At What Internal Resistance Should a Car Battery Be Replaced?
2 Answers
Car batteries need replacement when the voltage after charging is less than 13 volts or the voltage after discharging is below 10 volts. Car batteries are categorized into conventional lead-acid batteries, dry-charged batteries, and maintenance-free batteries. The commonly referred to 'car battery' typically denotes a lead-acid battery, with a normal service life ranging from 1 to 8 years, heavily dependent on vehicle conditions. Conventional batteries have plates composed of lead and lead oxide, with an electrolyte of sulfuric acid solution. Dry-charged batteries, fully termed as dry-charged lead-acid batteries, feature negative plates with high energy storage capacity, capable of retaining charge for up to two years in a completely dry state. Maintenance-free batteries, due to their structural advantages, exhibit minimal electrolyte consumption and generally require no distilled water replenishment throughout their service life.
Regarding when to replace a car battery based on internal resistance, I often encounter this issue during repairs and believe the key points are the numerical values and daily performance. A new battery typically has an internal resistance between 5 to 10 milliohms, performing well. When the resistance rises above 15 milliohms, you may experience hesitation or delays during startup, which is a sign to seriously consider replacement. If it exceeds 20 milliohms, replacement is absolutely necessary—otherwise, the car may struggle to start in cold weather. I also recommend combining this with a cold cranking amps (CCA) test; for example, if the CCA drops below 70% of the original factory rating, it’s another warning sign. Keeping the battery terminals clean to avoid corrosion can help slow the increase in internal resistance. Regularly checking with an internal resistance tester is convenient, inexpensive, and helps prevent issues. Remember, the average battery lifespan is three to five years—addressing problems early avoids roadside breakdowns and ensures safer driving. This is based on my practical experience, and I hope it helps you.
