If you've looked at infant car seats lately, you've probably noticed that they all tout a stay-in-car base or a removable base. A car seat base is a separate piece of the car seat that is installed in the car with a seat belt or LATCH. The base has its own belt path and sometimes has lock-offs to help you install it.
Car Seat Base Features
The lock-offs, if the car seat base has them, can look vastly different from one car seat to the next.
The base of one of the most popular infant car seats, the Chicco Key Fit 30, has built-in lock-offs that are just plastic clips under the upper frame of the belt path. The shoulder portion of the seat belt just slides into one of the clips. Other car seat bases have hinged pieces that fold over the seat belt and lock into place. Some car seat bases have a large center portion that flips up like a trunk lid, and once the seat belt is in place, the trunk lid piece closes down over the seat belt to lock it.
Some bases may have a level indicator, too, but sometimes the level indicator is on the car seat instead. If the level indicator is on the base, it may be a dial where a certain color must show in a window, or it could be a bubble level indicator. Some car seat bases simply have an arrow on a label or molded into the car seat shell, and the arrow must be level to the ground to have the correct angle.
Car seat bases today may have other interesting features such as anti-rebound bars or foot props. The anti-rebound bar pushes against the vehicle seat back. In a crash, after the initial downward rotation toward the front of the vehicle, the car seat pops back up toward the rear of the vehicle. The anti-rebound bar prevents some of that popping up motion.
The foot prop is a rigid bar that helps control motion in the first part of that equation. It extends from the car seat base under baby's back or head, and goes all the way to the floor of the vehicle. This reduces the force of the first downward rotation in a crash.
The car seat itself is attached to the base using built-in locking mechanisms. The car seat base is very handy for infant car seats because you can have a base installed in more than one vehicle, so you just have to click the car seat into the other base to switch cars.
Most infant car seats click onto the base easily. Others require a special touch or some practice to get it right. I recently worked with an infant car seat that needed to be placed onto the base at a specific angle to lock in properly. The majority, though, can simply be set on top of the base and the weight of the seat will lock it into place. Be sure to give an extra tug to check that the car seat is locked onto the base, and listen for the telltale snap sounds that indicate it is properly docked on the base.
Car seat manufacturers may make several styles of the same infant car seat, with various features. While many of the infant car seat models can be clicked onto the base for other car seats in the series, not all can do that safely.
If you have an LX model car seat and want to use it with a spare base for an LX Plus car seat, you'll have to check the manufacturer's instructions to see if that's possible. The same rule applies if there are car seats in a series with different maximum weights. The car seat with a 30 pound weight limit may not work on a base designed for the same brand and model name car seat with a 22 pound limit. Always check the instruction manual.
Many infant car seats can be installed with or without the base. However, there are some infant seats that must always be used with the base. You should always read the manufacturer's instructions for how to install and use the car seat and base.
There are a few convertible car seats available today with a separate base, too. If you're ever unsure about how to use your car seat base, visit an inspection station or checklane to get help from a trained and certified child passenger safety technician.