I bought a few LIR2032 batteries from E-bay at $1.50 each.
They're rated at 40mAh, and I've done some basic discharge tests:
Constant-current 10mA from 4.11V to 3.3V: 3 hours and 15 minutes.
Constant-power (through very low-power boost converter) 0.05W for the same voltage range: 2 hours and 23 minutes. (I realize that constant-power isn't the best measure, but it's what I'm most likely to use them for so I added this test)
I started them at 4.11V because that's what the voltage settles at after charging them all the way to ~4.18V and leaving them untouched for a couple of days. They got here at around 4.05V, so I assume that this is what they'd settle at after a few weeks untouched. After the test they bounce back to 3.5V in about 5 to 7 minutes.
This is a very neat and fairly cost-effective little battery for small gadgets (like sensor loggers). Their rated 40mAh is "close enough" to their actual capacity.
My question is (finally): why can't I find these batteries from brand-named companies? I can only seem to find them off-brand on E-bay (or aliexpress), and I wouldn't be surprised if they die off after about 100 cycles. Is this a type of battery that chinese companies often use but which didn't get traction in the west? Do they have an alternative name in the west?
I found panasonic cells with a similar capacity, but they all have tabs/leads:
https://www.digikey.com/product-detail/en/panasonic-bsg/VL-2320-HFN/P083-ND/119937https://www.digikey.com/product-detail/en/panasonic-bsg/ML-2020-G1AN/P046-ND/431505(that last one would be perfect if it *didn't* have tabs)
The closest I managed to find was this:
https://www.digikey.com/product-detail/en/illinois-capacitor/RJD2032C1/1572-1617-ND/6159135It's 85mAh, but it's also *$10*...