I work in the commercial and industrial uninterruptible power supply industry.

We use a lot of batteries. Some things to consider based on my observations and talking solely about lead acid type batteries.

If the batteries go 6 months or more between charges the likelihood of thermal runaway increases. Thermal runaway is when the battery overheats under use (charge or discharge) to the point it goes up in flames. That said, does it happen all the time? Nope. But if it happens you know the customer is going to blame us and potentially cost the company millions of dollars in damages.

The device we use to maintain and charge these batteries costs way more than anything you are going to use, even our cheapest model. They are as scienced out as possible and are continuously monitored and updated to prevent issues, see the millions of dollars mentioned.

We recommend 5 year replacement intervals for the VRLA batteries, similar to a maintenance free, and 20 years for flooded cells, similar to old school add water type batteries. Now these are in climate controlled environments using state of the art technology with regular testing and maintenance. Something you in your garage cannot match.

With all of this there is still the, very, occasional issue with thermal runaway. So, I would never leave my battery on any kind of continual charge, especially unmonitored and unobserved.

I would go thru whatever monkeyshines I needed to unhook my fully charged battery during storage. I would attach a charger when I came back to prep it for the driving season and disconnect when I left, if I were the type to store a car for the season. I real life, I just drive them.

But what I never do is leave them on a charger and walk away.