It’s not often that an accessory is perfect. Yet this Mophie design gets as close as it possibly can, building inexorably to something of a huge caveat: its price. But hopefully this look through the tech and design will convince you that this is worth $100 or so, if you occasionally live on the ragged battery edge…

The concept and sales pitch first though – who is this aimed at? In my life, the following is a semi-regular occurrence, maybe once every few weeks. I’ll be on a big day out, either business or pleasure, and I’ll be on the way home and my phone (in this case, the Galaxy S9+) is on its last legs in terms of battery, after mountains of camera, GPS and general use. Just when I want entertainment or a Twitter fix on the train home, the battery is down to less than 20% and I’m getting worried.

The traditional solution is to plug in a powerbank and, trust me, I have plenty of these. And they work just fine. The only downside is that you’re then carrying and using a smartphone with a cable hanging off it for the next hour and this is inelegant, to say the least. The Mophie Juice Packs, here in a classic two-part design, carry the day by slotting onto the phone directly and supplying a boost charge with no dangling wires.

You’ll be wondering about the little green things – they’re rubber nipples designed to brace the S9+ securely inside the pack and stop any possible rattles.

True, when in place, the Juice Pack significantly adds to the weight (an extra 100g) and thickness (an extra 7mm) of the phone, but I don’t think Mophie expects an owner to use their phone in the pack 24/7. The use case here is that you’re heading out on a longer than usual day and your absolute priority in terms of phone use is that you want to have plenty of battery capacity. With this Juice Pack ‘on’, the S9+ has around 5500mAh in its tank, and that’s more than enough for the heaviest possible daily use. Maybe even for a weekend’s hiking?

So yes, you have to decide ahead of time that you’ll need to (pre-charge and) ‘don’ this accessory, but the solution does work. When not in place, the Juice Pack either stays at home or lives in my briefcase, ready for slapping on at (say) teatime and then I’m good to go for as long as needed.

Of note is that the Juice Pack can be charged separately, i.e. without the phone. So it’s ready for action independently.

What’s impressive is how few compromises Mophie has made. In the past, I remember their Juice Packs (and clones from other companies) coming with microUSB input whatever the phone inside, and I don’t remember them supporting Qi wireless charging – after all, a Li-Ion battery does a great job of blocking Qi charging. But here they’ve wired the internal electronics for both USB Type C input, with some Quick Charge capability (up to 9V, 1.5A, at least) and with Qi, thanks to an extra coil layered underneath the extra battery.

Squeezing all this into an extra 7mm isn’t trivial, but the Juice Pack case is beautifully moulded and, when assembled around the Galaxy S9+, the whole feels a little like an S9++, i.e. an extra ‘plus’.

Aside from the physical compromises, you do lose some physical access to the fingerprint sensor and it has to be operated more with the tip of the finger than the pad. In practice, this means that it still works but isn’t quite as convenient. So I end up powering on with the side button and then ‘smart unlocking’ or using a PIN or pattern, using the fingerprint sensor just for authentication for payments.

But it all works. Pop the cased S9+ on a Qi pad and it charges away happily (at up to 1A). Plug in a USB Type C charger and the current is split between phone and Juice Pack, i.e. they both charge at the same time. And, crucially, when you’re charge is running low on the phone, just long press the flush button on the Juice Pack’s back and the LEDs animate to show you that the accessory is now charging the phone. The S9+ Juice Pack has an extra 2070mAh, though you’ll lose a little of this in the transfer process. As a rule of thumb, you’ll be able to claw back 1750mAh or so from a fully charged Juice Pack.

What of physical obstructions? The Galaxy S9+’s headphone socket is ‘covered’, but there’s a pretty wide hole in the Juice Pack to allow jack insertion – you’ll only come a cropper if you have a right angled jack. The bottom speaker is also covered, but there’s a moulded acoustic baffle here that deflects the sound to be ‘front-facing’, and this works really well. Ditto sound heading into the bottom microphone.

The Juice Pack only comes in black and with a matt finish that does tend to collect finger grease marks – but then so does any case and that’s the whole point of a case, surely – to take the knocks and dirt so that your phone doesn’t have to?

All of this design, technology and fit and finish doesn’t come cheap though. The caveat mentioned is the price and there’s no getting round the fact that the best part of $100 (or local equivalents) is on the high side for an accessory – especially when a power bank of similar capacity would be 20% this price (at most).

Whether the Juice Pack is worth it to you will depend on your use case. If you need such a beast only once a month (or less) then live with the power bank and a dangling cable. If you need extra capacity in a more streamlined form, as here, once a week then this could be an excellent investment. In my case, the once a week rule absolutely kicks in and on such days I swap out my Spigen Rugged Armor (reviewed by me here) for this Mophie Juice Pack, knowing that I’m then heading out the door with a 5500mAh (or thereabouts) phone that can withstand anything the day has to throw at me.

You can buy the Mophie Juice Pack for the Galaxy S9+ here – and at local vendors and distributors, no doubt.