I haven't run into this exact problem, but some of the outboards have unregulated voltage, and since a rectifier's job is to take the alternating current (AC) and with diodes change it into direct current (DC), I'm wondering if there is a break where your battery isn't in the charging circuit even though it (the battery) might be still connected to the starting and other circuits. Do you know what your voltage is at the battery terminals while the outboard is running?
The battery probably acts like a bit of a voltage regulator if this engine doesn't have a large amount of charging current capability - but if the battery is out of that rectifier circuit it might be able to go to a higher voltage. Just a thought.
If you have a multimeter handy, you might unhook the rectifier and try some resistance measurements. I found a few troubleshooting procedures with pictures on this site (it's not perfect, but does give a general starting point):
http://www.myoutboardengine.com/rectifier.htmlI have heard that some "not so old" outboards didn't have voltage regulators until the manufacturers finally decided to add them in more recent years - so it might be possible that 16 volts is not totally out of range - maybe was alright before we started adding electronics such as depth finders, but I would think that it would not be that great for the battery regardless unless there was enough load to normally keep that voltage down a bit. Good luck!
