Before you buy a 16V - if you haven't already - take a look at this.
http://www.cfmvoltguard.com
We have a number of them in cars, and they are working very well.
The ignition and trans brake will always see 16V even if the battery is low.
You will notice the engine starts much easier because the ignition always sees 16V - no drop in voltage when you hit the starter.