Since I already have a dumb battery charger (which I very rarely use anyway), I went the other route and got a battery analyzer instead.
This is the analysis my 2015 Innova Diesel's battery. It's made by Oriental, type 80D26L (133 mins RC | 68 AH) and it's approaching its 3rd year.
They say that once the State of Health (SOH) goes down to 55-60%, you should consider replacing it. Letting it go below 50% could make for a slow or no-crank condition.
To me, this is less guesswork as I don't have to wait for telltale signs of a dying battery. I just do the analysis every time I'm doing routine maintenance under the hood.