What is the Normal Internal Resistance of a Car Battery?
4 Answers
Car starting batteries generally have the following internal resistances: 12v38ah—8.20 milliohms, 12v65ah—5.80 milliohms, 12v100ah—4.50 milliohms. Below is relevant information about battery internal resistance: Introduction to Battery Internal Resistance: This refers to the resistance encountered by current flowing through the battery during operation, generally categorized into AC internal resistance and DC internal resistance. Since rechargeable batteries have very low internal resistance, measuring DC internal resistance is affected by electrode capacity polarization, which generates polarization resistance, making it impossible to obtain the true value. Measuring AC internal resistance eliminates the influence of polarization resistance, providing the actual internal resistance value. Reactions on Battery Plates: The chemical reactions of lead dioxide and lead inside the battery essentially involve electrochemical reactions between the active material on the plates and the dilute sulfuric acid electrolyte, generating electric current.
I've been driving for over a decade and usually check my battery myself. The internal resistance typically ranges between 2 to 10 milliohms when normal. If it's too high, exceeding 15 milliohms is dangerous, causing the engine to shudder or fail to start during ignition. Using a professional internal resistance meter gives the most accurate reading, as ordinary multimeters won't do. I recommend checking it once every year before winter sets in. If the internal resistance is on the higher side, it indicates battery aging or terminal corrosion, requiring immediate cleaning or replacement. Don't wait until you're stranded on the road to regret it—towing fees are much more expensive then. Keeping the battery dry and ensuring tight connections can extend its lifespan, saving money and ensuring safety. In short, low internal resistance ensures your car won't let you down during cold starts.
I'm a car DIY enthusiast and often tinker with car parts. The normal internal resistance of a battery ranges from 2 to 10 milliohms, and it's more accurate to measure it with an impedance tester. If the resistance is too high, say above 15 milliohms, starting the car will be noticeably delayed or fail. I bought a portable testing tool online for just a few bucks. Make sure the vehicle is turned off before measuring. Increased internal resistance is usually due to battery aging, dirt buildup, or improper use. Keeping the terminals clean can delay the problem. If you notice difficulty starting the car, measure the internal resistance first before rushing to replace the battery to avoid unnecessary expenses. Internal resistance is related to voltage, so be cautious if it drops below 11 volts.
I'm most concerned about driving safety, especially when picking up and dropping off kids at school. A high battery internal resistance is abnormal—it should normally be below 10 milliohms. Otherwise, the voltage drops quickly, and the car might fail to start in freezing weather, leaving you stranded on the road. Just last week, my neighbor's car stalled on the highway, costing $200 for a tow truck. Checking internal resistance requires specialized equipment, so average car owners shouldn’t worry too much, but keeping battery terminals clean and minimizing short trips helps. Abnormal resistance often signals a dying battery—regular maintenance can prevent sudden breakdowns. Safety first: fix issues promptly before they escalate.
