Today I started the battery balancing procedure. In order to keep all the lithium cells at the same charge level, so that no single cell is overcharged during charging or over-discharged during driving, they have to be belanced. That is be at the same level of charge. After they are balanced I can charge the whole pack together.
The balancing procedure I use is called bottom balancing. This means that the cells are given the same charge level by discharging them to the same resting voltage close to empty. To do this I put together a "bottom balancing aparatus" consisting of a Powerlab 8 battery workstation, a 12V lead acid battery, a 12V heater fan, and a PC with a control software.
The "bottom balancing apparatus"
The Powerlab 8 discharges each cell to a voltage of 2,75V using a constant current/constant voltage schema. This means that it starts discharging each cell at a constant 40A until the cell reaches a voltage of 2,75V. If I would just stop discharging at that point the cell would recover and the voltage would quickly rise. Therefore the Powerlab successivley lowers the current to withhold the 2,75V and it does so until the current has dropped to about 4A and then the procedure stops. At this point the cell voltage starts to rise but not so much it would have done if I would have only used the 40A discharge rate. When I measured one cell about 10 hours after discharge the voltage hade risen from 2,75V to 2,97V. I guess that when I have done all the cells I might have to make another round with a lower current, maybe starting at 4A, to get them even more balanced.
So what is the lead acid battery and electric heater fan for? Well, when discharging a cell the energy has to go somewhere. So the Powerlab, while discharging a lithium cell, charges the lead battery, which is also the power source for the Powerlab. And the lead battery will eventually become fully charged. So the heater fan discharges the lead battery. In effect, the energy from the lithium cells are transformed into heat that warms up my garage! So far I have a manual switch on the fan and I turn it on when the battery gets close to full and off when it gets close to empty. An automatic thing for this would be nice!
The PC software tracks the discharge procedure and makes pretty graphs.
The discharge of the first cell which was first charged to 3,5V
The very first cell I charged to 3,5V before discharging to see what capacity it had. My first idea was to do that with all cells, but it just takes forever! The charging took about 2 hours and then the discharge another 2,5 hours. So after that I only discharge the cells. This takes about 1 hour 20 minutes per cell so balancing all 68 cells will take a couple of days!
I charged the first cell to 3,5V which is
not fully charged. And then I discharged it to 2,75V, which is
not fully discharged (2,5V is supposed to be fully discharged). And I manged to discharge 109 Ah out of the cell which is almost 10% more than its nominal capacity of 100Ah. Looks good! I hope all the other cells are as good. I will make sure no cell has a substantially lower capacity than the others by frequently measuring all cells voltages during the first charge of the whole pack.
