Typical car battery, 12V. While connected to the car, It read 12.38V on my multimeter. Using the absolutely same multimeter, I measured it once I disconnected it from the car – voltage went to 12.56V. This is in the span of 5 minutes. First measure was taken outside at 1C, second at home at 18C.
Car sat for 1 week straight before disconnecting it. I charged it and reconnected, after sitting for one day voltage was back to 12.4V, thus I eliminate some sort of crazy parasitic draw as else after 1 week it would be dead, not same as after one day.
Is that type of drop to be considered normal?
Best Answer
Yes, this is perfectly normal. A battery has an internal resistance. When you add a load on to it, it becomes a resistor divider.
For example, assume you have a 12V battery and it read 12V exactly when measuring it. Imagine this battery has a 0.2 Ohm internal resistance. You then add a load of 10 Ohms.
Use the voltage divider equation [Rload/(Rbat+Rload)]*Vbat = 11.76V
This is why you get a variation in voltage measurement when you measure it with and without a load. Other environmental factors can play a part, such as temperature, but the main culprit in many cases I think you will find is the internal resistance.