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Thanks for the comments, please keep them coming, & those might have been good points, except:
  • Electric Power Steering only uses power when you steer with the key on, when everything runs off the drive battery.
  • My brake motor still comes on (with the EPS fuse out) sometimes just by opening the driver's door to get in, which definitely drains the 12V since the key is not on, but...
  • After 101 hours (4.2 days - VERY long weekend!) parked unplugged with the alarm off* it was 12.0v (50%) started fine & stayed stone-cold while charging itself, indicating fairly low current.
  • Nearly 12 hours into my 2nd test, with the EPS fuse in & the alarm armed, I already had to open the driver's door, running the brake motor, & it's still at 12.7v (97%). This is like a long work day parked unplugged, or if you street park at home, it's like arriving at 9pm & leaving at 9am (so far - test ongoing, on track to last nearly 5 days).
  • IF everything else is equal, lifespan is less for a smaller battery, BUT: The smaller one is AGM with 150% higher cycle life than OEM, for its size*. It's further from damaging heat of the onboard charger & radiator. It also has about 5 times the charge current tolerance for its size, so *at 1/5 the size it would have about equal current tolerance. It's actually even smaller than that, so only time will tell: It would take a year to find out that it lasts a year!
Longevity isn't such an issue at only $20 to replace, but if you want to play it really safe, for nearly the same cost & weight savings, you could get a 12V with DOUBLE the capacity. Even QUADRUPLE the capacity isn't much more.

*IF it's safe, you can use the metal key to lock the car with the alarm off.
 

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It is probably a good time to point out in this discussion, that just going to one's local auto parts store and buying the recommended size battery will work fine for the vast majority of users for 3 to 4+ years of carefree service. No need to go through extraordinary measures. . .
 

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Right. You might even say the CHEAPEST recommended size would work fine. Failure has been reported in under 2 years though.

When I checked, the cheapest OEM replacement was 5.5 times the cost of my current replacement, in case you don't consider two bolt-on adapters as "extraordinary measures", especially since they eliminate the need to lift a pretty heavy item in & out. Also I noticed a big improvement on speedbumps/dips.

So I will continue my current test parked unplugged with the alarm armed, post an update, & then post subsequent updates as it ages.
 

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Just to be clear, I think your experiment is interesting. The point of my post was more geared toward someone who may need to replace their battery and came across this thread - falling down the rabbit hole and end up thinking replacing the battery is way more complicated than it really is. Things I consider extraordinarily measures: opening the hood to charge the car, adding foam to the battery compartment, removing the plastic cover under the hood, locking and unlocking with the key and not the fob etc. These are not what I consider ordinary things. I understand why you do them, but they are really not needed or expected of the average user - ie. someone who expect to replace the 12 volt battery after 3 or 4 years.
 

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Thanks, & agreed 99%*: The 12v is quite cheap & easy to replace with a $110+ OEM equivalent.

* My 1% disagreement: It can die without warning in under two years. Therefore you need to choose between one or more extraordinary measures, such as the following:
  • Replace it every 23 months for $110+
  • Carry a $40+ jumpstarter
  • Rely on AAA (don't know the cost - can't afford it).
  • Pop the hood while charging ($0)
  • Add a piece of styrofoam ($0 from trash)
I'm presenting a $20-$40 alternative: Replace when needed, with nothing else required (no jumpstarter, no AAA, no hood popping, no foam). Well okay, like any gas car, when it nears the end of its life expectancy you'd want to pop the hood to test it every few weeks, unless you'd rather just throw in a new one for less than a tank of gas.

Midnight edit: $20 unit is on track to last 2 years, even with a full 4-day weekend parked alarmed unplugged every week. As above though, it will take 2 years to really know for sure if it lasts 2 years, but if it doesn't, even I don't care, since it's only $20!
 

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Sufficient warning would seem to solve the primary issue: When our 12V starts to die there's no engine to hear cranking slower, & our display warning doesn't come on until it's so dead it may not start! So after the short 2 years in which it can die, we need jumpstart access like a $40 jumpstarter (that we periodically check for charge), or $52/yr AAA, or jumper cables & a friend who's always available.

Instead, I will now get advance warning from this $22* Battery Tender test light. My quick & easy installation is pictured below, but it would be best located somewhere prominent in the cabin, wired to the dome light circuit so it shows its green, yellow, or flashing red light for 20 seconds when you open the door. Otherwise you'd want to hit the button to check the color every few months before turning the key.

It flashes red below 11.6v, which still starts the car fine, but might be a good time to start shopping for a new 12V battery.
10.0v is reportedly the car's own warning level, where it sometimes still starts, even though 10.5v is considered "0.0%" charge for these lead acid batteries!
Battery Tender.jpg
* with the required SAE connector. (underscored terms work on Amazon search)
 

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Warning at 10.0v on OBD was reported by someone discharging the drive battery in order to reset the % gauge. I believe it was fiatatat.

A possible inherent problem with any 12v battery warning on this car is that as soon as you turn the key one klick for any warning to show, the drive battery supplies 14+ volts! That's why I suggest activation by just opening the door. No matter what, to be safe we need extraordinary measures (like adding a test light or keeping a jumpstarter charged), so we may as well save some money!...:

$20 BATTERY UPDATE: I'm now even more confident recommending it. 1-time purchase of $18 adaptors, & then only $20 to replace the battery, probably after 2 years or more*.

*I'm nearly 3 non-stop days into my 2nd, harsher test (parked unplugged with the alarm on). As expected for lead-acid, the slow voltage drop is VERY steady, putting it on track to go 11.5 days & still start fine. Also on track to go 5 days before it hits 20%. It's supposed to be able to cycle that deep 100-120 times. Even if you only drive weekends & let it sit 5 days/week 52 weeks/year, 2 years is 104 cycles.

EDIT: It is also on track for 7.5 days to hit 11.0v, at which time the drive battery reportedly starts recharging it, possibly with too much current. I'll try to take it that low to check, but even IF it's the case, one could simply get the $29 double-capacity battery, for 15 days parked.
 

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IMPORTANT UPDATES to $20 battery discussion are posted here:
 
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