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The one thing I don't yet understand is the difference (to battery life) between manually maintaining the 12V battery charge periodically vs. having its charge maintained automatically from the traction battery.

Is it an issue of going a long time without charging the PHEV via AC, and given the lack of alternator, discharging the 12V battery too deep too often?
The benefit of "smart chargers" is their ability to determine when desulfation is required and provide a brief high charge to perform that function.
The DC to DC converter will not perform that function - in fact, an alternator will not do that either.
12V lead acid batteries like to be fully charged at all times and hybrid/EV vehicles do not regularly do that whereas an alternator on the ICE cars will attempt to keep fully charged.
 

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The one thing I don't yet understand is the difference (to battery life) between manually maintaining the 12V battery charge periodically vs. having its charge maintained automatically from the traction battery.

Is it an issue of going a long time without charging the PHEV via AC, and given the lack of alternator, discharging the 12V battery too deep too often?
Yes, relatively deep discharge and not always fully recharging. My takeaway from discussions read on this forum is the charge/discharge cycle or curve the Niro PHEV puts on the 12V is rough on a lead-acid battery, particularly one not designed for deep cycle use. That charge behavior leads to reduced capacity and ultimately a shorter service life than it could be. For this reason I replaced mine with a relatively expensive battery designed for the purpose. Periodic use of a "smart" tender can apply a particular charge profile to help undo essentially the damage caused by the PHEV's apparentlty poor (re)charging behavior. Here's an example from Noco: NOCO - Battery Sulfation - Support

I recently used such a charger to try and bring back an apparently flat motorcycle battery. At the start I was watching with a voltmeter and saw it applying relatively short, higher-voltage (20V maybe) pulses, gradually making them longer and with lower voltage. After a day or two the charger indicated it was able to get the battery back to full, though I haven't yet tested it. One caveat is the high pulse voltages during conditioning can damage the electronics of a still-connected vehicle, so the mfr. says to first disconnect it.
 

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2019 NIRO PHEV EX
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Look at this website and read the advertising about why the BatteryMinder and devices like that can extend your battery life

BatteryMINDer 1500 | 12 Volt Maintenance Charger Desulfator (batteryminders.com)
(Scroll down the website to see all the information)

Here is just a snapshot of some of the information there.

"Quadruple Your Battery's Life
The SmarTECHnology™ Plug 'n Run 1.5 Amp battery charger/maintainer system features a full-time automatic pulse desulfator that can help extend a battery's life up to 4 times its normal life when compared to new batteries that are not being properly charged/maintained over their lifetime. Just plug it in and the BatteryMINDer® takes care of the rest.
The Right Charge Every Time
An automatic high-low temperature compensator is installed in the BatteryMINDer® to ensure that batteries will never be overcharged or undercharged, regardless of the operating temperature. This ambient temperature sensor is installed for a -4°F (-20°C) to 122°F (50°C)."

I couldn't begin to explain better than these people or their product cut sheet.
 

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Any general suggestions about this situation and using a battery charger?
Any suggestions on a specific smart charger?
Do you need to remove the battery to do this? (I might go ahead and do this anyway because it's quite rainy here now and I don't have a garage).
I have NOCO Genius 1 $30. it works like a charm and intelligent to boot with support of most common batteries. make sure to get one with AGM and Lithium supporting charger. I use for keeping my hibernating vehicles during the winter months but have used on my wife SUV to get the car started. if the battery is less than three years, you should be able to "save" the battery as NOCO does self-recovery of a completely dead battery. you will know within an hour or two if the cells are savable. NOCO1 only has 1 amp so it will take a while to recharge but this is more than sufficient for DIY. higher version of NOCO will charge/check at faster speed. and you can charge both on and off the vehicle.
 

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most of the modern vehicles in the past few years now uses AGM types for accessory battery except for Niro HEV which uses Lithium. so getting a standard lead acid battery is not recommended as it will not hold/perform to meet the requirements.
 

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most of the modern vehicles in the past few years now uses AGM types for accessory battery except for Niro HEV which uses Lithium. so getting a standard lead acid battery is not recommended as it will not hold/perform to meet the requirements.
I'm not so sure AGM is as "standard" as you think.
The HEV does use lithium but it is not the HEV battery that is a problem. It is the PHEV and EV batteries that are lead acid and they need the extra help from a smart charger.
 

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It is the PHEV and EV batteries that are lead acid and they need the extra help from a smart charger.
I'm just starting to remotely track it, but so far looks like my PHEV will itself charge a brand-new 12V AGM battery only to ~91%. Best I can tell that's while actively charging the HV battery or driving - assuming one believes Kia's indication of the percentage. It appears when the car is "on," headlights "off" and not moving, it may still charge. I have also read on this forum car on/headlights on does charge.

Also, in a month or two when I put a little Noco Genius 1(A) on it for a recondition and push to 100%, I will first disconnect the negative terminal. For one, the voltage pulses I've seen during conditioning of another battery are too high to consider leaving it connected to the car. At best I would expect a too-high battery voltage DTC to be thrown, and at worst outright damage to expensive electronics.
 

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Also, in a month or two when I put a little Noco Genius 1(A) on it for a recondition and push to 100%, I will first disconnect the negative terminal. For one, the voltage pulses I've seen during conditioning of another battery are too high to consider leaving it connected to the car. At best I would expect a too-high battery voltage DTC to be thrown, and at worst outright damage to expensive electronics.
You might be overthinking this charging stuff. If the car is "off" and you are not charging the traction battery, I doubt there is any circuit of consequence that is connected to the 12V battery.
You may disconnect the 12V negative from the battery when connecting the smart charger but in my opinion, you are wasting your time.
 

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If the car is "off" and you are not charging the traction battery, I doubt there is any circuit of consequence that is connected to the 12V battery.
Since when this car's off we have live from the 12V UVO, the near and far field fob radios and anti theft to name three off the top of my head... For a maintenance item to be done only every couple months or so (and by your suggestion, thank you!), I'll chance wasting a couple minutes for the certainty of not damaging anything.

Another factor I didn't at first mention is that for a low-current charger like the 1A Noco, even a small extra trickle load could alter behavior of the conditioner's controller. Likely, maybe not. Certainly possible.

Last, I can't bring up the link right now, but I am pretty sure Noco themselves also write it's essential to disconnect the battery from the rest of the vehicle.

If someone believes just clamping on and plugging in is all good - possibly from prior experience with Kia/Hyundai vehicles, more power to ya. No pun intended. I do maintain charge on my boat and motorcycles that way. For each of them, when turned off there's a physical switch breaking the circuit.
 

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I might be a good experiment for you. When I hook up my BatteryMinder 1500 to my 2019 PHEV NIRO EX I never disconnect the battery. I use the chassis ground and the + terminal in the fuse box in the front engine compartment. I have been doing this for 2 years every 4 months with no bad results yet.

The one exception was when my 12v battery completely discharged in 2019 I did remove it from the vehicle before charging it. It voltage was 5.1 volts before the BatteryMinder bought it back to life. It was the original battery, and it is still working fine after it was bought back to life.
 

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The one exception was when my 12v battery completely discharged in 2019 I did remove it from the vehicle before charging it. It voltage was 5.1 volts before the BatteryMinder bought it back to life. It was the original battery, and it is still working fine after it was bought back to life.
That is amazing as 5.1V would have been the death of your battery if it had been 2 - 3 yrs old.
 

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Are there any gotchas with disconnecting the 12V PHEV battery for hours? (e.g. any loss of data, or need to input a stereo anti-theft code as was the case in my old Honda)

(Thanks to all who've contributed to this informative thread!)
 

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AGM replacement will run $200-$300. all DIYer should invest in $30 battery tester instead of unnecessarily spending $ on replacement. if the AGM battery is less than 3-4 years, it should be salvageable. Another $30 for NOCO1 charger [or equivalent] will even further save you $ with just one time use.
 

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Are there any gotchas with disconnecting the 12V PHEV battery for hours? (e.g. any loss of data, or need to input a stereo anti-theft code as was the case in my old Honda)

(Thanks to all who've contributed to this informative thread!)
I don't think there will be any "gotchas" but since I do not have a PHEV (with use of smart charger), I cannot definitively answer.
 

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Techy is right there are no gotchas when the 12v battery is disconnected - at least on a 2019 PHEV NIRO.

I was pleasantly surprised back in 2019 when I had to remove my completely discharged 12v battery for 2 days for our BatteryMinder to bring it back to life. Put it back in the car - the car came to life and my settings were fine after 2 days with no 12v battery - I don't even think I had to redo my radio presets.

After this happened, I did buy a Noco Battery Booster so that if my battery died, I would be able to start the car immediately - even with a dead battery. The Noco Battery Boosters have a setting so that it will start the car even with a completely dead battery.
 

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After this happened, I did buy a Noco Battery Booster so that if my battery died, I would be able to start the car immediately - even with a dead battery. The Noco Battery Boosters have a setting so that it will start the car even with a completely dead battery.
Yes, if voltage is below 3V, boosters will not operate but they all have an over ride feature that allows boosting when basically dead. If the over ride feature is used, the polarity safety is not available so if the cables are connected incorrectly, there is definitely the probability of sparking and blowing up the battery and/or booster.
 

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Another quick note to this thread- one of the post could confuse individuals that the NIRO EV or PHEV use an OEM AGM battery. Unless KIA changed this year NIRO EV and PHEV are using Closed Maintenance Free Lead Acid batteries.

Replacement AGM batteries will work if an induvial wants to go that way
 
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