I had the same issue with charging it on the road, often do several thousand mile trips etc... my solution was to buy a spare mains charger from Viper for the plug, and an old USB charger cable swapped the ends over after checkng the current pin outs with a multimeter and have a 12volt cigarette lighter to USB adapter I use for charging phones etc. with me anyway so it's just the cable thats needed. Voltage is the same as the mains charger, and it has worked fine for 2 years now.