Why (theoretically or technically) you need to load a battery to get good voltage measurements?
First off I am back. I will post later about the limits I have now. But to answerer the question:
Loaded vs. unloaded voltage:
The total voltage in a circuit it equal to the total of all the individual voltage drops in the circuit.
LOL… what heck does that mean?
The voltage in a circuit will divide itself across all the individual resistances in a circuit and add up to the total supply voltage.
Let’s look at a very simple circuit.
A 5 volt battery and a 5 ohm load.
What we have is 5 volts across a 5 ohm load so all the voltage would be “felt” across the single resistor (well not really but hang with me for a bit and I will explain). In this circuit we would have 1 amp flowing… right? I = E / R 1= 5/5 The voltage drop across the one resistor would be (E = I x R) 1 amp X 5 ohms = 5 volts The power, or heat from that one resistor would be 5 watts (P = I 2 x R) 1X1X5 =5.
Now let’s take that same circuit and add 2 resistors:
5 volt supply with 2 – 2.5 resistors in series:
Each resistor will “drop” 2.5 volts across it. So 2.5 volts plus 2.5 volts will still equal the 5 volt supply, and we would still have 1 amp flowing in the circuit. You can prove this with ohms law. 5 volts total with 5 ohms total still equals 1 amp total (I = E / R) 5/5=1. Now to see the voltage drop across each resistor you would use:
E = I x R 1 amp X 2.5 ohms = 2.5 volts. Each resistor will only “feel” 2.5 volts. When we add all the drops in the circuit we get 2.5 + 2.5 = 5 volts.
The power (heat) from each resistor would be (P = I 2 x R) 1 X 1 X 2.5 = 2.5 watts.
OK, one more circuit to make this clear.
5 volt supply with 2 resistors that are 1 ohm and 4 ohms in series.
We still have a 5 volt supply and still have 5 ohms total so we still have 1 amp flowing, right? BUT the voltage drops will be different. The one ohm resistor will drop 1 volt
(E = I x R) 1 = 1 X 1 and the 4 ohm will drop 4 volts. (E = I x R) 4 = 1 X 4. Total voltage drops equal 1+4=5.
So, as I said before. The total of the voltage in a circuit will always equal the sum of the individual voltage drops.
Sorry for the tech stuff above, but I had to do that to answer the loaded voltage question.
If we look at a simple mod circuit we have: 1-battery, 1- atty or carto, and one switch. Correct? And in a perfect world we could say, we have a voltage supply, a resistor (atty), and a switch, but we do not live in a perfect world. What we REALLY have is….. A voltage supply, a resistor (atty) , a switch, AND the internal resistance of the battery. I bet you thought that the battery was a voltage supple and not a resistor….It is both.
When you measure unloaded voltage your circuit would be …. Supply, supply resistance, switch and meter. The meter “looks” like a HUGE resistor to the circuit and therefore “sees” almost all the supply voltage. Remember in the above circuit the 4 ohm resistor saw much more voltage that the 1 ohm one. In this cast the meter is 10,000 or more ohms, and there is very little current flowing, so almost all the voltage in the circuit is seen buy the meter.
When you measure loaded voltage you are looking at the voltage across the 2-4 ohm atty minus the voltage across the batteries internal resistance.
The higher the battery resistance, the regulator chip, wire you are using, or even the switch resistance, you have the greater the difference in loaded vs. unloaded with be.