16 volts is way too high- - - -it should be 14.5 or a little more, max voltage against a fully charged battery. The high voltage has probably damaged the battery, which is now unable to hold a charge. Is it possible to check the electrolyte level on your battery? If so, you will probably find it way low, with the tops of the plates exposed. Find out why the voltage is too high- - -probably a wrong regulator setting if it's adjustable, or a shorted solid state regulator if you're running an internal-regulated alternator. Either way, you've probably got a cooked battery. Find and fix the charging problem before installing a replacement battery, but be aware that a properly-charged battery in good condition is necessary before you can make charging system voltage measurements that mean anything.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!