I've been after something like this for a while now and found one of these on play.com.
Simply recharge this small unit and keep it in your glovebox/purse etc.
If your mobile phone loses power completely and you really need it to make a call simply plug this into your mobile and it recharges the battery for you.
Motorola state that you can recharge a mobile phone twice or a bluetooth device up to ten times befor the unit needs a recharge itself.
Compatible with a large amunt of Motorola phones. RRP of £29.99