Sunday, November 9, 2014

Bottom Balancing

While the Porsche is over at Pro Automotive getting prepped for the conversion, I have plenty of time to get the battery pack ready. I'm following the EVTV battery bottom balancing routine to get each cell to nearly the same voltage in a discharged state so that capacity variances are cancelled out when empty. This eliminates the need for an active battery management system with its associated safety and cell damage issues.

Since each of the 96 cells will get three passes with different discharging profiles, this is the setup I'll be using to automate the process. The red and black device is a Revolectrix  Cellpro PowerLab8 which controls and monitors each discharge cycle. Behind it is an old 12 volt lead acid battery, actually the starting battery removed from the eBugeye when it was converted back in 2010. The PowerLab8 will dump the current from the Lithium pack into the lead acid battery as it bleeds it off from the cells. The Dell laptop is USB connected to the PowerLab8 to give a more friendly interface. This was a MAJOR concession on my part. Anyone who knows me knows that I have a sincere and monumental aversion to anything Micro$oft, but the PowerLab8 software is Windoze-only, so there it is.

I found after doing a couple of cells that the lead acid battery couldn't accept any more charge, so I added a 12 volt AC inverter that lights a desk lamp to bleed off just enough current to keep everything working. You can see the inverter behind the laptop next to the battery.

I plan to keep the cells in the three bank configuration, so with a little help from my new engine hoist, I'm getting them in position next to the workbench and in reach of the cables.

The process is like watching paint dry. The first pass discharges at 25 amps down to 2.75 volts per cell, and takes roughly an hour and a half. The second pass at 10 amps once again finishes at 2.75 volts and takes about a half hour per cell. The final pass uses a CCCV (Constant Current/Constant Voltage) profile that takes the cells down to 2.75 volts at 5 amps, then holds them at 2.75 volts as it reduces the current until it reaches about .35  amps. All of that times 96 cells, so It's a good thing that the car is still in the shop.

Jack Rickard has reported that these cells are a bit "squishy", they keep bouncing back up around three volts at rest. I'm shooting for three volts plus or minus a few thousandths. The first bank seems to be settling in between 2.988 and 3.04. We'll see where they are after a week or so.

Stay tuned for the next installment with something a bit more interesting ...

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