Charging issue

[Andyjack]

Only had my luna quasar a couple of months and been away nearly every weekend so far and its been fine with no issues but last weekend when packing up in the morning i went to use movers to hook up to car and they were dead. When i checked the battery was dead but it had been hooked up to mains all weekend. I got home bought a new battery which as been hooked up for three days now. if i checked the voltage while plugged into mains its showing 13.5 but when unplugged it shows 2-3v any suggestions please.