The Compass tricycle is prepared/wired up to accept a second battery if desired. It ships with what seems to be two batteries, however the right one is an empty case. The left battery is the viable one and is retained on the tricycle with a locking mechanism. When using the tricycle make sure the power switch (on the left battery) on the tricycle is switched on.
If the 2nd battery upgrade kit is purchased, the lock mechanism will have to be installed on the existing battery dock on the tricycle. This requires opening the battery dock by removing 6 phillips bolts. The lock mechanism is installed using 2 phillips bolts. The battery dock is then closed back up and new battery inserted and locked into place. The batteries will have to be charged independently from one another and most likely the key for second battery will not match the first, so they will use different keys.
To install the new battery you will have to either replace the entire battery dock (that includes the lock mechanism) that came with the battery or you transfer the lock mechanism to the battery dock that's already installed on the bicycle. I find it easier to install the new battery dock as it already has the lock mechanism installed.
In order to do so you will need access to the underside of the tricycle by flipping it on its side or upside down. From the bottom of the battery tray/dock you will find there are 4 cap head screws that hold it to the axle carrier. Once those are undone with a 4mm allen wrench the battery tray comes off. Install new tray by installing the same 4 bolts.
In front of the battery dock is a smaller black plastic box, inside that is the y split for the wiring lead that comes from the battery dock. This is opened with 4 phillips screws seen from the underside. Inside you will find the means to disconnect the old battery dock and hook up the new one.
Use zipties to retain the wiring harness to the frame so that it does not hang down or interfere with any moving parts. Once this is done properly make the connection at the y-split inside the connector box. Make sure all bare connectors/wires are insulated with electrical tape or heat shrink as not to cause a short circuit.
Close the connector box back up with 4 phillips screws.
Now you have the means of locking both batteries securely to the bicycle and choosing to ride with either battery (or both) as a power supply. Remember that each battery has its own switch, allowing you to use whichever battery you choose. It does not matter whether you use both simultaneously or one at a time. If you do choose to use one at a time, it is advised that once one is depleted, you switch it off while the other is on. Keeping the depleted one on, while the fresh one is on might result in negative affects to the depleted unit.