Inconsistent Idling after Battery Replacement?

2011 KIA OPTIMA
115,000 MILES • AUTOMATIC
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JAYFIVS
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I replaced the battery to my 2011 Kia Optima earlier and upon cranking it back up, it started right up and idled around 750RPMs. Letting it sit for a few minutes, the RPMs bounced between 500-800 in park and my engine would kind of "stop" and "go" causing small shudders and vibration.

Is this normal or is there another issue I should check for?
May 25, 2026 at 3:37 PM
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What to check next

Diagnostic guidance. Suggested checks and possible repairs are separate from the owner's reported results.

Have the battery state of charge, charging-system operation and loaded voltage drops across the battery connections, cables and grounds checked. A terminal can feel tight while the electrical path still needs testing. Compare charging measurements with the vehicle's operating conditions and service requirements. The reported readings do not by themselves separate an alternator fault from a cable or connection problem; the later no-start should not be dismissed as ordinary idle relearning.

Guidance references and applicability

Source discussion: question 1712798 https://2carpros.com/questions/inconsistent-idling-after-battery-replacement--1712798 Supports the reported history and outcome only. Follow-up guidance is editorial synthesis; unverified specifications and diagnoses are not adopted. Fluke: Diagnosing voltage drops https://www.fluke.com/en/learn/blog/automotive/electrical-automotive-troubleshooting Supports loaded feed and ground testing. Use matching wiring and vehicle-specific test limits.

Read the replies · Discussion summary and reported outcome

AI-ASSISTED GUIDANCE · REVIEWED BY 2CARPROS
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STRAILER
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This is normal as the computer is relearning the engine idle map. I would just drive it and the problem should go away, here is a guide to help you as well.


https://www.2carpros.com/articles/engine-stalls-idles-too-high-low-after-dead-or-replaced-battery/

Let me know how it goes
May 26, 2026 at 9:38 AM
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JAYFIVS
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Thanks for the response; but now we have another issue.

Even with a brand new battery, my car drive fine this morning then while idling in a parking lot, my warning lights flickered and flashed on again and after turning the car off, my car didn't start again.

Does this point more towards an alternator issue; or what could be causing my battery to completely drain even after a brand new one was put in last night?
May 26, 2026 at 9:41 AM
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STRAILER
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It sounds like you have a loose or bad connection at the battery, here is a guide to help you fix it.


https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked/

Let me know how it goes.

May 26, 2026 at 12:49 PM
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JAYFIVS
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Thanks again for the quick response; but I checked my battery terminals as soon as my car died and both were tight as could be.

It drove fine for a few hours this morning before dying down and not starting back up parked in a parking lot.

This is why I'm guessing it might be an alternator issue, as the battery is brand new and still won't start after a few hours of driving.

Any other advice or ideas?
May 26, 2026 at 12:51 PM
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STRAILER
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Does the battery cables have a after market end on them? These fail due to corrosion all the time FYI. Once the car is running lets check the alternator. This guide will show you how.

https://www.2carpros.com/articles/how-to-check-a-car-alternator/

Please go over this guide and get back to me.
May 27, 2026 at 7:53 AM
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JAYFIVS
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I did have to change out my positive terminal due to a crack a year ago; but there was no corrosion or anything on it.

We jumped the car and got it running; battery tested around 12.2v before cranking and after the jump start and keeping RPMs around 1500 in park, only slowly climbed to around 13v.

Turned on lights and air conditioning for a load test and my dashboard warning lights started to come on again; so I'm definitely feeling at this point it's the alternator between the voltage only reading 13 after a jump start and revving the engine, and the warning lights coming on as soon as I click on the radio, air, and lights.
May 27, 2026 at 7:59 AM
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STRAILER
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Yep, that can be the alternator is weak but I would remove the battery terminal end, and get a new one, when the end goes bad it will cause this issue. There will be corrosion under the cable end clamp that you cant see.
May 27, 2026 at 9:01 AM

Discussion summary

Inconsistent Idling after Battery Replacement?

After battery replacement, the owner reported uneven idle, followed by flickering warning lights and a no-start. The owner reported about 12.2 volts before starting and approximately 13 volts after a jump-start, with warning lights returning under electrical load. An aftermarket positive-terminal end and the charging system were both discussed.

Reported outcome: No confirmed alternator or terminal repair was reported.

No charging-current, voltage-drop or complete battery-test results were supplied. Corrosion under the terminal clamp was suggested, not observed.

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