martin if you check the info on the charger it will charge only 1 battery at a time on up to 4, so you can put the 1 battery in by itself, if it is way low on charge it will take longer to activate the charging lights but if you check the battery you will see that it is charging and coming up slowly, and once it reaches a certain level the fast charge light comes on, i use my battery tester to keep an eye on it once it is back up to par it will work fine but the problem im having is the chargers are locking into the fast charge mode and not shutting down to trickle charge when it reaches capacity.
hope this helps I sent an e-mail to comp and waiting for a reply
sarge