Hmm this is a long post, but in theory this could all work. So what is a rest heater. After you are driving and the car has reached operating temperature you can turn the car off and engage rest heating. This allows you to heat the cabin for about 15 minutes before the battery runs out or the timer stops it. To do this, the following needs to happen: Car needs to reach operating temp. Electric water circulation pump is turned on to pump hot water from engine to heater matrix. Heater fan runs. Now, with a circulation pump fitted to a car that is electric (auxiliary pump), Couldn't the same be achieved with a car that doesn't have this feature if it had the battery power to do so. If a secondary battery or battery upgraded with the extra capacity needed. Wouldn't this be pretty cool for a camper van conversion. I know of some people that have converted their cars to campers and this would reduce the engine usage. Obviously you couldn't just have the engine off all the time as the engine would cool down and battery run out. So here is my "ideal" setup... Car runs and then you pull up for "the camp" you switch the engine off and use the rest heating for a while (10 mins?). Then allow for cool down. Then engine start and heating back on (automated engine start would be pretty cool). Allowing for however long the battery takes to charge up from whatever it lost using rest heating. Then engine off, rest heat and cycle continues. Now, rather laughably so, i just realised that a modern "stop-start" car does all of this for you (minus circulation pump i think...). This just works out of the box . But for those who sleep in their cars on a camping trip in the cold it would help save quite a bit of fuel instead of running the engine on full constantly. But then the cost of getting this to work, it would probably be better to use a car with start-stop functionality. I did think of having an FBH (fuel burning heater) but it's a 1.3 litre, it warms up quick anyway but then a REST heater with FBH would be cool. Imagine not needing an engine at all for heating. But after writing this whole post I then realised if you were to camp in a car why not wear warm clothes *sigh*
The 15 minute limit isn't because of battery. It's simple. Your heat source is the engine. The engine hasn't been running for the past 15 minutes and is being actively cooled. You simply run out of heat
Also, this limit would depend on the size and materials the engine is made of. A massive V8 is obviously going to hold more thermal energy than a little 3-cylinder.
And aluminum vs iron’s specific heat capacity. An aluminum engine is going to lose more heat to the surrounding air than iron is.
Also may depend on how many components are in the engine bay; the more "stuffed" the engine bay is, it may contain heat more easily. Whereas an older engine bay has fewer components (so that you can see the ground below) may dissipate heat faster. If you want, you could stick attic insulation everywhere....what could go wrong?
Overheating leading to the insulation reaching ignition point causing a fire which destroys the car. Well, that idea went up in flames.
Same for most of the world, which is why we just use insulations that don't contain asbestos, and may still be fire resistant to some degree.
