Riding on trails in winter:
Fat bikes were designed for riding in snowy conditions, but tend to do best on trails that have been groomed. They can get bogged down in loose, deep snow. If it's a shared multi-use trail, it's good to be considerate of other trail users. Avoid riding over cross country ski tracks, and avoid any trails that are overly wet and muddy, as it will contribute to soil erosion that damages the trail. Some places have entirely banned ebike access due the negligence of a few bad riders.
As good as the fat tires can be for snow, they're terrible for icy surfaces. The only thing that sticks to ice is studded tires. A few companies make studded fat bike tires (Vee Tire, quite notably), wide studded tires don't grip the ice nearly as well as a narrower studded tires (it's all a matter of how much weight gets concentrated onto the tire's contact patch). When it comes to choosing tires, there is a much broader selection of 26"x4" winter tires than there is for 20"x4" tires.
Riding technique is also more critical in winter. Even with best equipment, you're still riding on something that is slippery. Your speed, weight distribution, and careful brake modulation are all important factors in maintaining control of the bike, and enjoying your ride. The skills you learn in winter will translate to much better bike handling skills when it's mountain biking season in the summer.
Riding in the city in winter:
If you're riding on icy streets, your best bet is to use a bike with a narrower studded tire. When it comes to traction on ice, you want to concentrate all the weight into a narrow contact patch for the best grip. Much like using a fat bike in the snow, you still want to pay attention to your speed, weight distribution and braking. Hard braking can cause the tires to shed studs, and frozen pavement is far less forgiving than a snow covered trail. Plenty of companies make 26"x2" tires that can fit either the DJ City Bike or the DJ Mountain Bike.
More cities have been recognizing that increasing numbers of people use their bicycles in winter, and have responded by doing snow removal on bike paths. Despite that, you may still run into routes that haven't been cleared or have recently accumulated snow. Snow removal can often make main roads and paths much narrower than normal. You may find it easier to ride on a quieter side street, instead.
Because of the reduced daylight hours, it's important that you're visible to other road users. It can be helpful to wear a reflective vest, and carry some extra lights with you, in the event of main light failure. Please note that the DJ Mountain Bike and Fat Bike models currently do not come with tail lights. The controller on the Mountain Bike has a tail light connector as part of its wiring harness. If you install a rear rack on the Mountain Bike, it can be fitted with the same tail light as the City/Folding Bikes. The DJ Fat Bike has no tail light provisions with its controller. The DJ Mid-Drive Fat Bike has no internal lighting system at all. It comes with a basic battery operated headlight that attaches to the handlebar, but we would consider upgrading to something more powerful that has a rechargeable battery.
Electrical system and winter:
The LCD will have a slower response time in the cold, and the LCD segments may show up darker than normal. It can operate in conditions as low as -20°C.
Although the electrical connections are designed to be water resistant, water can sometimes seep into the connections and cause electrical problems. A really good idea before the winter riding season is to go over every connector in the wiring harness, spray some contact cleaner inside the contacts, then put some dielectric gel on them to keep corrosion off. You can also wrap the connection with some good quality electrical tape for an extra measure of protection. It's also a good idea to routinely inspect the main battery terminals and the power connector to the motor. These both need to handle high levels of power. Any corrosion that gets inside can cause heat damage to the terminals.
Care and maintenance:
Road salt can cause serious corrosion issues on the bike. It's a good idea to rinse off the salt when you get home, dry the bike, and check that the drivetrain is properly lubricated. When it comes to wet and slushy conditions, the DJ City Bike is the only one that has full coverage fenders. Upgrading to full coverage fenders would be a good idea for any of the other bikes. It's strongly recommended to store the bike inside. You'll have much fewer maintenance problems with a bike that isn't left to sit outside in bad weather.