https://www.amazon.co.uk/AA-Battery-Charger-Maintainer-Batteries/dp/B00DW6RHN2
I've been using the AA one for 5 years now.
Zed is used daily but 2 other cars used infrequently so I top them up every few months.
My Kia is on charge as we speak
It's meant the Bosch S4 battery in my Mini is still going strong after nearly 9 years.
But the original battery in my Diesel Kia still died after 8 years despite regular maintenance.
Nothing lasts forever I guess.
The AA charger is only 1.5 amps so very slow, and won't work if the battery is fully flat but has been reliable for me over past 5 years and was alot cheaper than a Ctek one.