Are you just interested in the current figure, maybe load up the starter until it stalls and get the current value, or are you concerned about some kind of performance issue? The batteries you are using are sufficient to power the starter, you may find a limit to how much torque the starter has to turn over an engine, probably more of a limitation than electrical energy. Run time on the starter will vary according to the load, whether you can get 10 starts or 50 out of a fully charged battery pack.