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Determining the Voltage of an Unknown Battery in an Old Truck

There is an old truck that has a 6 or 12 volt battery in it which has very little charge left in it. The hood cannot be opened, so to charge the battery, someone uses 3 car batteries (12 volts each) which are placed in series. The reasoning is that if it is either a 6 volt or 12 volt battery that is in the truck, then the 3 batteries in series will certainly charge it up because there will be "extra voltage" in the circuit. Not wanting to blow out the battery in the old truck from an over current, there is placed a resistor of 12 Ohms in series with the circuit from the 3 batteries. Judging from the heat of the resistor this person believes that 12 watts of heat is being dissipated by this resistor. Determine the old truck battery voltage.

Solution Summary

Brief calculations show how to determine the voltage of a truck battery with little remaining charge.