Why 12v battery?

Why 12v battery?

I'm just curious why Tesla chose to use a 12v accessory battery for accessories rather than using a dc-dc converter to generate 12v from the main battery pack. The 12v battery will wear out over time and will need replacing, whereas a dc-dc converter would last longer. Any ideas?

GeekEV | January 31, 2013

I can't speak specifically to Tesla, but for other EV's the 12v battery is used to power the main computers. If there were some kind of a malfunction, or you completely drain the main battery pack to empty, you would be unable to charge it or power on the car to get any error messages. It's just easier to have a dedicated battery for this purpose.

shop | January 31, 2013

I understand your point, but here's what I was thinking. In the 8 year life of the main battery pack, odds are that it will never discharge to zero. However it is almost guaranteed that the 12v accessory battery will die and need to be replaced. So I don't quite see the utility of the 12v battery. Now maybe the two batteries act as backup to one another?

Brian H | January 31, 2013

Driving accessories off a 300+V source is crazy. Much easier to use a single 12V source and top it up occasionally.

shop | January 31, 2013

Well you wouldn't use 300V, you would use a DC-DC converter to convert it to 12V. In fact that's what they must do now to charge it, unless they only charge it during plug in time. I've heard people say that the 12V battery charges during driving time, which also strikes me as odd - I would just charge it when plugged in.

Anyways, I was hoping someone with some engineering knowledge of the Tesla internals could illuminate us. Maybe I should look at old Roadster forums, as I'm sure this has been discussed back then too!

jat | January 31, 2013

Let's say the main battery gets discharged to the point you have to completely disconnect it to keep from damaging it (obviously, the self-discharge rate will eventually kill it, but it will buy you more time). If you have disconnected it, how does your BMS system bring everything back up (or even let you charge it)? Having a cheap long-lasting 12V battery seems like a much better solution than greatly complicating the system design to avoid a very low-probability but potentially catastrophic situation.

Superliner | January 31, 2013


The main reason we used 12V batteries in BEV conversions aside from the obvious, was simply to have 12V storage capacity to handle peak loads and keep alive memory for clocks, keyless entry, radio station memory, body control modules etc. It also enables you to size your converter accordingly (read more cost effective solution) if it does not have to be large enough to handle peak or surge loads or complex enough to handle the few two or three millivolts for ignition off leach loads again clocks various memory etc. on it's own.

As far as "only charging" the Model S 12v battery "while plugged in" (which it does anyway) as opposed to while being driven (which it also does) is because without any charging while underway you would likely run out of power if say running wipers, heater, seat heat, and lights etc. such as driving on a cold rainy night and the 12V part of your car would die without dc converter boost, much like driving an ICE with a failed alternator eventually it dies usually in a matter of hours with high accessory use.

Not to mention the Feds which require some functions to be redundant or work in a worst case. hopefully it would not happen but a bricked pack or high voltage failure at night would or could leave you along the road with no flashers lights etc. "hence the 12v battery for reserve onboard storage".

There are probably a whole host of scenarios where the case could be made for doing it this way as opposed to a stand alone dc/dc converter. At the end of the day I suspect it is just a more cost effective solution for Tesla for regulation compliance as well as customer satisfaction.

Just my .02 Some is fact, some is a little more than I know lol!!

shop | January 31, 2013

Thanks superliner, good answer. Basically cost (cheaper dc-dc converter), plus simpler power path for essentials (less likely to fritz out), plus redundancy. I'm guessing that if the 12V battery died, the smaller DC-DC converter is big enough to take over and handle the essential loads (car computers) while turning off non-essentials (like seat heaters).

Brian H | January 31, 2013

Multiplying DC-DC converters is a bad idea. Using one 12V battery as a buffer is much easier, cheaper, and more reliable.

Flaninacupboard | February 1, 2013

Imagine there is no 12V battery. The main contactor to the HV pack is switched off. There is no power to any systems in the car. How do you unlock it? How does it detect your key in the car? How does is switch on the main contactor? It can't and doesn't. All EV's need a 12V battery.