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My low mileage 2016 civic recently needed a jump. After I got home I did some testing and suspected the alternator because immediately after ignition the voltage jumps to 14.x volts but then within a minute drops down to around 12.4v and it stays there as long as I cared to idle the car. I got as far as removing the alternator (more difficult than I expected) before I found out it's a "smart" alternator and that the behavior observed may be as designed. No warning messages on the dash, no OBDII codes that I could detect with a $20 reader I bought from amazon.

So is this "normal"? It seems very early for my 4 year old / 40K miles car to need either an alternator or new battery. And either way it's disappointing that he car doesn't give any kind of error codes, either a battery warning light or a service light for the alternator (which communicates over the LIN protocol; apparently the car and alternator chat back and forth with some regularity and in detail?).

PS. Very minimal corrosion on the battery lugs, wouldn't be enough to cause problems on my previous car.
 

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Without a proper diagnostic - and as a shot in the dark based on the things you're relayed here - I believe it is your battery. The 51R is a small battery and its placement in the engine bay isn't super favourable for heat and vibration. You don't mention where you live, but I live in Minnesota, and with the extreme cold winters and brutally hot and humid summers I am really thrilled to get three years out of a battery.

Four years on the OEM battery is pretty darned good in my opinion. If you seriously feel that there is reason for concern I would advise you to have a thorough electrical system diagnosis done, but I think that after spending $100-$150 on that they will come back saying:

You need a new battery. Start with the easy stuff.

Mike
 

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I'm in southern california, about as different a climate as you can find :)

my toyotas (rav/camry) usually get 6/7 years of useable battery life, using the cheapest walmart brand. so this is quite a change of pace. But I'm not doubting you - starting with the easy stuff is definitely what I wish I had done before taking the alternator out :-(
 

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My low mileage 2016 civic recently needed a jump. After I got home I did some testing and suspected the alternator because immediately after ignition the voltage jumps to 14.x volts but then within a minute drops down to around 12.4v and it stays there as long as I cared to idle the car. I got as far as removing the alternator (more difficult than I expected) before I found out it's a "smart" alternator and that the behavior observed may be as designed. No warning messages on the dash, no OBDII codes that I could detect with a $20 reader I bought from amazon.

So is this "normal"? It seems very early for my 4 year old / 40K miles car to need either an alternator or new battery. And either way it's disappointing that he car doesn't give any kind of error codes, either a battery warning light or a service light for the alternator (which communicates over the LIN protocol; apparently the car and alternator chat back and forth with some regularity and in detail?).

PS. Very minimal corrosion on the battery lugs, wouldn't be enough to cause problems on my previous car.
Batteries are prone to premature failure for many reasons. I recently washed my 2018 Si and thought I would just pop the battery caps and see how full each cell was. The car had actually started to sound like the battery was going to die when I started it. Well, I guess it didn't shock me when I saw the battery (each cell) was ½ empty . The so called "Dealer Prep" is BS, they dont check squat. So the cells being half full cause the exposed Lead "if you will" to create deteriorated material that falls between cells and bottom of the battery decreasing the the cells integrity. So check your battery electrolyte level about every other oil check(only use distilled water to fill, never tap water). So 4 years is actually a pretty good life for a battery. I live in Phoenix and you can expect 2 years tops, thats why I watch it so close. I am actually going to get a Dry cell. Had one on my Mustang and it lasted for years and they usually have a lifetime guarantee, they are a little pricey but worth it to me.
 

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Discussion Starter #5
that's a good idea to check the cells, I had been thinking I should but haven't gotten around to it.
 

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My low mileage 2016 civic recently needed a jump. After I got home I did some testing and suspected the alternator because immediately after ignition the voltage jumps to 14.x volts but then within a minute drops down to around 12.4v and it stays there as long as I cared to idle the car. I got as far as removing the alternator (more difficult than I expected) before I found out it's a "smart" alternator and that the behavior observed may be as designed. No warning messages on the dash, no OBDII codes that I could detect with a $20 reader I bought from amazon.

So is this "normal"? It seems very early for my 4 year old / 40K miles car to need either an alternator or new battery. And either way it's disappointing that he car doesn't give any kind of error codes, either a battery warning light or a service light for the alternator (which communicates over the LIN protocol; apparently the car and alternator chat back and forth with some regularity and in detail?).

PS. Very minimal corrosion on the battery lugs, wouldn't be enough to cause problems on my previous car.
You can also go to Autozone boys and buy a $4.00 Hydrometer and check the specific gravity (basically the ability for your battery to "recharge") level in your battery. Sounds like I would have the Autozone boys come out to your car and test it. Takes 5 minutes and they will even replace it for you, great service. No extra charge.
 

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My low mileage 2016 civic recently needed a jump. After I got home I did some testing and suspected the alternator because immediately after ignition the voltage jumps to 14.x volts but then within a minute drops down to around 12.4v and it stays there as long as I cared to idle the car. I got as far as removing the alternator (more difficult than I expected) before I found out it's a "smart" alternator and that the behavior observed may be as designed. No warning messages on the dash, no OBDII codes that I could detect with a $20 reader I bought from amazon.

So is this "normal"? It seems very early for my 4 year old / 40K miles car to need either an alternator or new battery. And either way it's disappointing that he car doesn't give any kind of error codes, either a battery warning light or a service light for the alternator (which communicates over the LIN protocol; apparently the car and alternator chat back and forth with some regularity and in detail?).

PS. Very minimal corrosion on the battery lugs, wouldn't be enough to cause problems on my previous car.
Just had the exact same thing happen. Out of the blue the car would not start. AAA jumped it and I drove it straight to the dealership. Very dead battery. 4 years and 50,000 miles. The AAA guy said that these small batteries just don’t last.
 

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I had a couple of experiences where my 2017 Civic Si had been sitting in the garage overnight and when I attempted to start it, it was dead. The interior light, if on, was a dim yellow. I have a charger and hooked it up at 10 amps (not the jump start amperage nor the trickle charge, in between). After 15 minutes the charger would show fully charged and it would start. It was finally diagnosed (just before the warranty expired) with a bad battery.
 
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