Why Do Two Car Batteries With Identical Voltage Readings Have Completely Different Carstarting Ability
Posted in Car Batteries | Asked on Apr 17, 2010
My car battery was turning my car over too weakly. Its voltage reading was 12.8 volts with the car turned off after sitting a while after being driven. I bought a new battery and before installing it in the car, voltage tested it. It was also 12.8 volts, no doubt because it had been sitting on the store shelves for a period of time. I expected my car to initially start slowly, just like the old battery, until it was charged up either by me later or by the car’s alternator.
With the new battery installed, the car started like a brand new car… like a rabbit! Totally better than before. The difference from the old battery was like night and day!
Why was the start performance different between the two batteries with the same 12.8 volts standalone voltage reading?
What else do I need to test about the battery to tell when my car battery is running out of gas and needs to be replaced? Will my hand held Craftsman volt/ohm multimeter do the trick or do I need some other specialized tester?
There are 4 Answers for "Why Do Two Car Batteries With Identical Voltage Readings Have Completely Different Carstarting Ability"
it’s called amperage. Same power amt but different power output- i.e. the river flowing much faster. A small battery for a corolla is not suitable for a chevy 350 engine. The starters use different amp amts.
a volt reader is suitable to see if the battery is alive and to see how much output you’re getting from the alternator. Should be 14 volts. U only use the battery when u start the car, that’s it. Also, keep in mind old batteries have cells that are worn and may not supply the amperage needed.
To properly check any battery, whether its a car battery or a flashlight battery, it should be done under load. With no load, most batteries will show a normal terminal voltage. As batteries age, they develop an increasing internal resistance which causes the voltage to drop under load while still remaining high when unloaded. Check your battery with the engine off, both unloaded and with the headlights on; there should only be a small difference. Do this now with your new battery to learn how a good battery behaves.
The starting depends on cranking amps, not volts. The batteries could have identical voltage but the old one could be wore out. The new one may have for example 700 cranking amps or 550 cold cranking amps and the old one could have the same but may only be putting out half that or less. Voltage keeps your battery charged by the alternator, which of course you need volts to get amps. If your voltmeter is reading 12.8 volts, try testing it while you crank it, you can tell if it falls below 9 volts if it’s bad or not.
The best way to test a battery, by far, is to apply a decent load (way more than just headlights. more along the lines of what your starter pulls), and measure the voltage while doing so. A battery can be virtually useless for starting a car, but still have a good voltage reading while not under load, as your experience illustrates. (12.6 volts is the normal minimum on a charged lead-acid battery without surface charge)
Short of spending hundreds of dollars on a proffessional grade tester, the best affordable units are like these from Sunpro or Actron or Schumacher;
http://www.amazon.com/3180-100-Amp-Battery-Load-Tester/dp/B000F5HU6C
http://www.amazon.com/Actron-CP7612-Battery-Load-Tester/dp/B0009XQUJI
http://www.amazon.com/gp/product/B000AMBOI0/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B0009XQUJI&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0D37V4A3A0RSHEXKE5DQ
Not one of those dinky little units that looks just like your multi-meter, which pulls even less amps than your headlights, and then extrapolates the results. You could also disable the ignition on your car, and check the voltage while cranking your starter for 15 seconds, but you should have a good idea of what kind of shape your starter is in too, for that to be accurate. Next cool automotive tool for your shopping list, an Ammeter capable of measuring 100′s of Amps, with an inductive clamp.
Here’s a couple articles that came up on the Google search I did to find the testers, which you might find interesting;
http://autorepair.about.com/library/weekly/aa101604c.htm
http://www.batterystuff.com/tutorial_battery.html
I haven’t read all the way through them though, so I’d go with this article for the most reliable info, written by one of the leading writers in the automotive repair field;
http://www.aa1car.com/library/2003/us20310.htm
You can find a bunch of his articles on other topics here;
http://www.aa1car.com/library.htm