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Take the two srews off the top plate that holds the collar to the battery. You will see the small gold wire that comes from the GPS and plugs into the battery casing. Unplug it. You will see four small screws, one in each corner of the black battery casing. You might need an eye glass screwdriver because the screws are small. Once the screws have been taken out, you can pop off the top of the casing and you will see where the battery plugs into the top of the casing. Unplug the colored wires and let is sit for about 5 mins and then plug it back up and replace everything like it was.
One thing you can try before going through all the trouble of unplugging the battery is take a wire brush and lightly brush the contact terminals on the collar where you slide the charger into and this might fix the problem. When the terminals become dirty they don't make good contact.
You reference the 220 box, but does he have DC 30 or DC 40 collars? The box won't have an effect on the collars charging or not. The box will only have an effect on the collars transmitting to the box.