Hacker News Reader

Comment by gwbas1c | Hacker News Reader

I don't understand the obsession with coin cell batteries. Why not use an AAA or AA battery? There's clearly room for one. Coin cell batteries are a lot more toxic than ordinary AAA or AA batteries.

In this case, they had to jump through quite a few hoops to get 1-2 years of battery life. Is sampling every 30 minutes really practical in all applications where someone would want to use a TI sensortag? It's probably easier to just modify the sensortag to use a larger battery and get the sampling rate you need. Depending on the situation, figuring out how to use a C or D battery might be easier than all the compromises made.

My garage door openers have plenty of room for AAA batteries, or could use AA with minor modifications; but they use coin cells. I just don't get it.

View on HN

Coin cells are common in marketing of microcontrollers and the like and as such is somewhat a mark of reference. There are also a good amount of devices (eg BLE beacon trackers) where AA or AAA would be too large. But if you can, use larger batteries :)

Coin cells aren't the best for this kind of system. When the current is a few tens of mA (CPU + transceiver), even for a short while, the coin cell battery internal resistance goes up, lowering the voltage over the terminals. This means that as the battery gets older, the risk of a brown-out reset of the SOC increases, even if there is still capacity to run a load with lower current requirements.

You could to some extent reduce this by using capacitors to reduce peak current over the battery, but caps have leakage current too, so the actual lifetime could be less still. Damned if you do...

I don't think I've ever seen any design that didn't have decoupling caps across the power lines. I mean, the Murata parts I use have dozens of megaohms of resistance at DC, and the chip is off 90% of the time anyway so there's no energy loss keeping the E-field charges in the cap when the SoC is only drawing whatever miniscule amount of power it needs when it's asleep.

Anyway, TI has stuff like this specifically designed for these applications, so it's kind of a nonissue these days:

http://www.ti.com/lit/ds/symlink/tps62730.pdf

I think Mouser had them for like $0.50 or something.

You can't weld regular batteries into a lot of applications, and a lot of IoT applications are mobile and vibration sensitive. You'd be surprised how just a little vibration can really mess up your power supply as the springs bounce around.

CR2032s have a much higher surface area on the contacts, which is required to dissipate the heat from the weld as well as have more surface contact with the springs if you use a battery socket instead.

>You can't weld regular batteries into a lot of applications

Not true. Take a look at an Amazon Dash teardown. Stock AA (or AAA? Can't remember which) welded into place.

Terminal voltage on a coin cell is 3V, so you have to use a pair of AAAs, not just one. Ends up requiring ten times the volume.

Shelf life on lithium batteries is longer, and they don't leak, so you can ship and store sensortags with the battery preinstalled.

My impression is that it's way more of a logistics thing than a customer-value thing.

I chose sensors that use AAA batteries over coin batteries frequently. Give me a little bit more space usage for significantly better ease of use and reliability. The only thing I really see use coin batteries successfully are IR remotes.

Reply on news.ycombinator.com