An attempt at repairing a possibly malfunctioning FERM 77917 12V battery charger. The last time this was used there was a burned smell and some smoke seemed to appear from the device. There is a possibility the battery being charged at the time was shorted.
The front of the device. The indicator on the left shows the charging current. When at 0 the battery should be fully charged. The chart gives an indication of the charge time: a 20 Ah battery should take about 5 hours to charge, a 80 Ah battery would take 22 hours. The two buttons on the right are probably switchable fuses; when the current exceeds 10 A the button lifts and has to be pressed to reset the fuse.
An inside look. Not shown are the cables; the 220V power cable to the bottom right and two cables connected to the positive/red clamp and negative/black clamp on the left.
Sticker on bottom.
When connected to AC I measure 0V across the naked copper wires exiting the coil. This is both on the AC and DC setting on my multimeter. I later realize, because there is no visible corrosion, that this is not naked copper; it is insulated wire.
When instead I measure the connectors at the other ends of the same wire it gives me 11.7V AC. Strangely, the charge indicator is vibrating with an audible sound (probably the net frequency of 50Hz).
Foolish mistake! The clamps were lying on my desk besides each other making an electrical connection. When I turned on the device the charge indicator immediately went to max, indicating a short circuit. After this I noticed the sound was no longer present.
Measuring the coil now gives 14.4 V AC. Measuring the output shows 13.07V DC. Apparently the device is functioning correctly?
This is my oscilloscope showing the voltage of the coil output.
This is my oscilloscope showing the charging voltage (voltage on the positive/negative clamps). Not a very good DC voltage but apparantly good enough for charging batteries.
