Where is the chart for before charging--were they all exactly at 2.8?

Need to let them sit for 24 hours in order to read the Open Circuit Voltage (OCV), which is the voltage with no load or charge--the resting voltage.

 

Hi Kennybobby

They were all at 2.89.

I discharged them all together in paralell to 2.89 then disconnected them and let them sit for four days. Most did self-discharge, about 1mv per day.

I regrouped them so that the ones that were staying at 2.89v were in one group and did not charge that group. It looks like all the ones that retained their charge are at 3.13.

The others I grouped together in bands of +/-5mv. Then I charged each group in increments until they reached 2.89, which may why they seem to be in groups at specific voltages.

 

You didn't indicate what size cells these were, i.e. cell capacity A-hrs, but it looks like you only added about 15 A-hrs from 2.89 and the cells are resting up near 3.30 to 3.31. Full cells would be resting at about 3.33 to 3.34.

When i have bottom balanced and charged cells, i set the current and measure the time with a stopwatch. This is coulomb counting and you can know exactly how much you have discharged and charged in order to compare this with your rated cell capacity. i doubt that 2.89 is low enough to be considered 'bottom', but it may not matter if all the cells were within a few mvolts.

Measuring current and time is more important in my view to know the state of charge--i.e. don't try to put in more than the cell is rated to hold. The purpose of bottom balancing is to take the cells down to a somewhat known 'empty' level, e.g. somewhere around 2.5. Then add a known A-hrs worth of coulombs back into all of them at the same time while monitoring voltage such that none runaway. This also gives you a way to compare the capacity of cells over time to determine health and capacity degradation.

Hope this helps, good luck to you...

 

If any of your cells continue to drop in voltage with no connection to them then those cells are not going to work in a bottom balance situation. (I don't suspect a self discharging cell will continue to work very long in any situation, but top balanced with shunt regs you could keep them in balance until one fails.) The point of bottom balance is that they all run out together. Any self discharging cells will run out sooner by the amount they have self discharged.

Since I run no reg top balance I cannot really comment on what represents acceptable balance at the bottom.

 

I did these two measurements this morning.

but top balanced with shunt regs you could keep them in balance until one fails

I have a BMS but it has no internal fuses so I'm nervous about using it. It doesn't shunt though so maybe the fusing isn't such an issue. I guess I can do some serious trunking to protect the wires.

Right now I just want to see what capacity the better cells have, in the hope I can get 20km range or so out of the pack.

 

What would you consider an unbalanced situation?

What difference between voltages should we expect? And this difference should be noticed during charging/discharging or when resting?

Thanks

 

If you bottom balance such that all cells are within a few thousandths when below 2.9 volts you will have a pretty good balance.

As you add energy to the cells the voltage of each cell will rise in relation to their state of charge. The cells with the lowest capacity will rise the fastest. The cell that reaches a voltage exceeding 3.4 volts is most likely the weakest of the lot. The farther you get from the bottom balance point the greater the spread in values you will see because every cell is going to have a little different capacity unless they were all selected for capacity. Under the conditions we normally operate our batteries the maximum usable capacity of your pack is defined by the capacity of the weakest cell in the pack times the number of cells. If the cells are all in pretty good condition and the pack is at about a 50% State Of Charge (SOC) there will be about a 0.0004 volt difference for a 1% difference in state of charge. That number is specific to LiFe type batteries. So if the pack is in good condition and the pack is well balanced you will need a better than normal meter to even see the difference in SOC. The farther you get from center the greater the voltage difference will be. When the first cell reaches 3.4 volts you will see much greater differences in voltage for a difference of 1% SOC.

And to answer your question an unbalanced situation depends on your goals. If you are trying to run the battery until there is nothing left you are interested only in a bottom balance. This will protect the cells and they will all go dead at the same time making it unlikely that you will damage any cells through cell reversal. In the other case if you are trying to extract the maximum energy from the pack a top balance is in order. In this case every cell will be full at the same time when charging. You will get maximum energy from the pack because the average voltage of the pack will be a little higher with a top balanced pack. In this case trying to drive until the car wont go anymore will result in one or more ruined cells because of cell reversal.

Hope that was informative.

 

If the cells are all in pretty good condition and the pack is at about a 50% State Of Charge (SOC) there will be about a 0.0004 volt difference for a 1% difference in state of charge.

THanks for the reply and explanation! 0.0004V is a voltage pretty diffiult to measure. My center multimeter precision voltage is 1mV. And the DAQ's I'm using is something like that also...

But what voltage difference between the cell can we be sure that means an unbalanced situation? 0.1V during charging/discharging (under use) or 0.1V when resting for some hours?

The voltages I'm registering under load differ 0.023V between cells. But the daq's have an error associated which is less than 0.01V and I'm pretty sure the cells are not unbalanced... Even the BMS doesn't have a precision of 0.01V but has an error associated too.

 

Hi Sabahtom
After you bottom balance you left them and then measured the voltages

I would advice leaving them some more measuring the voltages every two days

If they stabilize - then you are good

Any cells that keep losing voltage probably means they are about to fail