Well, that's a double coincidence. I recently bought both a RadMini ST2 and a Ryobi cordless electric mower.
That reminded me to check the lithium battery of the hedge trimmer I bought in 2022.
The nicad battery on my B & D hedge trimmer lasted many years. When it deteriorated, I bought a B & D nimh battery, which was supposed to be a replacement. I soon had trouble, so I bought a brand that was supposed to be better. Same thing. I thought it must be the charger. New charger, same thing.
It turns out, the B & D charger, which was supposed to be good for both types, would drain a battery at a few milliamps. When the battery came down to a certain point, the charger would come on and top it off. That seems to be the best way to maintain a nicad and why the original battery lasted so many years. With nimh, there's a drain current but the charger never comes on to top it back up. It's best not to leave the battery on the charger. B & D never explained that. They surely made a lot of money manipulating consumers into replacing batteries and chargers.
Several times a year I needed to cut back an enormous hedge with hand tools because the electric trimmer wasn't aggressive enough. When I saw a more aggressive one marked down, I bought it even thought it was B & D. It's lithium. B & D said to recharge it after each use. I wasn't inclined to believe their battery advice. I'd charge it only when it began to slow down.
Your mention of a Ryobi mower made me think. I read 16.6 volts on the 20 volt battery. I don't know what kind of cells the B & D has, but suppose it's 5 in series, which could be 20 volts fully charged and 18.5 nominal. I put it on the charger for an hour or so, measured 18 volts, and it was still 18 when I checked after it had sat an hour.
If they published state of charge vs voltage charts for particular batteries, I could see if a battery was suitably charged for storage.