So many batteries
Tesla cars are renowned for their huge lithium ion batteries under their floor. Less well known is the fact that they also have a 12-volt lead acid battery, too.
Why 12 volts?
12 volt electrical systems have been used in cars since the 1950s. Headlights, turn signals, seat adjustment motors and heaters, the blower fan, all these components rely on a supply of 12 volts. While it is possible to produce these components to work at other voltages (48 volt systems are becoming more common – the new Audi A8 has one in addition to the typical 12 volt one), the ability to buy an enormous variety of components from existing supply is advantageous.
Why lead acid?
Lead-acid batteries have far less energy density than the lithium ion batteries that are used to power the traction engines of an electric car. Their advantage is that they handle the rigors of standing by in the cold, being drawn down and charged back up frequently, and are proven and cheap.
So what’s the problem?
As I’ve discussed before, Elon Musk and the companies he leads are, consciously or not, applying 40 Principle thinking to their designs. The fact that their cars have two redundant electrical systems must feel like a terrible hangnail. Surely the huge battery pack can be used for all the car’s power needs. Principle 6 – Universality
After all, the redundant system adds weight, complexity and cost. Furthermore, adopting a higher voltage for accessories will permit thinner wiring as the amperage can be proportionally reduced.
In the May 2017 earnings call, Musk revealed that the Model Y will be on an entirely new platform and dispense with the 12 volt system completely. Either suppliers will need to get on board with this change or Tesla will again need to build more components in-house.
“The wiring harness on Model S is about 3 kilometers in length. The wire harness on Model 3 is 1.5 kilometers in length. The wiring harness on Model Y will be 100 meters.”
While 12 volts may be too little, and lead-acid may be too heavy, there may still be need for some auxiliary electrical storage system, be it a smaller, higher voltage battery with higher energy density, or a capacitor. The reason for this is that the main battery’s voltage is well over 400 volts, i.e. lethal. In the event of a crash or during certain types of service, it is important to be able to isolate the big battery and this may require a small battery in practice, to control relays and power control modules etc.