DT Systems now supply a single Jack Cable with their systems which will plug on to the battery charger and charge the one collar only. Each Add-On collar purchased will include an additional Jack Cable to connect to the first, meaning you'll have either 2 or 3 Jack Cables joined together for a 2 or 3 dog system.
This has complicated things a bit if you lose a piece, so give us a call and tell us how many dog collars are on your system and what cables you still have and we can supply what you need to get charging again.