I regularly see complaints on this forum about 12V battery drain, which makes me wonder - why do the 350h and 450h+ (and other hybrids, for that matter) even have 12V lead-acid batteries?

Wikipedia numbers for battery energy density:

• Lead-acid is ~40 Wh/kg
• NiMH is ~90 Wh/kg
• LFP and sodium-ion are ~125 Wh/kg
• Lithium-ion is ~180 Wh/kg

It seems like a better approach could be to expand the size of the traction battery, power the car's low-voltage systems off of a voltage converter off of the traction battery, and charge up a supercapacitor if any energy spikes are needed. This would still work even if the traction battery loses half its capacity due to cold weather.

 

There is a DC-to-DC converter which converts 355V DC to 12V DC. When the car is on, the traction battery is supplying power to the 12V systems while also charging the 12V battery.

When the car is turned off, the traction battery is disconnected through a mechanical relay. You can hear this switching on and off when you start and stop the car. This is why the 12V battery is required, to supply power to 12V systems. (So that the key can be detected, the app can work, the car knows what to do when plugged in, etc.)

So the real question you need to ask is - Why is the traction battery disconnected when the car is off? I assume it is some sort of safety feature or some other specific design choice to disconnect the high voltage battery and switch to the 12V

 

An airbag deploy will also trigger the traction battery to be disconnected. You'd still want to have a 12V source for diag, opening doors, flashing hazard lights, transmitting SOS calls, etc.

 

As others have said, for safety you don't want a high voltage battery connected until the car is ready to drive.
I do wish they adopted a smaller 12V Li-ion battery like Tesla is doing now. It would save a good chunk of space and weight.

 

RC "toys", motorcycles, RV's, boats, and racecars have had LiFePo4 batteries for quite a while. Safest and most energy dense. I'm sure there are listings for our cars, but you may not like the price. Cold weather is a manageable minor problem with them granted.

 

I've been running an Antigravity (that's the name of the brand) LiFePo4 battery in my LX for a few months now and it's been great. The big question is how long it will last because lithiums don't like heat at all and it's mounted in my engine bay. But if you've got a hybrid NX, you can mount it in the back, away from the temperature extremes subjected to the engine bay. I would think a LiFePo4 might last for a very long time in that position.

 

A battery blanket may actually help against the heat also. Insulation works both ways. Might be interesting to see a thermocouple taped to the battery with and without a blanket in the heat of summer. Thanks for the reply.