We've been playing with PhoneSuit's Primo micro battery pack on our iPhone 3GS for a few days. As the name suggests, this thumb-sized battery is the smallest we've seen in its class, but somehow it still packs 800mAh of juice -- almost two-thirds of the iPhone's built-in cell. Feature-wise you'll only find battery level and charge status indicators on the front, and a mini-USB port on the back for charging. Unlike the slot-in case-style battery packs for the iPhone, this battery pack's dock connector is on a flat top, which makes it compatible with "all iPhones and most iPods." We also noticed a 2mm gap when plugged in, and apparently this is to allow space for your iPod's / iPhone's protection case -- too bad our iPhone didn't have one.
While the battery life lived up to its promise for us, this broad-compatibility design backfired on us -- the battery pack was thicker than our iPhone 3GS, so the overhanging sharp edge on the back often caught our trouser pocket while the phone was being pulled out. Alas, we were more worried about our battery pack coming loose than our iPhone's own battery life. Hopefully PhoneSuit will consider making device-specific adapters to give us a peace of mind, or maybe offer baggy pants at discount prices.