It’s the last day of August – nearly a month since the last update! I’d like to say things have been moving at a furious pace but that’d be a lie. It’s been a busy month with travel, the Monterey car week (I met Cristian von Koenigsegg!), along with improved work on PassionList. I have a build date from Caterham for the chassis – early December. In the next few days I’m going to sign on a shop in Belmont – finding appropriate space was more difficult and time consuming than I had anticipated.
I’ve also been doing quite a bit of research on batteries and motors and decided that I’ll be going with Li-ion battery packs and an AC motor. My reasoning follows…
Batteries
In looking at EV applications, we’re basically dealing with three different battery types – lead acid, Nickel-based, and Lithium-based.
Lead acid
Lead acid batteries are familiar to us all – they’re in our cars as starter batteries and in electric wheelchairs, golf cars, forklifts, and many other applications. They’ve been around for a while and a known quantity. Many of the first EVs ran off lead acid batteries, including the EV1 .
When looking at an EV application, here are the pros and cons:
Pros
– inexpensive – low cost per watt-hour
– low self discharge
– capable of high discharge
– performances well across a broad range of temperatures
Cons
– low specific energy (low watt-hours per kilogram)
– slow to charge
– limited number of full cycles
Lead acid were a good choice for a long time, however battery tech has improved and given us better options.
Nickel-based
Nickel-based batteries have been a popular choice for portable devices and are at least a look. Mostly we’re dealing with Nickel-cadmium (NiCd) and Nickel-metal-hydride (NiMH).
Pros
– rugged, high cycle count
– takes well to ultra-fast charging
– long shelf life
– low cost per cycle
Cons
– low specific energy
– needs periodic full discharges
– high self discharge
– low cell voltage (1.20V)
Lithium-based
Finally we get to Lithium- based batteries – in the EV space these are the ones of choice these days (at least at the OEM level)
Pros
– high specific energy and load capacity
– long cycle life
– short charge times
– low self discharge
Cons
– requires protection circuit to prevent thermal runaway (i.e. blowing up)
– transportation regulations
– relatively expensive
For my application, it basically boils down to the following – lead acid are too heavy, Nickel-based are heavy and require too many for my needs, which leaves Lithium-based – the next decision will be what specific chemistry. That will boil down to a few other factors, most of which will be constrained by whichever supplier I decide on. Before I embark on that path there’s plenty to decide however.
Motors
This is a slightly easier question – AC (alternating current) vs DC (direct current). Many DIY EVs out there went with DC motors for plenty of good reasons – they were easier to source, less expensive, simpler, and required less voltage. Times are a changing, and there are several good reasons to prefer an AC motor in a modern EV.
– Regenerative braking! DC setups require a specialized controller for this, whereas an AC system is symmetrical in this regard. This improves efficiency drastically.
– Better torque characteristics – AC motors have a wider torque band.
– No motor brushes – this improves reliability and decreases service needs
– safety – if a DC power stage fails, the entire pack voltage is applied to the motor. That’s not a good situation. If an AC inverter fails you simply go nowhere.
It’s no surprise that every OEM EV out there today runs an AC setup.
Much of this material is from Battery University and Metric Mind.