Today, I was able to do a complete charge cycle of a cell from the i-Miev at 40 amps. Using the CellPro6, I changed the charge preset from a generic LiPo to one used for A123 cells. That setting allows charge and discharge rates of up to 40 amps. I looked through the preset and changed it to charge up to 4.0V (the max allowed on this preset) and the maximum discharge down to 3.0V.
I had previously charged the cell to 4.2V. Once my cycle was complete, I saved and exported these two images from the graphing portion of the charger software. As you can see on the voltage chart, it starts at 4.2V, gets discharged to 3V and then charged back up to 4.0V. If you look at the Pack Capacity chart, you can see that when the battery was charged to 4.2V, the charger read a capacity of 44AH, but when charged to only 4.0V, the capacity is only 26AH. Lithium isn’t exactly linear to voltage is it?
Anyways, I’m pretty excited to see that I actually CAN charge and discharge this cell. While charging to 4.0V doesn’t exactly give a true reflection of pack capacity, being able to charge at a rate of 40A instead of 10A cuts my testing time from 10 hours to 2.5 hours! (I was also monitoring temperature during this test. The 12ga battery cables get a little warm from that much current. The cell itself only got very slightly warm to the touch.)
My next challenge is to safely bring up other cells from 0V to at least 2.55v (the minimum the charger requires to sense a pack is connected.) So far, two options that may be possible are to use a cell phone charger to very slowly bring up the voltage at low amperage or to perhaps parallel a bad cell with a good one with a resistor to limit current to something fairly low. If you have some good ideas to safely and reliably bring up cell voltage to the point that I can use the smart lithium charger on it, please let me know!
