Here's a tip about charging your Desire using the in-car cigarette lighter socket together with a USB lead. If you find that your Desire isn't charging correctly and even worse losing charge whilst plugged in, check out this thread Charging: USB and AC differences
You may be using a USB socket that will only allow up to 5v 500mh output. You need a USB socket that can provide 5v 1000ma (1amp). The above thread will reveal all you need to know, bottom line, the two data pins on the car USB socket need to be shorted out, for the Desire to believe it has an AC charge and therefore draw 1Amp.