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Why don't cars have 2 batteries? One for starting the car, and the other for all the crap that tends to kill the battery when you most need it?

Probably because it costs more money and nobody will pay for it. That's the best guess I have.

Some cars have do two batteries, one for start/stop and one for everything else. (For example, some Mercedes have these, Suzuki "mild hybrid" cars, etc.)

I know about the Mercedes but did not know about the Suzuki. My main point though is that this is not a feature in your bog standard Honda Civic or Toyota Corolla or Ford Fusion. I think it should be, it's a huge safety and convenience feature. That it is not in your average car implies it's probably too expensive for the utility it offers the average person.

Personally I solved the problem by buying a lithium ion car starter for both of our cars. Works great, but I really would prefer the cars just had some redundancy built in.

You just have to be picky when you choose cars; my twenty year old Land Cruiser came with twin batteries factory fitted - though rather than one for starting and one for all the thingamajigs, they went with two beefy ones in parallel - 2*105Ah makes sure it starts every time.

I have been contemplating using the second one as an auxiliary battery after installing a fridge in the boot, though. Basically a matter of fitting a voltage monitor and a hefty relay.

Two beefy ones in parallel don't really solve the problem. If you leave your 1-Amp (probably a gross underapproximation) headlights on over the course of ~200 hours, you'll deplete the battery.

-It doesn't solve the problem (which is why I am considering splitting them now that I have added a ~1A consumer to the mix), but it does postpone the problem for long enough to (in most cases) ensure that you will realise your mistake before it causes you not to be able to start your car.

Leaving the headlights on (~10A) as you leave the car in the evening will still let you start it in the morning, for example.

It's an add-on you can choose for cars, if you want to. A lot of service vehicles in Australia have 2 batteries for this reason. 1 for the vehicle, the second for chargers (USB, DC, etc), routers, fridges, cameras, etc.

Not around here. I've never seen it offered for a non top of the line luxury vehicle.

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