It's a rechargeable battery pack for the Chromatic that gives you another means to power the handheld, should you not want to deal with disposable batteries. I'd give this a 5 star rating if it weren't for the fact that you need to charge the pack in a separate charging station instead of just plugging the Chromatic into a USB-C port like you might expect to find with other, similar handhelds. It's a small inconvenience in the grand scheme of things, sure, but I can't help but wonder if this could've been designed differently.

I don't regret getting it, though. It's still nice to have even if the recharging process requires an extra step or two beyond what I'm used to nowadays.

I went through two sets of AA's before my Rechargeable arrived. Ever since it arrived, I have stuck with the Rechargeable. Maybe it's just my subjective impression, but it seems I get more battery life out of the Rechargeable, and I don't have to go through the anxiety of wondering whether my AA stock is running low. They do take several hours to charge, though, so I personally will be ordering a second one as a reserve.