How to Turn a Tesla Battery Pack Into a $40,000 Brick
Li-ion, Not Your Father’s Battery
This Tesla-to-brick story first appeared in the Understatement blog, authored by Michael Degusta. Some rich guy, too busy to read his owner’s manual, parked his six-figure sports car in a garage for six weeks while his home was being remodeled.
From the Tesla owner’s manual:
“Keep in mind that when the vehicle is left unplugged with a full Battery, the initial rate of decline can be significant. When fully charged, the Battery’s charge level can drop as much as 7% a day and 50% within the first week. When the Battery’s charge level falls below 50%, the rate of decline slows down to approximately 5% per week. Over-discharge can permanently damage the Battery.”
That equates to a ruined battery in roughly 11 weeks. In other words, for this Tesla to be damaged in six weeks, it was likely parked with a mostly discharged battery. Ouch.
So, what does that picture of my electric bike have to do with all of this? It’s powered by lithium ion batteries. You can’t let this kind of battery drop below a certain voltage or it will be ruined. I’ve been using these batteries for five years now and they are going strong.
Specifically, my pack consists of six Dewalt 36 Volt lithium ion power tool packs. Each pack contains ten cells connected in series. I connect pairs of packs in series and then each pair connected in parallel with the other pairs. Each tool battery comes with a built in circuit board called the BMS (battery management system). In fact, all lithium ion battery packs come with a BMS designed specifically for that pack.
Some BM systems continuously use power and if you leave the pack on a shelf too long it will drain the voltage down low enough to permanently damage the cells. The Dewalt BMS when being used on a Dewalt power tool will turn off the pack before it can be damaged but a tiny current may still be flowing in the BMS circuit board.
The Tesla uses 6,831 itty-bitty cylindrical cells compared to the Leaf’s 192 much larger prismatic cells. No two cells (batteries) are identical. Weaker cells in series with other cells tend to get hotter than their neighbor’s and begin to degrade faster, creating a feedback loop and a rapid downward spiral to failure. One bad cell in series with other cells will cause them all to fail in short order. The more cells you have in series, the worse this problem gets.
To prevent the weaker cells from causing a failure, the BMS continuously measures each cell and sends a little charge to any that are dropping lower than their neighbors. My guess is that with 6,831 cells, the Tesla BMS stays very busy, and uses a lot of energy. On the other hand, the Leaf owner’s manual also warns not to “leave your vehicle for over 14 days where the Li-ion battery available charge gauge reaches a zero or near zero (state of charge).”
To be perfectly honest, Tesla should have done (and probably will do in the future) a lot more to warn owners of this potential.