HTC One Max Power Flip Case.

The already huge HTC One Max gets even bigger (and juicier) with the $90 Power Flip Case

Flip cases are nothing new to smartphones, and HTC introduced a decent one earlier this year with the original HTC One Flip Case. We've got a new iteration with the HTC One Max, one that includes a built-in battery for a little extra juice.

The Power Flip Case serves as case to protect the body of the phone, a cover to protect the massive display — and it also adds an extra 1,200 mAh of power with a thin, flexible battery built into the cover. That's about a third more than you'll get with the 3,300 One Max alone.

It's not cheap, though. Here's a preview of the prototype we've got.

HTC One Max Power Flip Case.

At its simplest, the Power Flip Case is your basic flip cover. A rigid plastic shell on the back, a hinge on the left-hand side, and a thin cover to keep your screen looking good.

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The cover mostly impresses with HTC's usual attention to design detail. It's done in the same color scheme and build as the original HTC One flip cover. You snap the phone into the body without much effort, and it's had no problem staying put for us, though you might want to give a second look to the corners to make sure the phone's pressed into it all the way.

There are cutouts for the buttons and ports, of course, and for the secondary microphone. All normal stuff.

Where things get a little weird are with the way the hinge — the part that connects the cover to the body of the case — extends beyond just the side of the phone halfway onto the back. It's not part of the rear shell — the plastic body follows the curve of the phone just fine — but, rather, it tends to give the feeling that you should also be able to use this case a a means for propping up the phone for watching movies. We can't show you that quite yet, as our prototype is lacking the nubs to keep things properly held up. But that'll change. (We're going to hold off on a proper video until we get a finished product.)

Also, there's no mechanism in the cover to automatically turn the display off and on. Forget to hit the power button when closing the cover? You'll have to wait for the display's timeout to kick in. That's another serious oversight and a feature that's absolutely expected in an accessory like this. (It's worth noting, however, that we've seen high-end tablet covers fail this test, too, so HTC's not alone here.)

HTC One Max Power Flip Case.HTC One Max Power Flip Case.

As a battery case, things couldn't be easier. The HTC One Max has a set of contacts for pogo pins on the back, and that's how the case provides power. The extra thickness the case needs for the pogo pins is smartly hidden by an HTC logo, which also is a nice way of providing some branding when the rest of the phone is covered. Again, HTC's attention to detail shines through. You can't charge the case separately, though. It has to be attached to the phone for that.

And it's worth remembering that while any sort of extra juice is welcomed, you're only looking at about one-third of what the phone's got in the first place. So you're not doubling your usage or anything. But neither are you getting the extra bulk of a more traditional external battery case.

Is the Power Flip Case worth the $90 price? It's not a cheap accessory, for sure. But it also gives a little extra battery without ruining the design of the phone or making it even bigger. Surely that's got to be worth something.