Love your iPhone 4 or 4S? Sure you do. Hate that it can barely get through a full day without needing to be plugged in? Join the club.

One thing almost all iPhone users agree on: the battery life could do with some serious enhancement. But since this has been a complaint of the iPhone since its debut in 2007, it’s pretty clear that Apple doesn’t consider it a top priority.

But what are you to do if you depend on your phone to keep you connected for long periods of time? The first thing you should do is read our 12 Tips For Extending Your iPhone’s Battery Life. But if you’re still hungering for juice, you may want to consider Third Rail’s Slim Case + Battery solution. While not new to the market, the product is now shipping to Canadians from Amazon.ca for the first time.

Unlike other snap-on batteries or cases with built-in batteries, Third Rail’s product doesn’t force you to compromise on form in order to get the function.

The Slim Case is a sleek matte-black unit that provides a perfect fit for both iPhone 4 and 4S models. It adds only a millimetre to the phone’s sides, top and back with the only noticeable bulk being on the bottom edge where it lengthens the phone by just under a centimetre.

The slightly rubberized texture provides excellent grip and as any phone case should, it provides enhanced protection from bumps and drops. But the truly clever part of the case design is on the back, where 4 discreet slots accept a slide-and-snap on Smart Battery pack.

Rather than carry the extra bulk of the battery around with you all of the time (see the Mophie Juice Pack), or rely on bottom-mounted battery-boosters (Scosche iBAT2) that could easily damage your dock-connector port if it were bumped the wrong way, the Third Rail Slim Case lets you add the battery when you need it, remove it when you don’t, and while it’s doing its job, it sits where it should: on the back of the phone.

If the Slim Case system only did what I just described, it would be a worthwhile choice. But it has a few tricks up its sleeve that make it a no-brainer for travelling road-warriors.

The snap-on batteries have built-in power meters: a series of 5 green LEDs that indicate the level of charge at the touch of a button. The batteries can be recharged on their own via a supplied micro-USB cable, or they can suck their power through the Slim Case’s bottom-mounted micro-USB port (which BTW, doubles as your iPhone’s Sync/Charge port) while you charge both the external battery and your iPhone’s internal unit.

You can use the batteries to charge almost any other USB-powered device (MP3 player, Bluetooth headset or keyboard etc) through a secondary, proprietary port which connects to the included USB-A female adapter.

The Slim Case has a bottom-mounted switch that lets you control the flow of electricity from the battery pack to the iPhone so there’s no need to remove the battery if you aren’t ready to boost your iPhone’s battery just yet. The same switch controls the recharging of the battery pack from the micro-USB port.

Finally, the Third Rail battery packs are stackable – up to 6 of them can be snapped together  – to create a truly flexible charging system.

In practice, I found that a single snap-on battery could take my iPhone 4S from 30% charge to 60% over the course of 2.5 hours while the phone was still powered-on and in regular use, while it could bring the phone from 3% to 50% when the phone was powered down over about 2 hours. When you take into account the price, which is very competitive with other case-based recharging systems, and the additional flexibility Third Rail offers, the Slim Case system seems like a good buy indeed.

It’s not completely without drawbacks however. As with any case that covers your phone’s dock connector, you won’t be able to dock your phone without first removing it from the Slim Case. Since I like to charge my iPhone on a bedside alarm-clock charger, this was a little frustrating.

If you like to pocket your phone when not in use, you’ll find that the battery pack creates a lumpy shape and might not be very comfortable for long periods.

You’ll need to decide how you want to tote around your back-up batteries – attached to the case, in a separate pocket, or somewhere else?

The Smart batteries are rated at 1250 mAh, which means if you want to fully recharge your phone from zero, you’ll need two fully-charged Smart batteries.

Overall though, I think you’ll find Third Rail’s Slim Case an excellent choice for extending the life of your iPhone when out of reach of an electrical outlet.

You can buy the Slim Case from Amazon.ca with a battery for $78.64 or by itself for $39.23 and extra batteries are available too at $54.48 each.