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2 alts better than 1


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#1 ricka

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Posted 22 April 2019 - 04:17 PM

Opinions please... will adding another engine mounted charging alternator reduce the charging time by a significant amount?

What there is = yacht, 38hp engine, 1x120amp alternator, 130ah house battery, 80amp start battery, Next Step regulator, voltage sensitive relay.

The engine starts easily so start battery is always pretty full.

As a comparison if the second alternator was 120amp as well would full charge time be improved?
How much better would a 240amp second alternator be?
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#2 Island Time

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Posted 22 April 2019 - 05:11 PM

Nope. The batteries dictate how much current they will accept at a given voltage, not the alternator. Dont waste your $.
You already have a smart reg, and the single alt is already large for your (small) batt bank. The best way to speed up the charging is to ensure that the next step reg is set to the max allowable voltage for the specific batteries you have. 0.1 volts makes a difference.
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#3 bigal.nz

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Posted 22 April 2019 - 06:01 PM

Sorry if this is a hijack, but my question is so similar. The previous owner of our boat configured the engine (Perkins 4.108) such that the manufacturer engine alternator charges the starter battery and also sends the signal to the tach.

 

A Balmar 90A alternator charges the 350Ah worth of house batteries.

 

I was about to make a post asking what the rationale might be behind a two alternator setup?


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#4 Island Time

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Posted 22 April 2019 - 06:20 PM

Two Alt setups normally use the factory alt to charge the start battery, and add the 2nd to charge the house batt. If you use VSR's either alt can charge both banks. Some VSRs sense only one battery, and engage when that one reaches the switch on, others sense either side. This gives some redundancy. Good external regs can often run 2 or more alternators. Here are a couple of setup rules;

A single V belt should only be used on alts < about 85 amps. After that, twin belts or serpentine.

A properly sized V belt should be level with, or just proud of the top of the V. It should NEVER touch the bottom of the V.

Alternators use a lot of power - a 100 amp alt uses about 2hp to drive it. The mounting brackets must be strong and properly aligned with the drive pulley. Excess belt dust, vibration etc are signs of problems to come.
The single best upgrade you can make to the charging system is a properly set up external regulator. WITH temp sensors on the batts, and preferably on the Alt. Its not uncommon to reduce charging times by 30% over a factory regulator.
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#5 Island Time

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Posted 22 April 2019 - 06:23 PM

Further to that though, an external reg will push your basic alt hard if the batts are down. It may overheat and fail. The ideal is a hot rated alt that can run at or near its rated output indefinitely. But they are expensive. The most common alt I see failed in this manner is the Hitachi 115a one.
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#6 Crazyhorse

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Posted 22 April 2019 - 08:02 PM

I agree with everything IT has said apart from the twin or serpentine belt required for a greater than 85amp alternator. The trick I have learned from experience is loading. Ours is a standard 120ah alternator that runs off a notched "green" belt (USA) and there are no issues so long as the load is regulated. You can spend lots on smart regulators that run temperature sensors to both batteries and alternator which is basically a set and forget setup, great for those with loads of money to pay for it all or you can add a monitored system, a fraction of the price. Whilst the ZM4 of Graeme Polleys design cooked our first alternator, if run with a good amp meter actually does the job but you have to monitor whats going on. Engine start, the field is wound right back. You have to consider we are running a 120 amp alternator off a yanmar 2gm20 and whilst running two sounds better, it would kill a 18hp engine. Starting the engine without any load is ideal for it to lubricate quickly then bring the field current up...slowly! We have a "cheapanese" oven monitor on the exhaust and another probe glued to the alternator so no more burnt smells! Loading it at 20 to 30ah seems to work then increasing until the voltage eventually reaches 14.6 and the current starts dropping by itself showing the batteries (SLA) are reaching full absorption. Alternator temperature goes slightly over 44deg C at 900rpm. For us, it now works! The 2GM20 requires a new fly wheel to run twin or serpentine belts but a slow increase in voltage loading on the alternator is kinder on both the engine and alternator. I know smart chargers do this but this works without the added expense and smart chargers are not smart enough to know when I am using a 1000w anchor windlass which incidentally cooked our last alternator.
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#7 Island Time

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Posted 22 April 2019 - 08:09 PM

Yes CH, but what your system is doing is decreasing the alt output so it does not overheat. Plus the control systems you have are beyond most diy owners. The original question starting this threat was how to speed up the charging, and throttling back the alternator when the batts can accept greater charge is counterproductive.
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#8 Crazyhorse

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Posted 22 April 2019 - 09:09 PM

I disagree, I'm no rocket scientist (ask the ex wife!) and don't believe it's beyond the reach of most to get a setup that works for them. Speeding up charging, we have done it. A Hitachi 55ah alternator that comes stock standard drops out at 13.5v lucky if you get 13.8 which means the thermal controlled regulator had pulled the field back within minutes of starting. Ideal for the start battery but a 240ah house bank? Actually charging now as I type ( exhaust temp 31deg, alternator 40deg start battery at 14.6v 11.2a battery bank in the bow as attached!).

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#9 Crazyhorse

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Posted 22 April 2019 - 09:17 PM

Single belt driven 120amp alternator.

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#10 Island Time

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Posted 22 April 2019 - 10:30 PM

Everyone is entitled to their own opinions of course. All I can say is that if your batts are very low, and your regulator pushes your alt to over 100 amps for a long period - say 5 hours or more - the alt will melt, or the belt will fail.
If the output is short term, or the temp compensation cuts the alt back, then you’ll likely get away with this setup, but your charge time will be extended over what’s possible.
Many people do run those 120a alts on one belt, and have no issues, as they dont make them charge large banks from a low level. But the day you do, you can be in trouble.
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