Helpful tip-- to get the most life out of the batteries, keep them around 50-60% capacity while in storage and charge before use if you can. Lithium ion batteries degrade much faster if kept at full charge. Difficult to do if there's no charge level indicator on them though.
I have a pneumatic gun which is a pain at times since you're tethered to an air line, but it works for me given 99% of my greasing is done in or near the garage. I'll have to look into one of those battery operated ones one day.