Swapped out the voltage regulator on my outboard's alternator — massive difference in charging

So I've got a 2005 Yamaha F40 on a 26ft bilge keeler and the stock regulator was doing absolutely nothing useful for my leisure bank — a 200Ah AGM setup wired in parallel. Was seeing maybe 13.6V at the battery terminals underway, which as we all know barely touches the surface charge on an AGM. Swapped in a Balmar MC-614 external regulator and now I'm hitting a proper 14.6V absorption phase. Night and day difference.

The alternator itself is only a 35A unit (pretty weedy for a 40hp outboard honestly), so I've got a Victron Orion-Tr Smart DC-DC 18A isolating it properly from the start battery. That way the outboard's charging circuit isn't fighting two banks at once and I'm not risking dragging the cranking battery down if I've been a bit greedy with the inverter.

Has anyone else gone down the external regulator route on an outboard rather than just accepting the rubbish OEM setup? I'm also wondering whether it's worth looking at a higher-output stator rewind — I've seen a few people mention it for Yamahas but can't find much UK-specific info. Seems like the Americans are way ahead of us on this sort of modification.

@Shunt_Guy bold of you to trust a marine alternator to actually charge something — mine's basically just a decorative spinning accessory bolted to the engine block.

@Shunt_Guy this is really timely — I've been wrestling with the same issue on my narrowboat with a Beta Marine engine. The stock regulator is absolutely useless for properly charging my 200Ah Fogstar lithium bank.

Did you go with an external regulator like a Balmar or Sterling? I keep seeing the Sterling Power AltoMax recommended for marine use but not sure if it plays nicely with lithium chemistry or if you need to manually set the charge profile.

Also — did you notice any heat issues with the alternator running harder under proper regulation? I've read that pushing these older alternators with a smart regulator can cook them if the windings aren't up to it.

What voltage are you seeing at bulk now compared to the 13.6V before?

@Shunt_Guy 13.6V from a marine alternator is basically just enough to convince the battery it's being looked at — fitted an external Victron regulator to my van's split charge setup years ago and the difference was embarrassing, alternator was clearly sandbagging the whole time.

Worth checking your wiring runs too — parallel AGMs on a bilge keeler with anything less than proper cable sizing and you're just heating up copper rather than storing electrons.

@Emma1996 Beta Marine engines respond well to this same trick, just make sure whatever regulator you fit has a temperature sensor — alternators on marinised engines run hotter than you'd expect in an enclosed bilge.

@Shunt_Guy this mirrors almost exactly what I did with a DC-DC isolator setup on my van build before I moved to proper alternator charging — the stock regulators are calibrated for the starter battery, full stop. For your 200Ah AGM bank you want to see 14.4–14.7V absorption properly held, not just flirted with.

Worth looking at whether a Balmar or Sterling external regulator is viable for your Yamaha — the Sterling ProReg D is particularly well-regarded in UK marine circles and lets you programme absorption/float profiles properly for AGM chemistry.

One thing I'd add that nobody's mentioned: temperature compensation becomes really important in a bilge environment where battery temps swing considerably. Make sure whatever regulator you fit has a sensor lead you can stick directly on the battery case, not just ambient air in the engine bay.