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I had a solar one and it was never really up to the job.
Get a small CTEK trickle charger, they are great and do a great job, you can buy an extra connecter kit to hard wire to the batteries of extra cars if needed, I just swap mine over to the car I am not using.
Obviously an outside plug is ideal, I route the extension lead under the bonnet where it remains dry with the charger plugged in there. I have the CTEK MX 5.0
https://www.smarterchargers.co.uk/?gclid=CjwKCAjwm4ukBhAuEiwA0zQxk7bPvLS6kemI-wnLY3-6FstUVKIz4KYfMp7AvKx-CIlYxPftDD5TPxoCIrkQAvD_BwE
Has anyone tested those solar battery topper uppers to see if they actually generate any meaningful current that might top a battery up? I swear the one I have does fuck all (and I realise this is Britain and Britain doesn't deserve sun, but still, it's blinking so it's getting something).