Car Electronics And Audio

why 12v charger only supplying 9v?

this is a car battery charger, rated ouput is 12v 2.7A dc (4A rms) when i put a meter across the terminals it reads 9.7 volts. what could this be? i assume the transformer is breaking down somehow......i don't know.

This is with no battery connected?

With no battery, the supply has no filter cap, so the output is a full wave rectified waveform. When you put a large capacitor OR a battery across it, the waveform becomes a DC voltage equal to the peak of the AC, as the cap or battery acts as a storage device, holding the voltage at it's peak value.

If you read 9.7 volts, that means the RMS AC input to the rectifier is 9.7/.9 = 10.8 volts.
The peak equivalent is 1.4 x that or 15 volts, which is what you need to charge a 12 volt battery.

So it is working correctly.

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