openinverter forum

If you were writing code for a BMS, how would you define the battery pack as being fully charged?

Counting coulombs in and out would give you a good idea of SOC, but after a time an amount of drift will happen due to losses and you may not be charging to full or you could be overcharging and need to protect the pack.

Given that a cell maximum is 4200mV, would it for example be right to say if the highest cell voltage is 4180mV (20mV safety margin) and the lowest cell is 4170mV (10mV balancing tolerence) the pack is charged? Is this too simple?

Any ideas are welcome.

100%, once your top cell hits charge voltage limit, which I would set atleast 50mV below overvoltage limit.

This is where you set you 100% charged, so SOC calculation/coulomb counting reset. Then count down from there.

You can then do all sorts on the way down like monitor how much drift there is between cells and how much energy goes out before hitting or getting near the cut-off voltage limits. Using these variables things like amphour capacity compensation and state of health.

Do you not need to account for intercolation effects? I.e., the charge needs time to evenly distribute itself within the electrode. Can't the voltage drop as the battery 'rests/settles'?

Or because you're in the constant voltage/constant current (or constant voltage/constant power) phase of the charge cycle as you reach 100%, the interlocation effects are small.

Thank Tom and Christain,
Yep, I was only going to check the high cell voltage once the balancing had been paused and I did wonder if you need to wait a set time after charging to see if the cell voltage has relaxed a little and need to restart charging.

So I was thinking
Always check cells do not go overvoltage, if so Charger is turned and an error is recorded.
Start balancing at 4.05V
Pause balancing and read the highest cell voltage is not over the charge limit say 4.15V
if over or hit charge limit stop charging and re-test over a two-minute period, restart if drops below.
if below continue balancing and charging. repeat.

Of course, this is only to get to 100%, otherwise, the SOC count is used.

I would not charge to more than 4.1V per cell.
That said, yes you need to leave some time for the charge to sip through otherwise it is just the V raise because of internal resistance.

So if you use charge from 3.2V to 4.1V that IS your SOC count. 3.2V would be 0% and 4.1V 100%. Everything below and above will mess with your estimation of Ah content.
Or you can say you have 90Ah cells, but from 3.2V to 4.1V you consume 82Ah which makes that 100% of your SOC.
In winter however your cell will be able to provide only 75Ah which again makes 100% of your winter SOC...
Is it any clearer?

...NOT!

Arber333, thank you, yep clear. Some info I was looking at suggests taking the cell higher, but looking at other graphs they is no point in going higher than 4.1V.

Thank you all.

Just to reiterate what others have said here, the best option is almost certainly to charge until the top cell is at 4.1V. The documentation that came with my cells considers 4.1V to be 100%. The stress that charging above this point puts on the cells very likely isn't worth it for the extra capacity you might gain. You might want to charge a little over 4.1V so that the cells *settle* to 4.1V when the charging stops, but it's simpler just to stop at 4.1V and call it done.

One thing to keep in mind also is the cycling of charging.

The SimpBMS code requires a 200mV as standard drop of highest cell before it clears the charge blocker. This is adjustable in the settings, the idea behing this is two fold, in a stationary application keep the charge request from bouncing due to voltage sag on cells post charge AND to keep you from over filling the cells by continuously trickle charging them.