I'd keep running it down and recharge it on the impres charger until it does something or just flat out doesnt last more than a few hours
I have a 2010 pmnn4066a on 6550 thats now 10 years old and works just fine. Conditions on the charger fine and lasts around a day if you leave it on scan all day. The service life still shows 100% according to the xpr radio. Ive got another lithium on my 7550e that was new old stock, a 4407, the slimmest for the 7550, dated late 2016 bought last year that has an ever lowering service life, now its on 82, it was great closeout deal at the time and got it cheap new in the box and first used date was when I first used it