Rechargeable cells (A battery is a colletion of individual cells) a have a finite number of charge-discharge cycles.
In the past, Ni-Cad rechargeables used to develop a memory effect if not fully discharged, and were best looked after by completely discharging them to reduce this.
Indeed when I used to race electric model cars, there were many different chargers on the market that would discharge the cells first, then charge them. We kept them flat as pancakes between uses, and only charged them just prior to use. This was the accepted way of doing it, and it worked. We also only used each set of cells once a day, as the demands placed on them were so high. The discharge rate (Fully charged to nearly flat in 5 mins) used to leave them too hot to touch. Seriously. A lot of effort was put into making packs of 6 'matched' cells too, so the max voltage and discharge curve of each cell in the pack was as close to identical as possible. This let each cell perform at it's best, and give maximum power. They weren't cheap: I had 4 packs, fairly decent ones, and they cost me about £52 a set.
These days that's pretty much all dead and buried. Cells still have a finite number of cycles in them, but don't need discharging and keeping flat between uses. They benefit from being kept fully charged. Also, if a battery is half used, then charged, you are only using 'half a cycle' if you see what I mean. Discharging them first would up that to a full' cycle', and shorten the life of the cells.
Just keep them charged, and charge them as soon as you've finished.
🙂