I use the standard 1.5v Alkaline batteries. As long as they show a 1.4v or better on the battery indicator they've worked fine for me.

I change them daily. They primary will only last for one day, but the secondary should go for two days as it's not driving the solenoid. But they are so cheap, I change them both.

Don't forget to clean the battery contacts & springs on a regular basis.....

Richie