Why Does Time to Recharge Battery Vary So Much ?

This winter I have stored my new car in the garage and used only my second car. Every 3 to 4 weeks I connect a battery maintainer to recharge the new car's battery.
Usually that takes about 6-8 hours, but once it only took 3 hours and once it hadn't recharged after 12 hours.
Is it possible that a less-than-firm ground connection could cause this difference? At the start the maintainer always lights up to show it is connected.
Usually that takes about 6-8 hours, but once it only took 3 hours and once it hadn't recharged after 12 hours.
Is it possible that a less-than-firm ground connection could cause this difference? At the start the maintainer always lights up to show it is connected.
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Answers
Also, battery maintainers, unlike battery chargers, do not charge at a constant rate.
I don't like to leave an unwatched electrical device turned on if I can help it. I don't see any advantage in leaving it connected and turned on.
Thanks for replies.