When an battery case adds only just over half of the phone's capacity while nearly doubling the thickness of the package, it's time to rethink the utility of the solution.
Over the years I have purchased a few Mophie Juice Pack battery cases in airports around the country as I deployed to remote locations to get work done. For most phones, battery life has improved the last couple of years, but I think Samsung is a bit gun shy after its Note 7 fiasco and we are not seeing stellar battery life from its latest smartphone.
While I think the Samsung Galaxy S9 Plus, see my full review[1], is one of the best Android phones available today, after using it for more than a month it is clear to me the battery is just barely acceptable for a flagship. When I compare the Huawei P20 Pro, see our full review[2] that gets me through almost two full days of moderate to heavy use, running out of battery on the S9 Plus mid-evening is a terrible experience that actually adds a bit of stress to my day as I watch the battery drain without a power source in sight.
Mophie's new S9 Plus Juice Pack[3] can help make sure you get through that one full day, but it can't do much more than that. The Galaxy S9 Plus has an integrated 3,500 mAh battery that may get some through a full day, but not if you are actively using it and also taking photos and video. The Mophie S9 Plus Juice Pack only adds another 2,070 mAh so you are likely not going to get through that second full day with this $99.95 accessory.