If it dropped low enough to not work the ecu it would not turn the engine over, DC motors as in starter motor are VERY voltage sensitive.
I suggest you first clean up all connections, and make sure the cu is earthed where it should be, then as said check power at ecu while cranking to see if this is your problem, but I am 100% sure JAY is correct and it is a set up problem, just needing a little more fuel etc, or a bigger prime etc.
Ecu's will work down to around 6 or 7 volts, your starter won't.
tabetha