why dont they have changeable batteries on electric cars instead of slow plug-in charging?

There has been some talk about having batteries you can swap out (i.e. Project Better Place - see link below), but there are some major problems with that idea.

1) Electric car batteries weigh hundreds of pounds. How are you going to swap them out easily?

2) Every electric car has its own battery. Some use lead acid, others use lithium ion (but there are several different types of lithium ion), and some are even trying to use ultracapacitors. So a battery replacement station would have to have every single kind of battery ready to swap out. It's simply not realistic.

I've seen designs that take advantage of this kind of thinking, but as far as I know no cars have been designed or built with this technology. It's more like propane tanks that you can exchange when empty for a different tank that is already filled. You assume a small risk that the tank isn't as good as the one you exchange but it's minimal and with batteries it would be simple enough to check each bank to ensure it's working, doesn't leak and is safe. The deck or bank of batteries would indeed be heavy but it would be easy enough to design a mechanical arm to remove the discharged batteries and replace them with freshly charged batteries. This would require a lot of standardization so it would probably also require the government to mandate this. Then any charging station could offer a 'fresh' set of batteries if you don't have time to wait for your batteries to be charged.

Without a breakthrough in capacitor technology such charging would take more time than most people would want to spend at a station. But swapping out the batteries need not take longer than you spend now to fill your gas tank but there would need to be government funding and mandates to ensure enough charging station are built and that there is the necessary degree of standardization and inspection. This won't happen at all until we have a large minority of electric cars on the road unless the government funds and builds the infrastructure first, which is unlikely since it's broke.

The battery assemblies are extremely heavy and would take special equipment to remove. Not to mention that if you handle them incorrectly you stand the possibility of getting fried. As battery technology improves, so should recharge time. Consider also the cost of an "extra set of batteries", even if they designed a "quick change Pack".