I am running a full size motorcycle/atv battery.  One battery model is YTX7L-BS.  This is a 7AH battery.  As long as you run a lower draw then the charging system, this modification should last a good long time without taxing the lighting coils.  OEM lighting coils put out around 80 watts max.  My LED light pulls 43watts, LED tail light pulls 3watts, and the LED Whip light pulls 5watts.  Figure around 50watts total draw with my system.