How do you know which batteries to use? There's a fair few brands around and probably as many power ratings. From experience, I've found not to skimp on batteries.
There are a few brands on amazon that I've tried. One being a brand offering a huge 2800mAh in power, and 16 batteries all for around £15. So I bought them. The charge takes forever and without use the charge only lasts around a day.
The brand I love and use is Powerex. They're quite pricey but mine have lasted two years so far and I'll be getting more soon. They charge quite fast, last for ages without use and have been in my speedight for entire wedding day shoots without issue.
Another brand I'd recommend is Enolpe by Panasonic. These can be charged and left unused in your camera bag for an age without losing charge.
Batteries are a pretty important piece of equipment for us photographers. It's better to buy one expensive but reliable brand than having 50 batteries that hold charge for a day unused.