Ebbie,
I agree with Pobjoy, adding the reason that if you have a fire in a battery back there, that's a lot of energy, and you're not going to be able to extinguish it.
I have two Bromptons, which I could not recommend more highly - excellent bikes in general, and fold really well. They are worth every penny in utility and quality. I have had them for 23 years, and been all over with them. Two fit perfectly in the back of my C150. I took my wife, the two bikes, and a little baggage from Canada to Bahamas for a few days years back.
To this day, I ride my Brompton most days just for fun.
