70 Amp Alternator not enough juice for 3 stereo amps

[2002 LAUNCH SS OWNER]

Hey Guys I have a 2002 Launch SS with 320HP Indmar engine with stock 7si series GM marine alternator. Currently I am using 3 Optima blue
top Marine batteries with a battery switch, I run 2 batteries all the time on position 1 on switch and have a backup battery on position 2 in case I listen to stereo too long while floating.I have never run the batteries dead by doing this but I did cook my 10si Gm alternator that puts out 100 amps. I have 3 stereo amps total wattage around 1100 watts.I think the 10si series alternator does not cool itself as well as the newer 7si alternator which has a more breathable case and 2 internal cooling fans. So my question is would I be better off with a battery isolater switch that just separates 2 batteries,one for starting and running boat and 2nd battery just feeding amps?? I hear their is a voltage drop when you use a isolator?? Or has anyone upgraded their stock alternator to something that will bolt into existing bracket and still use a single V belt, I understand the newer boats use a 90 amp alternator that uses a serpentine belt, has anyone upgraded to that by replacing all the puleys?? Sounds like that would be costly. I also hear some people use two 6 volt golf cart batteries in series for stereo systems they say it increases your capacity but do they recharge as quickly as a Optima battery?? Going to check out another alternator rebuild shop soon and see what they can come up with maybe a 11si series, I would love to get around 120amps, I may upgrade #10 wire from alternator to starter with #6 wire if I get a new unit over 100amps

Any suggestions would be greatly appreciated I have all winter to sort this out.
Thanks Dave:)

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[wotan2525]

If you've never run your batteries dead then the alternator is doing it's job. What are you trying to fix? It sounds like everything is working as it should?

And... having large batteries can not "cook" an alternator. You had an alternator go bad. My guess is that your 10 gauge wiring wasn't heavy enough to carry 100amps. I run 00-gauge off my 160amp alternator to my starter and batteries.

[2002 LAUNCH SS OWNER]

Thanks for your reply, it was the alternator shop that sold me 100 amp unit that suggested the unit burned up from constantly charging 2 batteries while also
trying to supply enough voltage to stereo amps. I think I will install a isolator switch and upgrade wire size from alternator to starter and install a separate cable direct to battery switch.If I use heater while driving with stereo on voltage drops alot at voltage gauge on dash.I really would prefer a larger output alternator so I could do some tower lights also.It seems to me the more accessories you have installed a bigger alternator is required. As you mentioned your boat has a 160 amp
alternator,and I doubt that was a factory installed unit in a 1987 Saltare. Maybe it is like you say I just had a bad alternator and I am worrying about nothing, but having it break down during my vacation has got me looking into making it more reliable* Thank you for your advice that is why I posted to get ideas or solutions

[wotan2525]

No problem. My guess is that a voltage drop at the gauge is more indicative of wiring problems with your setup. Upgrading your wiring would be a good first step and will make any larger alternator just that much more effective.

[sybrmike]

Several of us have had luck with DB Electric online. I installed one of their 140A units with 4/0 wiring & the gauge barely moves while running blowers, 2 electric trim tabs, & 2 impeller ballast pumps.

[2002 LAUNCH SS OWNER]

I had a look at DB Electric web site,did you install Delco Remy 10si> 140a> 3 wire unit? I had a 10si 100amp unit in my boat and it only lasted 2 years. There is another unit they have under universal fit and also under Mercruiser link, it is a 160 amp unit which has the 2 inch foot mount and a cast in adjustment plate in a half circle where adjustment bolt usually attaches. This mounting flange makes it a small case alternator with high output and universal mount that has 6.5 inch spread between mounting holes. Part # 160-SE-1G, what I like about this unit is it has a more open case for breathing and 2 internal cooling fans, I think my unit failed from heat and being under constant full load.This new universal mount unit is $307.00 where as 10si 140a unit is $151.00. Have a look at their. site if you have the time and please let me know which unit you installed in your boat

[sybrmike]

Found my invoice - I installed the 10si, 140A, 1 wire p/n HO-ADR0334-140. I've only run it once during a lake test of my rebuild project, so can't speak to longevity under heavy loads. I switched on everything I had - 2 blowers, 2 electric tabs, 2 impeller ballast pumps, led dash/running/transom lights, and even tooted the horn:) and the gauge barely moved at speed. Also, double checked & I used 6 ga (not 4 as stated above) from the alternator, 2/0 on the batteries/starter/ground, and 4/0 to the VSR. I don't have a stereo wired to the second battery yet, but have 2 batteries with a VSR and switch. Don't know if the VSR was combining or not.

[2002 LAUNCH SS OWNER]

Mike I went ahead and orderd that universal mount 160amp alternator from DB Electrical, I have some really nice 1 gauge battery cables I removed from a 560volt DC battery bank from a UPS unit from work, so I will replace #10 wire from alternator to starter and also run a direct feed from alternator to battery switch. Should have new unit on Monday and have it mounted that night.I will let you know if unit fits existing Indmar bracket without any modification. It is goofy how they mounted alternator on this engine, they used belt tensioning hole for holding unit in place and 1 inch foot mount as adjustment mount for tightening belt.So it is like it is mounted upside down** Thanks for checking which unit you installed on your boat.

[sybrmike]

I debated about a second feed direct to the battery as well, but my single 6 ga to the starter and 2/0 starter to battery switch seems to work fine. I'm not gonna have a monster stereo (80A max), so I think it'll be o.k. but can add later if needed. Good luck with the funky mounting.