MY BIG MYSTERY-14.5 volts on first start.12.5 volts next start

I can’t figure it out. 2017 Hyundai Santa Fe Sport 2.4. I installed a new alternator and a new battery. I start the car in the morning and get 14.5 volts. I can drive around no issues. If I turn the car off and back on I get 12.5 volts and battery light and discharge warning on screen. If I leave car off all night and start it in the morning it’s at 14.5 volts again until I turn it off. What could this be?? Thanks!!

There may be a problem with the battery current sensor.


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Thank you. That was one thing that crossed my mind as the issue.

Is there a way to actually test that sensor? Would computer diagnostics actually point to that directly or just a generic low voltage/battery light?


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It’s almost like the alternator isn’t being placed in charge mode after you stop and turn the engine back on. Auto charging systems are more complicated these days, usually controlled by a computer algorithm. When alternator is in charge mode, that places an add’l load on the engine. So it can be undesirable to put alternator is charge mode, esp at idle or if some other engine load is turned on, like A/C. Computer decides when to turn the alternator on, based on what else is on, & how much it thinks the battery is charged at that point. There’s probably a way to tell if the alternator is being commanded into charge mode, probably first thing to check. It’s possible this is normal, car is avoiding charge mode for fear it will cause engine to stall, and the warning lights are just telling you that. If so, once battery becomes fully charged, after some more driving, this problem may go away on its own. the 12.5 volt measurement indicates battery isn’t fully charged. To test this theory, you might ask a shop to fully charge the battery overnight, using their battery charger. Check what your owner’s manual says about the charging system warning light also. Do those warning lights go out once you start driving at speed? Also try turning everything off that’s not absolutely needed, turn off A/C, defrost, seat warmers, window defoggers, headlights, etc.

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Thanks for your input! It’s interesting because I drove around for like an hour one day, without turning off the car(accessories on too)multimeter showing 14.5 volts. Then finally turned off car, checked battery which was at 95%, waited a minute, started car and battery light on right away. Used multimeter and got 12.5 volts while running.

If car sits(and maybe cools) I can start it and get no warning light and 14.5 volts. Maybe battery sensor not reporting correct temp??

Sensors in vehicles can be effected by heat and fail.

Once the sensor cools down it works again.


Makes sense. So you think it’s not a case of battery/car overheating and tripping the sensor, but sensor not working well?

Test the sensor when the engine is cold.

Test it again when the engine is hot.

If there’s a duty cycle measured when cold, but not when hot, there ya go.


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OP, are you certain the new battery meets the required battery specs for the car? On your vehicle, does Hyundai require that a new battery install also require the drivetrain computer be informed of the change?

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I believe it’s the correct one($250 from AutoZone, they better be correct!), but I will double check as well! I have ordered a new battery sensor(not cheap either for this type of car), but at least free returns. I will keep everyone posted in what I find when it arrives.

Thanks for all input so far!

I expect the way that battery sensor works, it has a small resistance, the computer measures the voltage across, and from that knows the current flow into and out of the battery, and which direction the current is moving. For example, in the diagram below, if rrrrr is the resistance, and the current would be moving from left to right b/c the + voltage is on the left. If rrrrr was 0.1 ohm, and the voltage difference was 0.1 volt, the computer would know there was 1 amp flow from left to right. By adding up all the current flows in and out, the computer can keep track of the current state of the battery’s charge.

Since it’s a pretty simple gadget, if it failed, the most likely the failure mode would be one or both of the voltage inputs isn’t making it to the computer. The second most likely, rrrrrrr somehow changed resistance. When you get the new one, suggest to compare the resistances of the two versions.

“+” voltage…rrrrrrrrrrr… - voltage

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That makes a lot of sense. I also thought it was interesting because I didn’t get any continuity reading from the negative terminal to the inside of the attached sensor(where the other lead clips in)??

Part arriving next Wednesday🤞🏼

The resistance (rrrrrrr in diagram above) is probably between the battery post and the harness wires that connect to the sensor ass’y. Click on the photo in the link above for a close-up view. There should be good continuity from the battery post to where the thick, black harness wires connect. There’s probably some resistance in that path, but it would be so small DVM’s couldn’t measure it. The little black plastic box likely contains some signal-conditioning electronics, so there may not be any continuity from where the harness wires connect through that box to the signal connector which presumably goes to the computer. In fact there’s pretty good chance there’s an amplifier in that box, and post-amplification, the signal (representing the measured current) is communicated to the computer via a pulse-technique, done that way to minimize electrical interference corrupting the current signal.

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Hello Everyone,

Received the new sensor early today and unfortunately battery light came on after turning car off and then back on. Is it possible the new alternator is malfunctioning, or would I not be getting the 14.5 volts sometimes?

Though car fires right up, could it be starter related?? I’m just at a loss right now. Thanks for all your helping thus far!!

I expect you’re going to need some Hyundai expertise to resolve. My guess, the power-train computer got confused by the battery and/or alternator replacement. It’s possible all that’s needed is to inform computer (via a scan tool) taht those parts have been replaced, causing the computer to recalculate its charging system algorithm parameters.

Suggest to explain the details of why the alternator & battery were replaced?

Also suggest to save the old battery current sensor.

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Rebuilt off-brand parts store alternator? Or a name brand new unit?

Some parts stores have alternator test fixtures. Used when a customer believes the alternator they parts store sourced is faulty right out of the box. Your symptoms are not one of the common alternator failure modes though.

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