TOPIC: Battery capacity for a 1984 115?

Merc 115 1984 but rebuilt with hi dome pistons. What capacity (AH) battery would you guys recommend? The boat runs minimal electrics (GPS, instrument lights & bilge pump when needed). None of my OE manuals list what capacity battery is recommended.

I know the charge is unregulated but I have a friend who runs his boat with an 850XS succesfully for a number of years on a small Odyssey gel battery (see here ) which I favour due to size & weight (lack of). The alternative is a Optima type (see here ) but its expensive & heavier (but very good). Using the boat on the sea I want reliability (I don't fancy roping that bugger over) but light weight is a plus too. I know some of you run lawn mower batteries - but probably not on an L6! I read on here or John's once that gel batteries are not a good idea with ADI ignition as they can damage the switchboxes?? I struggle to get this as the ADI system is alternator powered (Alternator Driven Ignition) so is totally independent of the battery whatever type you have fitted?

Hey Geoff, good to hear from you again! I remember the discussion here on gel type (Optima style) batteries having a severely reduced life on unregulated charging systems. It's been a couple years ago, so I'd have to search the archives for it, but I remember the general consensus was it was better to run lead-acid type batteries on the open charge system (older) outboards.

That being said, I also remember Bruce G speaking of running used or reconditioned (?) power wheel chair batt's and having quite favorable results I believe. Considerably smaller and lighter than a standard marine cranking batt.

I have been wondering about this as well. I have a 1967 Merc 500 and I wanted to use a large deep discharge battery so I could also use it to power a trolling motor. But, I was told the Merc charging system rectifier may not live long if a deep discharge battery is used due to the fact that it has a lower internal resistance. That is, it will accept a high amperage charge rate for longer and thus overheat the rectifier. In addition, the gel cells and AGM batteries will likewise accept a higher rate of charge (due to lower internal resistance) and therefore may not be a good idea for the same reason. The logic seeems sound, but I would like to hear from someone with real experience in the matter.

I agree that it is hard to see how the ignitions could be harmed by what happens in the charging system, but I don't have any experience with these old motors. Maybe some of the (new for their day) solid-state ignitions were trouble prone for other reasons and the batteries got blamed. Or, maybe there is some connection. I don't know.

I found the thread in question, at least the one I was talking about;
www.fiberglassics.com/fiberglassics-forums/home/main-forum/what-s-the-deal-with-marine-batteries#25410

Thanks for the link & the advice - lead acid it is then!

Smart Move: Lead acid batteries are very reliable, a lot less expensive than Gel or AGMs, and not as finicky. AGMs and Gels are both very voltage and temperature sensitive. You not only need a regulated charger, but ideally one that has a temperature sensor, that measures the temperature of the battery cells. Over charging and over heating ruins AGMs and Gels. Lead Acid is fairly insensitive to this. Their big drawback is their weight, but a properly maintained LA battery will last 5 to 7 years. see newboatbuilders.com/docs/BatteriesEverything.pdf Batteries: everything you need to know.