This started out as a feel-good article about another great experience I had with Tesla's extraordinary service program.

But it morphed into a detective story that apparently solves the mystery of why my 60-kWh 2013 Tesla Model S still suffered a significant "vampire" power drain, even after downloading new software that was supposed to tame the beast.

Almost from the beginning, the Model S has consumed a significant amount of power when turned off and parked. Last winter, shortly after purchasing my Model S, I measured this "vampire" draw at about 4.5 kWh per day--enough juice to drive about 13 miles.

2013 Tesla Model S electric sport sedan [photo by owner David Noland]

After long delays and some false starts, Tesla finally sent out software update 5.8 to the Model S fleet in mid-November. Among many other improvements, 5.8 was supposed to "significantly reduces power usage when (the car is) shut down."

However, in my case, the vampire draw still amounted to about 3.5 kWh per day, a reduction of only about 25 percent, even after installation of 5.8.

A number of other Model S owners--oddly, most of them 60-kWh guys--reported similarly mediocre results from 5.8 on owner forums.

On the other hand, many others, mostly drivers of the Model S with the 85-kWh battery pack, weighed in that 5.8 had had virtually eliminated their vampire drain.

What was going on here?

Big Brother calling

The morning before Thanksgiving, I was sitting at my kitchen table sipping tea and reading The New York Times when the phone rang.

It was the scheduling chief at my local Tesla Motors service center. "We just got an e-mail from Tesla engineering in California," she said. "It seems there's a problem with your 12-Volt battery. Would it be okay if we sent someone out to your house this morning to replace it?"

Onsite service by Tesla Motor technicians on 2013 Tesla Model S, upstate NY [photo: David Noland]

I was completely unaware of any problem with my 12-Volt battery. (I wasn't totally sure my car even had a 12-Volt battery.) But apparently Tesla's remote monitoring system had detected some sort of anomaly in the electrical readings from my car.

I agreed to let Tesla replace my battery, of course. Two hours later, a couple of Tesla "Ranger" technicians showed up at my house, and replaced the battery with an upgraded Exide model.

They also downloaded software version 5.8(1.49.25) to replace the earlier version of 5.8 I'd received only the previous week. And then they were gone.

A problem that I didn't know I had was fixed almost before I knew it, with zero effort or inconvenience on my part. Amazing.

If this is the future of automotive service, count me in.

81-hour sleep

Two days later, my wife Lisa and I flew to Memphis for the weekend to visit relatives. I figured it would be a good opportunity to get more data for my ongoing 5.8 vampire-draw tests.

So, before leaving, I charged the car up to its normal 80-percent setting, and unplugged it for the duration.

Onsite service by Tesla Motor technicians on 2013 Tesla Model S, upstate NY [photo: David Noland]

On my return three days later, I plugged in to charge the car back to its previous level. A kilowatt-hour meter in the circuit would read out my total vampire draw for the 81 hours the car had sat unplugged.

My first hint of good news came when I checked the "Rated Range" readout in the car. It had read 140 miles when I left, and was now down to 126 miles--14 miles lost in three-plus days. That worked out to only about 4 miles per day, far better than my previous range losses.

But mileage readouts are quirky, subject to the vagaries of temperature and algorithm. Only the kWh-meter would tell the tale.

And the tale it told was a happy one indeed: just 3.6 kWh of vampire power consumed in 81 hours. That was 1.1 kWh per day--only a quarter of my previous long-term vampire draw, and three times better than my initial tests with 5.8.

This latest kWh-meter test seemed to suggest that the vampire was dead. Or at least mortally wounded.

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But why?

Elon weighs in

My first thought was to credit the tweaked version of 5.8 that the techs had downloaded in my driveway. Looking to see if other owners might have had similar positive results with 5.8.(1.49.25), I logged on to the Tesla owners forum.

There I found a thread called, "Wow, this guy is a complete nut job that wrote a negative article."

The complete nut job, it turns out, was me. The negative article was the one I wrote here two weeks ago describing my tests with Version 5.8 that showed it to be only mildly effective.

Onsite service by Tesla Motor technicians on 2013 Tesla Model S, upstate NY [photo: David Noland]

Thankfully, this thread was on the same page as one entitled, "Vampire Loss Fix in 5.8 Not Working For Me," which featured a list of a dozen 60-kWh Model S owners who had reported vampire losses similar to mine with 5.8.

Buried deep in the barbs and catcalls of the "nut job" thread (a few in my favor, I might add), was a post from "CCbldg", who had apparently tweeted Elon Musk about my article. The post included three tweets in reply from Musk.

Onsite service by Tesla Motor technicians on 2013 Tesla Model S, upstate NY [photo: David Noland]

The first tweet said, "Doesn't sound right. If it is, we certainly need to fix it. Investigating..."

An hour later, Musk tweeted, "The writer of this article has a problem with his 12V battery, so main pack was constantly charging it."

Shortly thereafter came this tweet: "It was fixed this weekend, so he should try the test again."

I had already tried the test again, of course, and that certainly backed up Musk's hypothesis: my vampire was apparently a defective 12-Volt battery.

Onsite service by Tesla Motor technicians on 2013 Tesla Model S, upstate NY [photo: David Noland]

Mystery solved?

I subsequently found a post on the Tesla forum from an S85 owner who, like me, got a big reduction in his vampire draw when he installed a new 12-Volt battery. But he too had also upgraded to 5.8 (1.42.25) at the same time, so it still seemed possible the software tweak was his real vampire-slayer.

A Tesla spokesman, however, subsequently explained that all versions of 5.8 are identical in terms of their effect on vampire draw.

And so it looks like my vampire was indeed my car's defective 12-Volt battery

But does the 12-Volt hypothesis explain why so many Model S owners reported similar problems on the various owner forums? Do they all have bad 12-Volt batteries?

And what about the odd fact that most of the reported problems seem to be in 60-kWh cars? The Tesla spokesman told me that the 60- and 85-kWh batteries are identical, vampire-wise.

While I can now count myself among the lucky Model S owners who has largely been freed of the vampire's dreaded fangs, not every owner is so fortunate yet. And no one seems to be able to figure out why.

We'll keep you posted.

With luck, Tesla will soon fully solve this mystery and put a final stake through the vampire's heart.

Which will provide yet another accolade for CEO Elon Musk and his dedicated team: Vampire Slayer.

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