Originally Posted By: oilboy123
With an ohm meter? Won't you fry it........
Never use an ohmmeter to measure battery resistance, besides burning up your ohmmeter, you will short the battery which may cause it to explode!
Ahh… we all remember the days when our batteries could not start our cars. It could be due to a discharged battery, old battery, or a completely fine battery but the cable leads may be loose or sulfated, just happened on my sister’s car last week. In each case we are fighting internal resistance which will determine whether we will be able to start our vehicles.
You cannot use an ohmmeter because an ohmmeter supplies a constant current through a circuit, measures the voltage drop and calculates the resistance (R=V/I) We can borrow this equation, the battery in this case becomes the current supplier, we draw a certain amount of current, measure the voltage drop difference and calculate its internal resistance. Unlike a piece of wire, a battery will have a constantly changing resistance. As discharge happens electrolyte chemicals migrate towards the plates, the specific gravity of electrolyte drops, as both produce ever larger resistance. This is the same reason why old batteries or batteries with sulfated connections will have enough power to power your accessories, yet they will be unable to start the car. It is the same exact principle that your local mechanic uses to perform a load test on the battery.
Remember that over time, resistance in a cell or across a series of cells (a battery) will increase as no reaction is 100% complete. Due to this fact, each battery will sulfate over time and its electrolyte solution will decrease from the ideal 1280 specific gravity. This is the reason why we here all obsess about buying fresh batteries from vendors that have high battery turn around.
After a battery has been sitting for a while (4 to 6 hours) to remove surface charge, we can record its voltage, apply a specific current, record the voltage drop, then take the difference in the two voltages, divide by the current applied and we have our internal resistance. We are basically doing the same thing as our local mechanic, and are doing the same calculations as our ohmmeter would do on a circuit.
R=I/V
Lets say that a battery has OVC (open current voltage) of 12.60v. I take a 12w bulb which will consume (12w/12.60v)= roughly 0.99A. If the battery voltage drops to 12.55v then our difference is (0.05v/0.99a)= roughly 0.05 ohms of internal resistance.
Older batteries will produce a greater voltage drop with the same current applied than newer batteries as the internal resistance is much higher, and that is why older batteries have a tougher time starting engines on cold mornings. The prolonged starting is usually the first sign for most people that their battery is about to give out.