18650 Batteries Use

Hi, I have (had) a broken Power Bank unit that apparently had 50,000mAh capacity - pretty sure it was broken because the USB sockets were damaged physically. I cracked it open and it has 8 18650 batteries inside which I have been able to remove (and measured the voltage - they are currently at 3.6V). The batteries are unbranded and unlabelled in every way. I would like to re-use them in a robotics project that involves solar charging and I am looking for any advice on doing this and their safe use -

Is there any way I can get an idea of their capacity?
Is there any safe way I can figure out their charge and discharge capability without any given information? (Probably don't need more than 800mA and only for short periods at that)

The batteries were connected in parallel in the Power Bank and the max output was spec'd at 2.1A, though I guess that was more based on the handling circuitry built in, and designed for smart phone charging. The specs on the power bank case suggest each cell has around 6000mAh capacity, but I am struggling to believe that!!

I appreciate any advice!

The largest 18650 battery currently has a capacity of 3.5 AH , and thats a Panasonic NCR 18560G.
Anything bigger is a fake , and unfortunately there are lots of them

Maybe worth adding protection to the batteries (see this) if you have separated them from the charge circuit.

That video looks very useful thanks!

I have a few 18650s from laptop batteries.
I have put them one by one into a really cheap single 18650 phone charger.
('DIY Charger Box Case For 18650 Battery' on ebay)
I charged them up in the charger then discharged them by charging up my phone via a 'USB charger doctor' (again, on ebay).
Of course the figure that the charger doctor gives is at 5v having been boosted at something like 80% efficiency, but for my purposes, it is just fine as I can compare cells this way.
To get the real value for a battery I would guess you need to multiply by 1.4, but this really is only a guess.

The actual capacity of a battery will depend on start and end voltage of a discharge cycle - the further you go the more capacity you get but you'll shorten the life of the battery. The cheapo phone charger I use for testing goes from 3.05v to 4.15v, but you could go as low as 2.0v with some batteries if you dare though I suspect this remark will get replies with fire warnings. I have repeatedly gone down to 2.4v but then charge them back up in a fire proof area just in case. No flames ... yet.

Another parameter to check for is self discharge, which old and/or cheap batteries suffer from.
I fully charge the batteries, leave them to stand for an hour, measure the voltage with a meter, then measure them again a week later. If self-discharge would get them below 3v within in a year (multiply by 52), I won't trust them! (charge, internal short ... fire)

Thanks for the detail in your reply - yes we just really got down to it and started using the batteries with cheap PCB chargers, going fine so far. We want to power a Pi Zero W and (separately) an ESP32, and hopefully get to solar charging - but one step at a time!!

Instead of using a standard 3.7V to 5V booster, you could use a 7.4V to 5V regulator-probably a switching type, as linear regulators just waste power. However, charging might be harder. Maybe a few relays to change the batteries from series to parallel arrangement?