Hi @cmackinnon, you may take one of a few solutions to fixing your problem. First check to see if your battery is fully locking into the charger, if it is then you may have a dead battery. Check to see if your back-up battery is charging by placing it on the charger. If the back up battery is not working you most likely have a broken charger. Check out the DeWalt DW920 Troubleshooting page here if you have anymore questions.
[[DeWalt DW920 Troubleshooting]]