Using my multimeter, I measure the voltage across my battery terminals, (+) to (-), while the car is off. I get about 13 volts. Fine.
I turn the car on and take the same measurement to see if the alternator is charging properly. I get about 14 volts, which is about right from what I hear...but the polarity is the same?
If the battery is being charged, shouldn't I be seeing minus 14 volts since current is going into the battery?
** Nothing wrong with the car. I took these measurements b/c I wanted to see what voltage my alternator was charging at versus another car.Why doesn't polarity on car battery change during charging from alternator?The polarity will not change. You just have a greater voltage potential across the batter because of the charging of the alternator.
You would see a positive to a negative current flow if you were to measure current (remember you measure current in-line with the circuit). This would be done with a clamp meter, since the current traveling through your battery leads is too high for an in-line multimeter.Why doesn't polarity on car battery change during charging from alternator?The polarity remains the same. It's the direction of current flow that changes. If you want to see that change you need to install a current meter in line with one of the terminals.
From here we get into electrical theory.Why doesn't polarity on car battery change during charging from alternator?Cars run off a 12 volt system. The car, after being started actually runs off the alternator (unless) the engine is not running well. In that case, it draws from the battery just long enough for the alternator to kick in when you step on the gas, in effect, giving the alternator the RPM's it needs to supply power. Some engines running at low idle RPM's can have this discharging effect. The alternator supplies just over 12 volts which in effect operates all of the 12 Volt accessories and then the extra 1 or 2 volts of extra power is sent to the battery to keep it charged. The voltage regulator has a role of switching, sending power back to the battery when it's needed. It's a very simple process.The altenator is equipped with a DIODE.
This is the electrical equivalent of a check valve.
It will only let current flow in one direction. You are correct that the polarity changes but where you are checking is on the downstream side of the diode so you do not see the reversal. The 14 volt positve output is good. You will not see minus 14 volts unless you reverse the leads from your tester.
