You have stumbled into one of the biggest money grabbing scams in the feeder business. You buy a feeder and right next to them are a ton of gray(heavy duty) batteries. Take out the words heavy duty and insert the word junk. It is all designed to make you have to buy the solar pannel and it is on the accessories that stores and factories make most of their profit. The batteries and solar panels are just packaged and shipped from china at a huge mark up(and so were most feeders for that matter).
If you think you need a solar panel then you do need to use a rechargeable battery. Not a energizer of duracel copper top. However it is possible that if you would experiment with using a good quality battery to begin with you might find that you really dont need the solar panel at all.
This is not true in all cases as many feeders are designed to be very innefficient to begin with so you have to buy the solar panel and the cycle continues......