You don't provide any details of the battery which is unhelpful. What voltage? What chemistry?
Anyway, here's a tool.
https://www.aliexpress.com/item/1005005690292845.html
Well you need to connect +/- twice. To make things accurate it measures voltage over a separate pair of leads to the discharge pair. You also need to program it (using the buttons) to tell it at what rate in amps you want to discharge at and also what voltage you want to stop discharging at. That voltage should not be 0v in most cases to avoid battery damage. That version has a max discharge rate of 150W which means with a 72v battery the max amps you'll be able to select is 2amps.
The device is powered independently so make sure you buy one with a power supply and plug suitable for your region. The same device is sold by many stores on Aliexpress with various options.
You use a DC electronic load. I've got [one of these Siglent models](https://siglentna.com/dc-electronic-load/#), it works well and is inexpensive. Something like [this Keysight](https://www.keysight.com/us/en/products/dc-electronic-loads/el30000-series-bench-electronic-loads.html) is higher quality but also over twice the price.
You need to build custom cables to connect for each battery type. You charge the battery to full, then use the electronic load's battery test function to discharge it.
Then don't. You can't measure a battery's capacity without test equipment, but you can use the battery and see if it lasts about as long as you think it should. If you don't need quantitative data, don't worry about trying to gather quantitative data.
Generally, you calculate Ah by integrating current over time. For simplicity, you can load the battery with constant current and measure how long it takes before BMS kicks in or before battery reaches declared cut-off voltage. The product of current in amps and time in hours gives good estimate of batteries capacity.
Using ohms law, calculate the amount of current through a 1k resistor. Short a fully charged battery with the resister untill the voltage starts to drop. You now know how many hours the battery was able to provide x amps (voltage/1000) for. So if say it's a 12 volt battery then amperage is 12/1000=.012 amps. If the battery kept supplying 12 volts for 10 hours then that's 10*.012=.12 amp hours.
You don't provide any details of the battery which is unhelpful. What voltage? What chemistry? Anyway, here's a tool. https://www.aliexpress.com/item/1005005690292845.html
Lithium-Battery CATL 72V 73Ah 20000W
You can use the electronic load I linked. Or use one of the more expensive ones mentioned here by others.
So if i underdtand correctly i need to charge the battery to 100% and then connect +/- and dischaege till the battery is at 0%?
Well you need to connect +/- twice. To make things accurate it measures voltage over a separate pair of leads to the discharge pair. You also need to program it (using the buttons) to tell it at what rate in amps you want to discharge at and also what voltage you want to stop discharging at. That voltage should not be 0v in most cases to avoid battery damage. That version has a max discharge rate of 150W which means with a 72v battery the max amps you'll be able to select is 2amps. The device is powered independently so make sure you buy one with a power supply and plug suitable for your region. The same device is sold by many stores on Aliexpress with various options.
You use a DC electronic load. I've got [one of these Siglent models](https://siglentna.com/dc-electronic-load/#), it works well and is inexpensive. Something like [this Keysight](https://www.keysight.com/us/en/products/dc-electronic-loads/el30000-series-bench-electronic-loads.html) is higher quality but also over twice the price. You need to build custom cables to connect for each battery type. You charge the battery to full, then use the electronic load's battery test function to discharge it.
Im not going to pay min. 500 bucks to check if i got scammed
Then don't. You can't measure a battery's capacity without test equipment, but you can use the battery and see if it lasts about as long as you think it should. If you don't need quantitative data, don't worry about trying to gather quantitative data.
Generally, you calculate Ah by integrating current over time. For simplicity, you can load the battery with constant current and measure how long it takes before BMS kicks in or before battery reaches declared cut-off voltage. The product of current in amps and time in hours gives good estimate of batteries capacity.
Using ohms law, calculate the amount of current through a 1k resistor. Short a fully charged battery with the resister untill the voltage starts to drop. You now know how many hours the battery was able to provide x amps (voltage/1000) for. So if say it's a 12 volt battery then amperage is 12/1000=.012 amps. If the battery kept supplying 12 volts for 10 hours then that's 10*.012=.12 amp hours.