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2 alts better than 1


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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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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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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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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

 

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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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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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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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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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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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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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.

Quite agree (as the pages in here from our alternator past issues will attest!). Charging up as I type (again..in the fog). Started at 12.2v from last night (no wind). RPM 1600 and in 12 minutes have bought the voltage up to 14.5v on 240ah SLA and one 80ah start battery currently all charging in parallel. Currently doing 28.4 amps. Exhaust temp 31deg alternator rose to 44 now back to 39deg. One thing I have learnt is that the alternator has to spin at a high speed or won't cool properly. Still intend to fit a blower to it running off a thermal switch. I don't think this setup is really beyond most people who would be able to swap a starter motor or do an oil change? I wish yanmar produced big end pulleys with the provision for 2 belts and unfortunately I have never heard of any after market kit that bolts onto the existing fly wheel, the whole thing needs to be replaced. However, seems to work so long as the system is given a good charge up before running the anchor windlass and even then, don't run it continuously but in short runs. Heat gets to 55deg c when pulling up the pic!

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I wish yanmar produced big end pulleys with the provision for 2 belts and unfortunately I have never heard of any after market kit that bolts onto the existing fly wheel, the whole thing needs to be replaced. 

 

I would imagine that Yanmar would not be keen to put that much load on the front end of the crank. Unless the engine is designed to take that load, it isn't going to last long if you start swinging on the front of it.

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This is what Balmar says about belt sizing;

 

"Ensure that the belt or belts driving the alternator are capable of handling the alternator’s horsepower load. As a rule, a single 3/8" belt is capable of supporting up to 80 amps@12 volts, and a single 1/2" belt can support 100-amp@12-volts. Dual belts will typically support alternator outputs of up to 300+ amps/12 volts. Driving an alternator on an undersized belt will substantially increase the risk of belt slippage and premature belt failure. All 24-volt Balmar alternators require a minimum of dual 1/2” belts for optimal performance. Balmar offers a wide selection of AltMount serpentine pulley kits for marine diesel engines. Visit the Balmar website for more information and application charts"

 

Not far from what I said before

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From what I understand:

Current is the only indicator of charge acceptance/ SOC whilst charging.

Bulk = increasing voltage, constant current, Absorb= constant voltage Vset decreasing current. Bulk is usually over quickly.

Time at bulk to Vset  only indicates how flat your batteries were.

240Ahr batteries at 0.01C indicator means a tail current of 2.4A for 100%.( Probably

How long is it going to take to get your acceptance to that rate?

Otherwise you are doing a partial soc charging.

Which LA batteries don't like.

Correct me if I'm wrong.

You forget we are talking alternators. There is also a 400 watt wind turbine (more like 200w unless a cyclone is passing by) and 120w of solar panels.

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I would imagine that Yanmar would not be keen to put that much load on the front end of the crank. Unless the engine is designed to take that load, it isn't going to last long if you start swinging on the front of it.

That's the idea behind a gradual increase in power output so we don't need foot pounds of belt tention. Belt is still great (now 2 years). The alternator bearings would wear before the big end.

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This is what Balmar says about belt sizing;

 

"Ensure that the belt or belts driving the alternator are capable of handling the alternator’s horsepower load. As a rule, a single 3/8" belt is capable of supporting up to 80 amps@12 volts, and a single 1/2" belt can support 100-amp@12-volts. Dual belts will typically support alternator outputs of up to 300+ amps/12 volts. Driving an alternator on an undersized belt will substantially increase the risk of belt slippage and premature belt failure. All 24-volt Balmar alternators require a minimum of dual 1/2” belts for optimal performance. Balmar offers a wide selection of AltMount serpentine pulley kits for marine diesel engines. Visit the Balmar website for more information and application charts"

 

Not far from what I said before

All that's fine if you are running a V12 etc. This is a 18hp 2 cylinder long stroke diesel and running the alternator where we needed multi belts will overload both the alternator and engine. If the power output is kept at a controlled level where temperature is monitored and wattage slowly increased as the batteries reach a SOC where the current starts tapering off, wheres the problem in that? So far it's working very well.

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