Alot of newer style consumer battery chargers will not energize the cables untill they sence some voltage on the leads.
This is a quote from a Solar 1040C charger manual
"This charger is equipped with Auto On-Off Technology.
After connecting to a 12 volt battery, the charger needs to
sense a minimum voltage condition to begin charging and
it will stop charging when it reaches the maximum voltage
condition, but it will continue to monitor the battery charge
level. The charger will "hum" as it monitors. If the battery
charge level drops, the charger will begin charging again."