I would be wary of counterfeits. I got counterfeit Duracell 9V batteries from a well-known online retailer who co-mingles inventory. They dropped to about 5.5V after a month of use and, on closer inspection, were made of subtly different materials.

Not an issue if you buy unbranded, although I don't think there is anything special about "Duracell" batteries in terms of construction or quality control.

Store bought Duracell AA batteries (unlikely to be fake) have a tendency to leak and destroy devices in my experience.

Do you have any more details on how you identified them as counterfeit? I guess they looked good enough at first glance, so the details must be quite subtle.

I’m not confident of my ability to identify counterfeit goods so I have greatly curtailed purchases from that well-known retailer that commingles inventory. Pretty much anything that might risk health or safety is something I’m scared to buy from them, which eliminates most items that use electricity - even in low voltages, as batteries can have hazardous failure modes.

The battery died very quickly so I was inspecting the "best by" date to see if I had a bad batch. I then compared it to some new batteries bought at a physical store. I noticed the font and some small details were subtly different. The base plate (opposite the terminals) was plastic with a fine texture on the real product, and simple black cardboard on the fake. The stamping and forming of the metal cover was more crude on the fake as well, and the trademark copper was duller.

However, there's no way I would have noticed any of these without a genuine comparison, especially if it was still in the wrapping.

This was for a smoke detector, so definitely a safety issue!

Fun fact: the reason we get quality Chinese components is because we get the ones that pass QA in factories in China and India. Buy a TV in Eastern Europe, buy a TV in the US. The one from Walmart will be fine 99% of the time, won't have a small crack, won't have random transistors pop in a year.

Buy one in Poland or Bulgaria or Russia, and you have about a 75% chance it'll power up at all, and about a 50% chance it'll work in 5 years. Same TV.

Something is made in China or India. About half of the production is perfect and gets sent to "The West." Then about half of that is "it's probably fine, but didn't pass QA." That last 25% is straight up dangerous and defective, and that's what you're buying on ebay. As an example, half the phone screens for iPhones don't pass QA. Those are the ones that get sold on ebay.

Now, for most cases, you battery will simply live half as long. For a few cases, it'll leak, light on fire, or overvolt your circuit and destroy your device. Then all those savings on batteries you've had over the years will be wiped out and then some.

So you are saying that there's a QA process that somehow determines which devices will have a transistor pop in a year, and then they send those to eastern Europe? That sounds... unlikely. But then again, I know nothing of chinese QA. My assumption would be that it varies greatly between factories. There's cheap stuff from China, and there's expensive stuff from China, and the quality varies accordingly.

You don't need to know anything about the Chinese QA process. This is how QA is done for literally everything. What exactly do you thing QA is? The cheap batteries you buy from ebay are batteries that were rejected to sell at a store in the west, and rejected to sell at a store in the east.

I'm not trying to change your mind on anything here, as you seem to have your mind set before bothering to spend even a minute looking up something you stated yourself you know nothing about.

Wouldn't QA be done on components before assembly? How do faulty components end up getting assembled anyway?

It is depending whether the manufacturer demands stringent test for their components. Corner must be cut to allow something to be made cheap. This is the same for PCB used in TV/computer, or battery.

I don't believe this. I think western companies simply have higher QA standards so they qualify to be imported and this results in higher quality even if the product is manufactured in China. Meanwhile products for the Chinese market must pass less stringent QA. A company that would sell products that don't pass QA is committing fraud.

so you believe that the close to 50% of factory output that doesn't pass QA gets destroyed or fixed by a company in China, instead of selling it on ebay? oh my sweet summer child..

yes, it gets sold on the Chinese market as well. but we're talking about someone in the west buying the same component from China on ebay, that they would buy at a store in the west. It's the same component - it's just one that didn't pass QA to make it to the store here.

Yes, the "company" selling these products is committing fraud. welcome to china.

There are /very/ few realistic failure modes for a primary battery that lead to overvoltage.

No one said overvoltage is a failure mode. You took a list of things that can go wrong, for some reason created a cause-and effect between two random things on the list, just so you could correct someone?

a 9V battery should produce around 9V. There are plenty of recheargable 9V batteries sold that are over 9V, because 9V is either using lithium or nicad in series - none of which add up to 9V. The 9V lithium batteries I buy have a tiny voltage converter and deliver 9V. If I order that same battery from a random ebay source, they can easily save on the voltage converter and deliver 10.5V for example.

More to the point of my example, a bunch come off the production line with a Broken voltage converter. They don't pass QA and get sold on ebay.

Yes, I have bought those, as well as ones from Amazon. Either they are of very low quality (only lasts 3-6 months in my car keyfob) or just fake.

I was pointing out a cheap source for quality batteries.