If you have an shunt type ammeter you can use a battery charger and a length of bare NiChrome wire salvaged from an electric heater to determine how many ohms the shunt should be to make the meter read correctly. Use a couple of electrical “jumper” wires with alligator clips at their ends to vary the length of NiChrome wire, making it a temporary shunt whose ohms you can vary. Standard industrial shunts typically are 100 milli-ohms or 1.00 ohms.