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There is no precise mathematical relation between engine capacity and starter amps. Only some ballpark figure can be given. In general, the bigger the engine, the more powerful the starter and therefore the amps drawn by it. Besides, the actual current drawn depends on the starting condition. A recently warmed up engine on a warm day will draw much less current than a cold engine on a freezing day. Having said all that, a reasonable range would be 50-70A for a warm engine and more than 200A for the other extreme.
Another point worth noting is that the main rating of a battery is not in Amperes but in Ampere-hours, Ah for short. It is the product of current drawn from a fully charged battery and the number of hours it can continue to supply that current before being fully drained. For example, at least in theory, a 65 Ah battery can supply 1A for 65 hours, 6.5A for 10 hrs, 65 A for 1hr, 20A for 3.25 hrs, and so on. It doesn't quite work out that way in practice. The higher the current drawn, the lower its actual capacity in Ah. A real-world battery that can supply 6.5A for 10 hrs may be able to supply 65A for only 30 minutes or so.
The principle is very simple: just insert an ampere-meter (ammeter) in series between the battery and the starter motor. In practice, the high current and low voltage introduces some problems. The high current requires very thick wires, both to reduce losses and to ensure that it will not overheat. The low voltage of only 12V makes it doubly important that the wires and connections are of as low resistance as possible so that voltage drops across them will be negligible. A total resistance of 0.01 ohm will drop 1V at 100A, and a loss of 1V is quite significant in a 12V system. This is why we need perfect contact at a car's battery terminals.
Simplest way to test start motor current.
Use DC clamp meter 0...1000A with peak hold option.
Clamp meter on battery minus cable , go into car and start.
Read meter , what was the peak current value.
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