Battery question

Thanks everyone for taking the time to give me your thoughts.
As a result of the postings, I purchased a £9.99 multi-meter from Screwfix, and a 12amp charger on Gumtree for £20. I realised after I bought it that the charger has a "Voltage" display, so I'm not sure I needed the multi-meter.
Both my batteries charged up very swiftly, unlike with the previous charger.
12 hours later, they both have a 12.8 v charge, according to the multi-meter and a 13.5v charge according to the charger. Not sure why there's a difference.

I'll keep them topped up before I put them back in mid March.
 
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