This is the third Better Place Module that I have had the privilege to bottom balance and must admit, after seeing the aftermath of the fire at EVTV, am being a bit more cautious. When bottom balancing the Better Place Modules, they are always on a furniture dolly, so they can be quickly wheeled out into the driveway should anything go wrong.
As I described in my last post, the process is started by finding the lowest cell and then hooking up two 1000 watt halogen flood lights and bringing them down until the lowest cell is 2.9 volts. The lights are then disconnected and another voltage reading pass is taken to make sure that is still the lowest cell. The volt meter is then moved to the new lowest cell or left as is, if that is still the lowest cell. Then a 40 watt light bulb is used to draw the cell voltage down to 2.75 volts. Once the cell is down to 2.75 the light bulb is turned off and another voltage pass is taken to make sure that is still the lowest cell. Then a 5 watt light bulb is connected to the module and left on until the lowest cell comes back down to 2.75. This is the stage I am currently in, as once the 5 watt bulb is connected, the voltage starts coming back up and often goes close to 3 volts before it starts coming down again. I started this yesterday morning and after 25 hours of discharging, still do not have the low cell stabilized to 2.75 volts.
When the cells are drained, they seem to always drift back up but when they are charged seem to always drift back down in voltage, so once I get the low cell down stabilized at 2.75 volts, I start charging them all back up to 2.8 volts or what ever voltage the highest half cell is resting at. Remember, the Better Place cells are LiMn2O4, not LiFePo4 cells and operate at a higher voltage range, so when they are down to 3 volts per half cell, are pretty much depleted. My controller is set to cut off at 90 volts or around 2.8 volts per cell, so the module should never discharge lower that that.
While the module is being discharged, a post it note is stuck to my laptop to remind me and I check the module every half hour or so. I have found that since the LiMn2O4 cells are so "bouncy," the best way to get them to stabilize below 3 volts is to discharge at a very low level when approaching the desired low voltage.
Also, measurements were taken in the nose of the Fiero and the module will fit, so am thinking about how to mount it. To get the module in from the top, the hood will need to be removed but this may be the best way to install it.
Until next post,