I would suggest using 3 bridges. 25A or 35A.
Heat sink them. Use heat sink compound.
Some types of ECMs have a suitable dohickey in the brain can. This one is wired different, but use that chunk if it is in yours. Put heat sink compound between the chunk and bridges, and between the chunk and can.
Join the bridges 2 AC (~) terminals together, and connect a wire from the ECM to them.
Do that 3 times.
Combine all 3 bridges Pos together, they go to the battery Pos.
Combine all 3 bridges Neg together, they go to the battery Neg.
DanB's sketch is here,
http://www.otherpower.com/rectifier.shtmlG-