40-Amp testing a Miev Lithium Cell

by Ben N on February 25, 2013

ben02_cell06_40amps_charged to 4dotzeroV Ahr ben02_cell06_40amps_charged to 4dotzeroV Volts_VOLTAGE

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 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!




{ 4 comments… read them below or add one }

1 Jarkko Santala February 25, 2013 at 11:39 am

Don’t know about these cells specifically, but usually most of the charge is not in the last bit of the charge, but during the flat part of the charge curve. Getting 26 Ah at 4.0 V and 44 Ah at 4.2 V smells suspicious to me. I’d repeat and see what happens on the next go.

You could try paralleling the revived and 0 V cells and see if you can charge them together that way. I wouldn’t place my money on getting 0 V cells to perfect working condition though. Let’s hope I’m wrong. 🙂

2 Don Dakin February 26, 2013 at 7:07 pm

This is a great project. You are a brave and adventurous pioneer.

It’s like watching iMiev CSI…

Thanks for the videos and great documentation. Great job

I really hope you are able to get the car going because I really love mine. It’s a great EV.


3 JoeS. February 26, 2013 at 8:13 pm

Ben, thank you for performing this test. Looks like you may have some good cells and maybe you’ll be lucky at reviving some of the dead ones as well. I tend to agree with Jarkko, as there is typically very little energy in the cell between 4.0 and 4.2v as you’re leaving the flat part of the curve and it climbs fairly fast over time when charging and drops quickly when discharging between those two points. Mitsubishi conservatively specs 3.955v as their fully-charged voltage (ref: http://myimiev.com/forum/viewtopic.php?p=7380#p7380)

4 Ben N February 26, 2013 at 9:21 pm

Thanks Joe,
The next test I ran was taking the pack up to 4.1V. Maybe I’ll run future tests at 3.955v. Sure seems like a good idea for longevity. Would you happen to know what Mitsubishi uses for the BOTTOM voltage on these cells?

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