This is the type of battery and charger I was suggesting, lithium polymer battery weighs 2.2kgs comes with charger, has a 1000 to 2000 cycle and on light useage probably more. A very safe battery, better than lithium ion, you would need to make an adaption to the connection to the pump. Best of luck.
http://www.powerhousegolf.co.uk/acatalog/18-Hole-Lithium-Trolley-Battery-MISC036.html#SID=92
advertised at £129