No need to guess, here are the numbers from the Yanmar
However by using the decompression levers, you can start a 3GM30
with 'near' flat batteries
Decompress all cylinders, engage the starter motor and wait a few seconds while the engine rpm
builds. Keeping the starter engaged, compress one cylinder and usually it will fire. If so, introduce another cylinder and if it fires, you are almost there! Release the starer at the same time as bringing the third cylinder in.
The starter motor does not require huge amps to spin over a decompressed engine
and the energy stored in the flywheel does all the work