I've always just used the 6V battery.....and has always lasted forever! I have a moultrie....spin/spread style....after months of use I go check the battery, and the battery life meter still usually reads....50+ % sometimes still like 90% , new batteries maybe?

Ive found those solar chargers to be expensive and only to supplement....extend normal battery life, not replace the batteries....also in my areas usualy asolar panel wouldn't help, ie woods.....