| I like that idea. It's a little tricky to pull off, though. Adding cells in series increases the voltage (which the car might not be designed to handle) or adding them in parallel means they have to be well-matched to what the car already has. I don't know if anyone has tried this, but a solution I like for adding dissimilar batteries is to break the battery up into blocks that are linked in series, with each block being, let's say, 50 volts. Maybe the stock battery has five of these in series for 250 volts. If you add more blocks, it exceeds the voltage specifications. So what happens is that each block has a contactor (i.e. a big relay) that can bypass that block; it just looks like a 0-volt battery when enabled. Supposing you add two extra blocks for a long trip. Then you have a scheduler that disables two blocks at a time on a rotating schedule, so it always has 250 volts. If the added blocks have more amp-hours than the OEM blocks, that's fine: they'll just be enabled for a longer duration. Rotating batteries in and out can be done in a way that keeps their voltages roughly equal with each other as they're depleted. Maybe this is too complicated and too much of a hassle to be worth it in general (as opposed to just building a car with a big battery to begin with). I'm also not sure if typical relatively-inexpensive contactors can tolerate being switched on and off every few minutes while under load. On the plus side, you could probably just have a few kinds of generic "extra" batteries that work in a wide range of vehicle as long as they can handle the required current. They wouldn't have to be an exact match for what's already in the car. |