Interesting! My accessory battery went flat over the easter break as I went away and didn’t use the Twizy for 4 days (I have things connected which use around 2.7Ah per day, so I knew it would happen). I parked it at work where I could plug it in to charge up again so I wasn’t stuck.
However, it got me thinking about the “jump starting” idea Osbrook mentions above; using another battery to provide the power to turn on the “ignition” and charging circuits.
A hobby LiPo battery such as this: http://www.hobbyking.com/hobbyking/store/__21371__ZIPPY_Compact_5000mAh_4S_25C_Lipo_Pack.html
is small and light enough to keep in the boot (in a protective bag!). It’s the right voltage and has enough grunt and holds charge very well. With the right connectors and a bit of simple protection circuitry (diode and current limiter) it should do the job nicely as an emergency backup. I already have a charger which can top it up.
I wonder if anyone else would be interested? When I get all the bits together I can let you know how I get on.