So needed new starting battery for my 1992 Johnson 150hp outboard and forked out for a AGM combo starting/trolling battery. Two fishing trips later tach stopped working and could smell burning electrical and found my rectifier had melted very lucky outboard didn't start on fire. This pertains to outboards with rectifier/stator type systems, mine is a 35 amp system.
When looking up replacement rectifier CDI electric site has a warning to not use AGM batteries with older outboards. Appears you can only use wet cell lead acid batteries that you can add water to, maintenance free, glass matt, AGM etc have to high of resistance for older outboards. They can only be used with outboards wit alternators on them.
Just putting this out there to save someone some money my listed for $250 lucky I found it online for $169.