The Battery Charger would say that the battery was already charged or fault (most likly fault if that is an option). Unless of course you severly discharged the battery to the point where the voltage was below 6V (way past usable on a robot). Think of volage as the force or pressure on the electrons. A battery charger works b/c it has higher voltage or force than the battery so it forces electrons backward through the battery, thus charging it. When the voltage of the charger is equal to the battery, it is fully charged. If the battery voltage is higher than the charger (like if you hook up a 6V charger to the 12V battery), the battery will try to "charge" the charger. This could damage the battery charger (I can smell the smoke). Two 6V chargers in series (6+6=12) might worK
