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I have a four and half year old manual transmission car with start/stop technology. The car has required jump start once after I accidentally left the lights on overnight. I have recently noticed that when I press the cluth after the engine has been stopped automatically, the starting of the engine causes the lights to momentarily dim. I don't recall seeing this phenomenon when the car was new. The engine has never failed to start (apart from the one time when lights were left on overnight). The battery is of the type designed for start/stop cars but it is not AGM.

I became interested in the condition of the battery and asked the dealership to do a battery test. Their testing device, which I believe did a cold cranking amp test and open circuit voltage test, showed that the battery is like new but all it needs is a recharge. I'm somewhat sceptical of the results, because of the phenomenon of the lights momentarily dimming when starting the engine at stoplights. I don't believe there is anything wrong with the charging system of the car, either.

How long do lead-acid starting batteries last, typically? What is the failure mode? Failure during a trip, or failure to start on a cold morning? Should I replace this battery merely based on its old age, or should I trust the dealership battery test?

  • Experience tells me that its about three years on average. Operating conditions have a big effect on longevity. The lights dimming during starting is normal because the starter motor uses a lot of current. If the test at the dealership was OK then don't worry about. If you want to make sure just go ahead and replace it. Its been four years and a half already. – race fever Dec 24 '15 at 3:35
  • I replaced my dead battery last month. It lasted 4 years. The new one I bought had a 3 year warranty and an additional 2 years of prorating. The cheaper version was only 2/3 the price, but had no prorate, so that should give you an idea. – MasterHD Dec 25 '15 at 14:43
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According to the Battery University a battery will continue to have the same amount of CCA throughout most of its life (up until the end), but will lose capacity over time. What that means is, it should be able to crank the engine over at the same speed, but won't be able to do it for as long.

I don't think there is any real measure of how long a battery will last. Obviously, you can use the warranty which is provided by the battery as a litmus. If the total warranty is five years (even if part of it is prorated), that is probably the max you can expect it to perform at a reasonable level.

As a general rule, most batteries will last about five years ... At least the regular ones and not the elcheapo batteries. If you are anywhere close to that time frame and you are starting to see little issues, I'd plan on replacing the battery. There is nothing worse than a battery which fails to perform.

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I just bought a battery and it said the expected life of it right on the side of it. Mine said three years but I bought an expensive one.

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I think once you put a good, overnight charge on the battery you'll see improvement. A start/stop car puts a lot more drain on the battery, and gives less charging time. In a normal car, a very dead battery has lots of uninterrupted time to charge back up. Your battery has a significant load on it each time you stop.

That odd charging pattern might not be enough to recharge a battery that is older and not in a good state of charge already.

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The test is pretty sound. As batteries age they discharge faster but provide the same output current. They also charge slower. In a stop/start this will cause issues especially if its used frequently, like city driving.

You may also want to have the alternator and regulator checked, but its probably time to replace the battery. 5 years in your application is pretty great. You can keep going with it, but it will become less reliable.

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