Why Getting Your Battery Tested at O’Reilly Doesn’t Make Sense. (Bayesian Theory) (2024)

I recently got a dead battery after a long day at airport logistics. Once again I had spent the day dreaming about how everything could be automated. I thought about the added complexity of having to resort to bins that the plane food goes in. Realistically the whole business is self defeating. It’s kind of funny actually in like a really sad way.

After waiting an hour for my best friend from Roy to come jump my car (none of my compatriots had jumper cables) I drove home unmolested. I took a shower and finished my usual post work, post ISKCON 6 round reality break.

The next morning I work up around 4:45 for Aarti. After Seth getting up I hopped in the car to discover — Battery dead! Damn. After an oil change and getting the tire thing sorted out — This happens. Material reality blows, doesn’t it?

Why Getting Your Battery Tested at O’Reilly Doesn’t Make Sense. (Bayesian Theory) (2)

After contemplating the merits of buying a new battery and consulting with the usual authorities I decided to take it to O’Reilly. My roommate suggested them, saying that he had a good track record with them. Sounds good to me.

I pull in. I’m slightly nervous because I don’t want to stop the engine. It was a lot of work coordinating getting the car started. Luckily a perfectly nice salesman was (coincidentally) outside. He greeted me and ensured me that the battery could be tested easily and quickly and if the car died that he could jump it so I could get home. Groovy! What a gent!

He tests the thing — Dead! Oh no! I’m out another 100 dollars I don’t have right now. — Not after the Brian thing. Nope. Alright. Wait. Let’s try to turn her over just to see — She started! Nice! No need for a jump! Seth makes a clever remark, “Wait, if the battery is totally dead why did his car start?” An apt question.

The salesman very confidently replies that the battery may still turn over from the small amount of charge generated from driving from the house to the store. He ensures me that this is common in this situation and suggests buying a new battery. I’d love to — Can’t. Not until I get an ML contract and move out of this rotten city. I decided to drive home telling him I need time to coordinate some things.

Long story short it’s been about — 4 weeks now and my battery has been just fine. I took a 20 minute drive at high revs on the freeway to charge the thing and I haven’t had an issues. Was I scammed? Is the whole thing a scam? Maybe the detector always reads, “dead battery?” I’ve seen Dateline specials about this sort of thing!

Actually though — It’s not that at all! The meter is probably perfectly legitimate. It may even be 99%+ accurate. Here’s the thing though — Baye’s theorem tells us something interesting about accurate tests — In the population pool of instances of positive test results — There are going to be more false positives than real positives! That can’t be right!

In a simple demonstration of this — let’s assume the detector was 99% accurate — If we plug this into Bayes’ Theorem we can actually show that the probability of my battery being dead is actually only around 0.8. That’s for a highly accurate test. Factor in the fact that the test might have some unseen fault and the motivating factor of usual retail scumbaggery as prior values and I reckon the final probability to be closer to 0.3 or in other words — Not.

Why Getting Your Battery Tested at O’Reilly Doesn’t Make Sense. (Bayesian Theory) (3)

The question is though — Are auto parts places that do free testing aware of this? I mean — They should be. I can imagine some accountant somewhere having figured this out years ago. That’s the thing though — If you think about it, this is really how the whole of the car-repair industry works. A diagnostic of a part of the car reveals that some critical component is failing — — That’s great and all Mr. Mechanic but my good friend Thomas Bayes tell us that even if your diagnostic test is highly accurate — It’s probably not factoring in Bayes’ Theorem or prior probabilities AND neither is the customer! I wonder how many car repairs are actually necessary? I wonder how much economic inefficiency this creates. Ugh. I can’t wait to own a Tesla. Surely it will never break down. (Elon fingers crossed. Also; Hyperloop)

The moral of the story is — Bayes’ Theorem gives us some insight into probabilities. Things get even funkier when we factor in prior probabilities. If someone says, “Hey mate I tested your so and so and it’s broken — You need a new one” they are either a) Very clever and trying to scam you. b) Omniscient OR c) A well meaning friend/agent who isn’t very well versed in Probability Theory. At any rate — You get a diagnosis of cancer or the like — Get a second opinion! (and a third, and a fourth) Bayes’ Theorem might explain to use why Medical Misdiagnosis is the 3RD leading cause of death in the United States!

— Learn Math. It makes you smart. You can get a good job — Whatever. Forget that. It might just save your life. Stay clever friends. Stay clever!

Why Getting Your Battery Tested at O’Reilly Doesn’t Make Sense. (Bayesian Theory) (4)
Why Getting Your Battery Tested at O’Reilly Doesn’t Make Sense. (Bayesian Theory) (2024)

FAQs

What is the most accurate way to test a car battery? ›

A battery test is not about checking corrosion or making sure the plastic case isn't cracked. It's also not about checking voltage or how charged the battery is. The most accurate battery tests come from a conductance battery tester.

How accurate is battery testing? ›

That's why it's recommended to test every battery on every vehicle that enters a service drive or shop. But battery tests might not be 100% accurate if you're using test equipment that doesn't cover the full spectrum including state of charge, state of health, and conductance profiling, or if your process is wrong.

Does Oreilly's load test batteries? ›

Whether your car battery is in the vehicle or carried into the store, our trusted Parts Professionals can test your battery and help you determine its usable life. We are experienced and committed to keeping your car on the road, and can even install the battery in your car free of charge in most cases.

Do cheap battery testers work? ›

Most battery testers aren't that good

You can use specialized, inexpensive battery testers with a mechanical needle and a “good” or “bad” indicator. These are so simple that battery manufacturers have figured out how to build basic battery testers right into the throw-away plastic packaging the batteries come in.

Where is the best place to test a car battery? ›

Battery tests are fast, accurate, and available at every AutoZone in the USA.

Can a car battery test OK but still be bad? ›

It's possible for a battery to be tested as meeting or surpassing the rated CCA and still be bad. It's particularly true when the reserve capacity is no longer sufficient to crank the engine over for several subsequent starts. Alternatively, low CCA test results don't always mean the battery is bad or beyond help.

Can a car battery give a false reading? ›

If you test it immediately after use, you can receive a false reading from a unit charged by your car's electrical system, also called a surface charge.

Can battery health be inaccurate? ›

Battery health and how much charge is left is really just an estimate based on observation. It's not like a fuel tank where there's a physical means to determine how much fuel there is. Especially as they get older they can sometimes start showing inaccurate condition. That's usually solved by replacing the battery.

Why won't Oreillys change my battery? ›

If your vehicle's battery is inaccessible, requires extensive disassembly to access, or if the manufacturer requires a battery saver or system reset during installation, you may not be eligible for our on-site installation service.

Does O'Reilly do free diagnostics? ›

If your Check Engine light is on, don't take a chance on getting stranded, or worse yet, risk damage to your engine. Stop by O'Reilly Auto Parts and we'll test your Check Engine light codes for free. We can even print out a list of common fixes for the codes, and help you find a repair shop if needed.

Can battery testers be wrong? ›

A dead battery is easy to check and most testers are 100 percent accurate. The challenge comes in evaluating a battery in the 80–100 percent performance range while in use. Most service and repair workshops struggle to introduce good battery test procedures.

What is the most accurate test method for checking battery is? ›

Among these, the most common are voltage testing, resistance testing, capacity testing, and load testing. Unfortunately, the most accurate way to determine if a battery has gone bad and overall battery health would be to use all three tests: Voltage, Load, and Resistance.

Which test is most accurate in determining the condition of a battery? ›

A battery load test is one of the most accurate way of determining battery condition. A small wire brush is the best tool for cleaning battery terminals. As a battery becomes discharged, the percentage of water in its electrolyte increases. A dirty battery top can permit current leakage across its surface.

What is the best way to test battery condition? ›

Place the positive multimeter lead on the positive battery terminal and the negative lead on the negative terminal. Have a helper turn the ignition on for no longer than 15 seconds and watch the multimeter. If the battery voltage drops below 9.6 volts, this is an indicator of a weak battery.

How can I check if my car battery needs replacing? ›

This post will teach you how to tell if your car battery is failing so that you can get it replaced timely.
  1. Your Battery is Likely Old. ...
  2. Your Car Doesn't Start as It Used to in the Morning. ...
  3. You Frequently Have to Jump Start Your Car. ...
  4. Slow Engine Turnover is Another Sign. ...
  5. Your Battery Smells Bad. ...
  6. Your Battery Case is Bulging.

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