Parasitic Battery Drain

[g8dhe]

No that is the voltage produced by the alternator to charge the battery.
Once the engine has stopped, ignition off and any EHU & Solar charger disconnected, then leave it for 30 minutes or more and check the battery voltage then it should be around 12.6 volts - this shows the battery is fully charged, but it doesn't tell you how much charge the battery has taken only that it is full!
As the battery ages the capacity reduces. To check the likely state of charge you then measure the voltage with a load applied, ideally you need something that can draw a current similar to the starter motor many 10's of amps its called a drop test as you measure the amount the voltage drops with the load applied.
A cheap one https://www.ebay.co.uk/p/2308226882
An expensive one https://uk.rs-online.com/web/p/battery-testers/8372081

Without a proper "drop tester" the best you can do is put on all the headlights on full beam and look for a significant voltage drop with that much smaller load.

[Bob]

You could buy a lot of batteries for the price of the second one.

[willow2u]

Why not try disconnecting the battery overnight and see if still performs when you reconnect it the next morning

[Prospective Owner]

Hi All,
Sorry for the late reply, I've only just realised that there were more responses!!! Now I just feel old
I'll try all of the suggestions as soon as I can.
Thanks again and sorry again for being rude and not replying earlier.