Fi Series 3+ Review: The Long-Battery GPS Dog Collar, Weighed Against Tractive

The Series 3+ finds its position with GPS and reports it over AT&T's LTE-M network, with roaming in the US, Canada and Mexico, according to Fi's product page and network support article. At home it hands off to a lower-power link: Fi says the tracker prefers the Base, then your phone over Bluetooth, then 2.4 GHz Wi-Fi, and only falls back to GPS and cellular when none of those is in range (connection guide). That hierarchy is why the battery lasts so long, and it also means the collar is only as quick as the link it is on.

Fi claims the Series 3+ antenna delivers GPS tracking twice as fast and more accurate than the Series 3 (upgrade article). No independent source has measured that claim.

The best independent evidence comes from Treeline Review, which ran the Series 3+ for months on two off-leash dogs in Colorado alongside a Tractive DOG 6 and a Garmin collar. Its tester found Fi's locations slightly less accurate than Tractive's and Garmin's, though not in a way that would affect finding a lost dog within cell range, and connection times slightly slower. Default location updates came roughly every five minutes, with live tracking every few seconds once Lost Mode is on. Safe-zone exit alerts were described as remarkably accurate, while hike maps sometimes had missing sections.

For most owners the practical question is coverage. Fi uses one carrier, so check AT&T coverage where you walk before buying. Treeline's tester had no slow-connection problems in an area where AT&T phone service is poor, but one tester in one region cannot speak for every town.

Battery life is the Series 3+'s strongest point. Fi rates it at up to three months with low use, four to six weeks with moderate use, and up to two weeks with heavy use, while Lost Mode drains it in about two days (battery article). Those tiers depend on how much time the dog spends away from the Base, your phone and home Wi-Fi.

The independent result supports the claim. Treeline Review reports that its Series 3+ lasted about ten weeks in testing. In the same tester's use, the Tractive DOG 6 lasted about 25 days (Treeline's Tractive review), so the Fi needs charging far less often.

Charging is less convenient. The tracker only charges on the Fi Base, a round dock that also acts as the home Wi-Fi and Bluetooth hub, and a full charge takes around three hours (charging article). In the box are the tracker, Base, band and a USB cable but no wall adapter, so you need a 5 W or higher USB charger (in-the-box article). The sealed battery is not user-replaceable, according to the safety manual filed with the FCC. If you travel with the dog, the Base has to come with you.

Behavior tracking is the Series 3+'s headline addition. Beyond steps and sleep, Fi says the collar uses AI to log eating, drinking, barking, licking and scratching (behavior article). The app turns these into daily timelines and can alert you to unusually vocal or quiet days.

Fi is careful about what this means. Its support page says the data syncs through your phone, Wi-Fi or the Base with a delay of up to one or two hours, is not captured during walks, and does not diagnose medical conditions. Treat it as a trend line that might prompt a vet visit, not a health monitor.

Independent evidence on the behavior categories is thin. Treeline Review named the Series 3+ its pick for advanced activity tracking and noted that it records walks and drives automatically and tells them apart, which the Tractive DOG 6 does not do. No reviewer has checked the eating, drinking or scratching counts against observation, so we cannot say how accurate they are.

Apple Watch integration is new on the Series 3+ (upgrade article), which is useful if you want alerts on your wrist during walks.

A Fi membership is required to activate the Series 3+ and for every remote feature, including GPS location, escape alerts and Lost Mode (membership article). The hardware itself is included with a first membership. In the US, Fi sells prepaid terms of six months, one year or two years, adds a one-time activation fee, and sets a six-month minimum commitment, per its product page. New US and Canadian members can take a 30-day trial that ships a certified pre-owned unit and converts to a one-year plan unless canceled (trial terms). Read those terms before signing up.

Our guide to what GPS pet trackers really cost after three years puts these plans side by side with Tractive and others. The short version: with Fi the membership is the real purchase, and the longer terms cost less per month.

The app itself gets good marks. Treeline Review found it less buggy and faster than earlier Fi versions, with unlimited family members allowed on one account. Its tester listed one con: the app needs to run in the background on your phone, which is how the collar uses your phone as a Bluetooth relay on walks.

The tracker module measures 81.5 x 30.3 x 11.5 mm and weighs 47 g without the band (product page). It locks into Fi's own band, so you cannot move it to your dog's existing collar (band article). Fi sells bands from XS for dogs under 10 lb to XL for 95 lb and up, and says the collar is not optimized for cats (size guide).

On water resistance, Fi's support page gives the conservative figure: IP68 tested at up to 1.5 m for up to 30 minutes, plus IP66K for high-pressure water, in salt, fresh or chlorinated water (waterproof article). The product page's marketing claim of a far deeper salt-water test does not match its own support documentation, so we rely on the support figure.

Fit is the weak spot in independent use. Treeline Review found that the included band does not stay adjusted when you need to tighten it, and its tester bought a third-party adapter. Fi lists a flashing night light with a color you choose in the app (light article), which helps on evening walks.

At that weight the module is heavier than clip-on rivals, so owners of small dogs should check the XS and S sizing carefully.

The Amazon listing in the offers table on this page bundles the collar with a six-month membership. Other listings bundle twelve months, so compare the membership length, not only the headline price.

The closest rival is the Tractive DOG 6, covered in detail in our Tractive DOG 6 review. In Treeline Review's side-by-side use, the Tractive connected faster and located slightly more precisely, clips onto any collar and roams across more carriers, while the Fi lasted about ten weeks per charge, far longer than the Tractive, and logged walks automatically. The Life360 Pet GPS is lighter at 28 g but rated for 14 days in its standard mode (Life360 help center).

Fi also now sells the clip-on Fi Mini, which fits any collar and works for cats, and the satellite-capable Fi Ultra, launched in July 2026 (Engadget). The Series 3+ sits between them as the long-battery, collar-integrated option.

Value comes down to battery versus flexibility. For an owner who hates charging, with a dog that stays within AT&T coverage, the Series 3+ is good value over a two-year term. Faster alerts, any-collar fitting and multi-carrier roaming make the Tractive the better buy.