4 wheeler winch

I have a winch on my honda 4wheeler. I don't use it often therefore the battery is not always at full capacity. Sometimes I do alot of winching which drains the battery. It will still crank but the winch gets so slow. During the times that I know I will be using the winch, how could I take an extra battery and hook it up to help the situation? I was wondering if I could rig a lead with a cigarete lighter plug and back feed into the system.? Any ideas?