here is the longevity problem-
They were made as starting batteries.
So, the 18v alt would run over the 16v post, the car would run over the 12v post.
Now, what post do u think will be in use more?
The 12v post... It has to run the car.
The 16v post is there just so the alt can pass over it and reach the battery bank of 16v in the rear.
The idea was unique but the conclusion was horrible.
What happens is the majority of 16v equipment pulls it's current from the rear bank so the front battery's 16v side is usually topped off.
The 12v side however always uses more current so it discharges more...
So you have 6 cells that make up the 12v side and 2 cells that make up the 16v side.
Since the 2 cells is what the alt passes over, if those 2 cells are fully charged... then the battery will not accept a charge to allow it to top off the battery like it should!
This causes an imbalance which eventually leads to failure.
the onyl solution i know to prevent failure is to periodically connect the hc16v to a charger and let it equalize before charging. This is expensive and not convenient.
This is why powermaster does not like the design at all and have tested it as well. this where the information comes from that i read a long time ago.
they make a step down transformer but mostly for racing so the 12v output is low, only 25A continuous.