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One of the main problems is that batteries (all types) lose a fraction of their capacity, every charge/discharge cycle. Therefore, in practice, the battery packs would be anything but standardized.

I recognize this. By standardized, I mean same size, terminal arrangement, and mounting.

The battery in my computer costs ~$100, and tells me how many times it has been cycled, and its current absolute charge (watt-hours, not percent). I would expect no less of a car battery. It might add $5 - $10 to the cost of the battery. You would only pay for the watt-hours you actually get, the price would not be the same for every battery.

The real problem with cars with only one battery is that we expect to start out with it fully charged every morning. This is the equivalent of filling your gas tank every day whether you need to or not. Having multiple batteries which are only used one at a time, with automatic switching, allows the equivalent of buying $10 worth of gas if that is all you can afford today. Not quite as subdivideable as gasoline, but a whole lot closer than one battery.

You should look at the Chevy Volt and the use of A123 systems batteries.

http://www.a123systems.com/#/home/cordless

The cells from A123 last about 10 times longer than standard lithium ion and they don’t flame out either. You can buy them now in a DeWalt power tools and other things.

The Volt will go 40 miles with no gas and then a generator kicks in and you can go 260 miles more.

At 50 to 60 MPG!

Recent developments in battery-gage technology would probably make what you're proposing feasible. Until now, we've only had voltage-based gages & they aren't very accurate in practice -- especially under the conditions that an automobile has to operate in.