire bank

Reconfigure in a parallel configuration and hook up a bench power supply adjusted to 3.65 volts with either five or 10 amps of current

Allow it to continue charging until the current drops to 0.00 amps

Now let it hold there for a few hours at least... I prefer to let it hold there overnight personally

I like to allow it a small resting. And then repeat this last step

This is a super quick way to do it and you still get all the benefit of a top balance starting from 3.2 volts...
 
There is a difference between Top Charging cells in Parallel to building a 4S pack with BMS and top charging it that way.
Sadly, so few RTFM and think they know more than the engineers & chemists.
 
If you discount the time connecting and disconnecting it should be quicker. When top balancing groups of cells you always have to wait for the slowest cell to get to the top. Also it could be said that theres a detrimental effect to holding the other cells near the top voltage for long periods. I think doing them in batches of 4 is probably a good balance. It does depend on your charger amps though.

You would always finish off by charging them altogether but this should be a quick affair if they are all fully charged individually.
 
Steve_S said:
There is a difference between Top Charging cells in Parallel to building a 4S pack with BMS and top charging it that way.
Sadly, so few RTFM and think they know more than the engineers & chemists.
Click to expand...
If that is directed at me, I think you need to reread what I wrote...

I build the pack in 4S

Do a quick charge to near 100%

Then reconfigure into parallel... And charge to 3.65 volts and hold it there overnight

PS... If you're not directing it at me I apologize... If you are directing it at me I would say, "sadly, so few people have reading comprehension skills... ".


Also either way... No hard feelings.
 

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squowse said:
If you discount the time connecting and disconnecting it should be quicker. When top balancing groups of cells you always have to wait for the slowest cell to get to the top. Also it could be said that theres a detrimental effect to holding the other cells near the top voltage for long periods. I think doing them in batches of 4 is probably a good balance. It does depend on your charger amps though.

You would always finish off by charging them altogether but this should be a quick affair if they are all fully charged individually.
Click to expand...
Yeah but there's something to think about here

When you have a bunch of cells in parallel the current is going to travel where it's easiest for it to travel and that is going to be the cell that is the lowest on charge overall

In fact if you have a bunch of cells in parallel and one cell that is lagging behind a little bit the other cells are also going to be transferring current to it

Ultimately it comes down to there being a differential between those cells and the resistance increasing is voltage goes up on this chemistry
 
kolek said:
I believe @Steve_S said somewhere it's faster charging individual cells than groups of cells.

Meaning, for example, it's more than 4X faster charging a single cell than a group of 4 cells.

Is this true, and how much faster is it, roughly?
Click to expand...
I mean faster than what? Of course it's faster to charge a single cell than multiple cells in parallel, all things being equal...

But at the end of the day if you start out with four cells with a total aggregated charge of all cells working out to say 60% just to throw a number out there... Then it wouldn't matter if one was at 55 one was it 65 and the others at 60...

You still have to charge the same amount... Whether you split it up between one salad a time or all the cells

I think charging the cells individually to to 3.65 volts does not necessarily get them in a proper balance... They'll be close Yes... But allowing them to be in parallel allows some current sharing between individual cells in that same parallel string

And this allows them not only to naturally balance between each other as they're setting with no charge being applied but also allows each individual cell to naturally increase in internal resistance allowing the cells that are lagging behind to get more current until they all reach 3.65 volts and hold there for a while
 
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