How close should voltage be after top balancing?

[abwillingham]

After top balancing is compete, how close in voltage should each cell be? Should they be exact, down to the mV? Or is it common for each battery to be off by some percetange?

Thanks
Tony

 

[MisterSandals]

Or is it common for each battery to be off by some percetange?

Are you top balancing batteries or cells?
Are you measuring with them removed from parallel connections?

 

[abwillingham]

Top balancing 4P. Measured either way. Which is the preferred way to measure, connected in P or individually?

 

[MisterSandals]

Top balancing 4P. Measured either way. Which is the preferred way to measure, connected in P or individually?

Sounds like you are using cells connected in parallel.
If you measure them still connected in parallel, you won't get an accurate cell level voltage.

How high did your charge your cells?
How long did they settle?
What voltages are you seeing for each cell?

Without any info about what you have, what you did and what you are measuring, its hard to give you any advice or recommendations.

But to answer your questions, i'd say:
fairly close, no, yes.

 

[abwillingham]

Let me rephrase the question. When top balancing 4 cells in P, doing everything exactly has it should be done, how close in voltage should the individual cells be after completion, measured the proper way?

 

[OffGridInTheCity]

LifePo4 has a very flat charge/discharge curve and lithium-ion is not nearly as flat - this makes a difference in the discussion.

In the Lithium-ion case - the 'allowable' delta between cells is really a function of what is OK for your operation as long as the individual cells are healthy (e.g. no self-discharge for example).

As an example - I have 84 packs of 260ah each and I don't bother to balance any closer that 0.04v - e.g. 40mv. During the cycle, the packs diverge up to 80mv max difference and then back to 40mv - and repeat this every day. This is OK since I operate within 4.0v/cell max and 3.54v/cell low. The highest a particular cell would get is 4.08v and on the low side 3.46v - well within safe/long-life operating ranges. It's been 6 months since I've last balanced - and this is the current view mid cycle today - perfectly OK / lack of balance shows healthy cells.


For LifePo4, I understand its better to have 'very close' top balance - more like 10mv max? - because you can't balance mid-discharge curve. But even so as long as your cells are healthy and no 'extreme cell' gets to dangerous/short-life min/max of the series - a little bit of difference is OK day to day to day.

This topic can be one of those OCD 'things' where some focus on continuous, perfect balance when - to me - its really about
1) Am I operating with healthy cells - e.g. no self-discharge, sagging, are they maintaining day to day the same.
2) Am I operating safely / getting the power out of the battery than I want consistent with life-span expectations.

 

[time2roll]

They may settle at different rates and come to a rest at different voltages. Less than 10mV is fine, less than 20 is OK too. Post your actual results for best comments.

The real tell on balance will come after a few cycles that the cells stay in balance at both the bottom and top.

 

[John Frum]

They may settle at different rates and come to a rest at different voltages. Less than 10mV is fine, less than 20 is OK too. Post your actual results for best comments.

The real tell on balance will come after a few cycles that the cells stay in balance at both the bottom and top.

When commodity cells are top balanced its not reasonable to expect them to stay balanced to the bottom.
Unless you are power ball lucky, the capacity of the pack will be determined by the weakest cell.
That cell will nose dive into the low knee before the others and hit 2.5 volts first.
Hopefully the cells will re-converge as the pack is re-charged.
The cells tend to stay better balanced if the weak cell is not drawn down too far.
By too far I mean that you should stop discharging when the weakest cell hits ~3.0 volts.
That will determine the usable capacity of your pack.

 

[OffGridInTheCity]

When commodity cells are top balanced its not reasonable to expect them to stay balanced to the bottom.
Unless you are power ball lucky, the capacity of the pack will be determined by the weakest cell.
That cell will nose dive into the low knee before the others and hit 2.5 volts first.
Hopefully the cells will re-converge as the pack is re-charged.
The cells tend to stay better balanced if the weak cell is not drawn down too far.
By too far I mean that you should stop discharging when the weakest cell hits ~3.0 volts.
That will determine the usable capacity of your pack.

Here's a picture of this for lithium-ion - but I believe it applies to LifePo4 as well in it's own way.

In the spring, I was fooling around seeing how many days I could run the house 'continuously' without shutting off the inverter. To do this, I lowered the cut-off voltage on my pack on a couple of those days and when I took it below the normal 3.5v cut-off down to 3.35v/pack on a particular day - this happened. Clearly, 3.35v is on the loosing side of the lithium-ion discharge curve knee for my cells.

Notice 4 packs *sagged* by 1.7v (170mv) !!!

However, once I returned to normal operation with 3.5v cutt-off, all returned to normal as show above a couple of posts above and the balance returned to be within 0.04v and 0.08v thru the day - I did not need to 'rebalance' anything.


If you look at a discharge curve, once you past the 'knee' smaller and smaller amounts power will dramatically affect the voltage. Some cells may be a bit different than others and it will dramatically 'show up' at this portion of the discharge. This does not necessarily mean that you have bad cells - just don't discharge down that low .