At below $20, I think the Rui Deng (it seems now simplified to ‘Riden’) meters are acceptably good. I do have several RD USB meters (“RD USB 3.0 comprehensive tester” which is an older model / UM25C (the earlier version, there is a later version with a few minor changes listed but I haven’t tried that) / UM34C / AT34), but have not really tried using them for measuring capacity, so am unable to comment how good it measures capacity.
I use ZKE-Tech EBD-USB+ meter connected to a Windows PC to check powerbank capacity, both input (charging the powerbank) & output (discharging the powerbank), as the EBD-USB+ can be a USB meter and also a load tester. Downside is that the EBD-USB+ requires connection to a computer as it does not have any display.
Regarding Riden’s meters, the UM25C is actually a more ‘accurate’ meter than the UM34C (the “C” is the “communication” models — with bluetooth), there is an Android app (I’m not using iPhone so I don’t know if there is an Apple app). It also has a basic Windows software. I’ve seen some 3rd-party apps for the Riden meters, but haven’t tried them though, for instance, one of them (I haven’t tried personally) can be found here (Windows / Linux): GitHub - kolinger/rd-usb: Web GUI for RuiDeng/Riden USB testers (UM34C, UM24C, UM25C, TC66C)
UM34C supports up to 4A whereas UM25C supports up to 5A. The UM25/34 will detect QC2 / QC3 protocols in use, but it does not have ‘protocol auto-detection feature’ unlike in many of the more advanced (and more expensive) meters.
I would suggest the QWay U2p (newer 4-button version) if you can find it, this supports QC4/PPS detect.
I have also tried the FNirsi FNB48, which seems to have similar feature set at the U2P (maybe has a few more), but FNB48 may still have some bugs as it’s newer and may not be that well-tested.
I still like the ZY1276 USB meter although it doesn’t have new features (eg. QC4/PPS).
I haven’ tried some of the newer USB meters though (ZY1278, ZY1280, KT002), which are more expensive and I think these may be slightly more ‘premium’ than the QWay/Fnirsi.
The Riden are more basic, but generally does the job fairly well (but regarding your ‘capacity test purpose’ = something I haven’t really tested, as I have been heavily reliant on the EBD-USB+ meter/tester. (The EBD-USB+ probably can fit in your $24 budget if you look around, but the EBD-USB+ requires a Windows PC to work best, otherwise by itself, the device does not have any display)…
