Hi , I am not used to dynamos , car electrics in general . So buggy not used for a while and battery flat 10 volts , power pack on - brumm , on tickover with , ignition light off , still 10 ish volts until reved up 13.8 volts . So I either have to go fast every where or solve the problem , if there is one ! Brian , owner of Pearl .

Obvious but is the fan belt tight enough

If ok check connections then battery. Charge it and see if it holds it's charge

Anything less than 12.6v fully charge means the battery is poorly, probably lost a cell. 
Paul

Check - Check and battery on charge overnight , thanks for input .

Do you have a clock or radio with a memory. Both will pull the battery down within a week. I usually connect mine to a trickle charger for example winter 
Paul

Hi , no nothing that would drain the battery but I am going to fit a battery isolation switch while the battery is out .

Dynamos work with rpm at low speed it will not charge so your battery is still flat and need an overnight trickle charge. The dynamo tops up the battery it will not charge a flat one.