Today I was checking the voltage between charging terminals going to the battery of emergency lantern using a multimeter.
The multimeter showed a DC voltage of 9V and an AC voltage of 4V.
Could this AC voltage have damaged by old battery?
The DC voltage while being measured was not constant and the decimal part was changing in a cyclic pattern.
I only saw 3 diodes on the board. Is this a half-wave rectifier and is the multimeter mistakingly showing the ripple of half-wave as an AC voltage.
Please see the image of the circuit.
Best Answer
It's fairly common to see a lead-acid battery charged using rectified AC. As long as the charging current isn't beyond the capability of the battery, it will 'work'. If there isn't a series resistor somewhere, or some primary-side limiter, the winding resistance of the transformer could be what's limiting the charging current.
A handheld multimeter is sensitive to 60Hz AC, so yes, your DC reading was likely skewed by the low-frequency ripple.
The circuit you have drawn is a full-wave rectifier.
If there isn't any explicit current-limiting protection in the charger, it is possible that the charger can become damaged if subjected to long-term overload. Wall-wart adapters have similar failure modes (usually the transformer goes high-impedance).