If you’ve been following along, you’ve seen why we want to make use of a 48V battery pack. Now the question is a matter of where or how to get and/or build one. There are three pretty obvious choices here:
- Buy prismatic lithium iron phosphate (LFP) cells, connecting 16 cells in series. This would be the route that normal people would probably consider first.
- Buy a Chevy Volt battery out of a junk yard (or an “auto recycler” in politically-correct terms), open it up, and re-configure it.
- Buy a used battery from an EV or hybrid other than the Volt
I’m not a normal person, so the first option is out! Actually, cost is what puts the first option out of serious consideration. I had planned to go that route, but kept putting it off because of the expense required to do it. There are advantages to LFP–it’s generally considered less susceptible to thermal runaway, the cell voltage stays virtually unchanged whether near full or empty, and they’re available with bits and pieces to make connecting and monitoring them pretty straightforward.
But there are disadvantages too, beyond cost. They need to be kept at essentially room temperature to avoid degradation. They still need to be managed thermally, but there’s no easy way to do so. As mentioned, they’re expensive, and virtually all of the available cell manufacturers are located in China.
When we take a look at the Volt battery pack, a number of features stand out. First of all, it’s made in Holland, Michigan. At least as far as I’m concerned, that has value: the battery is produced very close to where it was designed, and there’s obviously a close connection between battery design/manufacturing and the vehicle itself. (A well-written article on that subject can be found here. ) That generally translates to a higher quality product, and the attention to detail as I disassembled the pack was obvious. Every fastener had two sets of inspection marks. Everything inside was neat and clearly labeled. Being such a big undertaking for the General, the batteries were very conservatively rated initially, and performance to date has resulted in newer software that increases the range of these vehicles.
It’s also far cheaper than LFP cells, even before you consider all of the interconnecting hardware. It conveniently breaks down into 48V sections (the as-built 3-parallel, 96-series configuration would be 384V, which is more than we want to work with). It’s also liquid cooled in the factory configuration, which we’ll talk about in detail later. Voltage and state of charge have a nice linear relationship, which makes battery and cell monitoring much easier (another future topic).
What about other EVs/hybrids? With the exception of some plug-in hybrids, none have battery capacities big enough to be of interest. Among plug-in hybrids, such as the Ford Fusion Energi or C-Max Energi, battery packs are about half the capacity of the Volt, and aren’t as easily reconfigured. The only EV manufactured in sufficient volume to warrant consideration is the Nissan Leaf, and batteries for it can be found for about the same cost-per-kWh as the Volt. But as an EV, we can be sure those batteries were regularly charged and discharged, where many (most?) Volts spend a lot of time operating as a gasoline-electric hybrid where the battery isn’t being regularly used at its limits. In other words, in dealing with a junk yard battery, we’re much less likely to find a “worn out” battery in a Volt. Like the others, the Leaf’s battery just isn’t as easy to reconfigure.
Next time, we’ll start disassembling the battery pack!