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Old 09-05-2020, 08:56   #16
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Re: Alternator

A hydrometer (ie specific gravity) will NOT tell you much about the SOC of a battery unless you know what the specific gravity was when the batteries were brand new and fully charged. Ive seen big deep-cycle batteries shipped with an initial SG from about 1.245 to as high as 1.290 depending on the application and the operating environment.
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Old 09-05-2020, 09:50   #17
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Re: Alternator

I have had two of those cheap Hitachi alternators that you can get on Amazon for $75. The first one 55A, it was only producing around 35A. I opened it up and converted to external regulation, then was getting 55A at start, slowly falling as the temp rose. At some point one of the bearings failed (I assume I did not put it back together properly). Next I got an 80A alternator with a high voltage set point. I have been running it for about two years now, it produces 65A max and I am just too lazy to convert it to external regulation. I have ordered a 100A alternator to keep as spare, which I expect will produce around 80A when cold. It does not take that long to replace if needed.

There is some logic to buying expensive alternators with external regulation for a bullet proof setup but the following a true for any alternator:

- 12V units are max 70% efficient, most are in the range of 50-60% efficient at typical charging rpms.

This means that an alternator that makes 1,000W (14V x 70A) will produce approximately the same amount of heat in the engine room. That is a lot of heat to take out on top of what the engine makes. Just imagine a 1,000W heater in the small engine compartment.

The key alternator characteristic is the set voltage, you want it to be around 14.4V or higher to be able to push the current to the batteries. So, make sure you get such an alternator.

- With a typical 440AHr bank, discharged to 60%, the alternator needs to put in around 100A before the SOC rises above 80% and battery acceptance limits the output. This means that you do not need hours and hours of run time at full power, just 1.5-2.0 hours is enough and this can be distributed during the day. So, in practice it is not so bad.

I am not saying that a proper charging system is not beneficial, all I am saying is that for most coastal uses, a cheap alternator will do just fine.
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Old 09-05-2020, 15:58   #18
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Re: Alternator

Besides alternator temperature another limitation is the vee belt running over the necessarily small alternator pulley needed to keep the alternator speed up at an engine fast idle to a place where the alternator can generate both the amps and enough airflow with its fan(s) to keep itself cool. If you can long term get more than 50 or 75 amps with a 1/2" belt without belt dusting or without coolant pump bearing failure from over tightening the belt, more power to you. ( QUOTE )

Yea Bill I've had that battle. PO had the wrong v section belt & belt dust is everywhere. Had to tighten daily & thought it would wear in eventually. Well on recent 10 week cruise it wore in alright & sat properly in the v but then it broke, so it wore in but wore out! lol. There was a vee belt with teeth at the bottom in the spares box that had the right profile & after I fitted that & sprayed with belt dressing the problem went away ok but realised I would need a double belt setup if I wanted a bigger alternator. What I think I'll try next is to run the original Hitachi 35 amp as well as the Bosch 55 amp simultaneously. The Hitachi only has a 13.8v VR whereas the Bosch has a 14.8v so the Hitachi will stop first. Thanks for your help

Another factor is the wiring between the alternator and the battery on charge and whether or not your regulator has a voltage sensing terminal connected to the battery terminal. ( Quote Raymond R)

No voltage sensing terminal just the B+ Raymond. Think the wires are ok as havent noticed any hotspots but will check again. My GUESS is Wotname might be on the right track suggesting the batteries wont accept more but need more investigating. Thanks for your input.


A hydrometer (ie specific gravity) will NOT tell you much about the SOC of a battery unless you know what the specific gravity was when the batteries were brand new and fully charged. Ive seen big deep-cycle batteries shipped with an initial SG from about 1.245 to as high as 1.290 depending on the application and the operating environment.
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Ah, thanks for the heads up Bycrick. I thought hydrometer was supposed to be a better way than resting voltage ( about 12.6v incidentally)

I have had two of those cheap Hitachi alternators that you can get on Amazon for $75. The first one 55A, it was only producing around 35A. (Quote Pizazz)

Well Pizzaz thanks for your real world experience. Our 55 amp Bosch alternator is the same as they installed in Ford pickups in the '90's for NZ & Australia so it was probably still made properly. I hope now I've done the bearings it will outlast me. It had a 14 volt VR originally but we tricked it with diodes in the field wire to put out 14.5v but that vr failed & I have replaced it with a 14.8v one I got online. It actually goes to 14.8v
I think I would buy a used older style alternator now thinking about it as it wasn't difficult to do the bearings & test it. At $75 if you get 2+ years out of one I can see the attraction of an Amazon one tho. I'd buy 2 & keep one as a spare after I tested it.
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Old 09-05-2020, 16:49   #19
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Re: Alternator

Measuring the specific gravity is a good thing. It will help you sort out cells that are "unequal." If you KNOW the initial specific gravity, you can build a graph of indicated SG with state of charge. If you really want to measure SG, spend $30 on a refractometer. Its far more accurate and much simpler to use reliably than a hydrometer.

I once met a fellow cruiser who was a professional brewmaster whod worked for many of the biggest. His comment was that hed carried a hydrometer for 35 years like engineers used to carry slides rules. He still had trouble getting reliable readings consistently and had bought a refractometer the minute he could get one.
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Old 09-05-2020, 18:25   #20
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Re: Alternator

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Originally Posted by Bycrick View Post
Measuring the specific gravity is a good thing. It will help you sort out cells that are "unequal." If you KNOW the initial specific gravity, you can build a graph of indicated SG with state of charge. If you really want to measure SG, spend $30 on a refractometer. Its far more accurate and much simpler to use reliably than a hydrometer.

I once met a fellow cruiser who was a professional brewmaster whod worked for many of the biggest. His comment was that hed carried a hydrometer for 35 years like engineers used to carry slides rules. He still had trouble getting reliable readings consistently and had bought a refractometer the minute he could get one.
Thanks for schooling me up. Thinking more on it, the thing is it wont make much difference having a refractometer for better accuracy as I'll use the batteries until they can't keep fridge going overnite or start the motor & then we have another pair of 6v 225amp deep cycles that will be pressed into boat service.
Wisely or not I'd rather spend the $30 on beer but I was happy to learn something new.
Wisely or not
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Old 09-05-2020, 19:51   #21
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Re: Alternator

I have a Chinese and German alternator here for my Yanmar. The chinese have been making alternators for a long time. I punish mine with my LFP bank and it is going strong.

80amp alternator is about 3hp plus, at full whack, so is a decent load for a smaller engine. This is where either external regulation or something like a B2C or Alternator to Battery charger come in handy to reduce the load by turning down the power
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Old 09-05-2020, 20:19   #22
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Re: Alternator

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Originally Posted by Compass790 View Post
Na Wottie it doesn't!
When fridge comes on whilst alternator putting out 28amps @ 1600 engine rpm, our usual at anchor charge rpm ( fridge draws 8 amps roughly but dependent on battery voltage ) alternator output does not change.
Although my knowledge of electrickery is minimal it occured to me to check that or perhaps
Thanks for the lesson about state of battery affecting alternator output. Can I work that backwards? i.e. seeing how the batteries do accept 35 amps does that suggest they are not totally knackered? Or am I making a link that isn't there?The hydrometer says they are ok when charged & they seem to be good enough for our 50-60 amp over 24 hr needs & start the engine no problem. Or does it suggest an alternator fault? bearing in mind I can't remember trying fridge on at max revs just at anchor charging revs. Tho I probably did try that, I just can't remember
I did realise the battery state does have some effect as after about 90 mins charging at anchor from a 12.1 or 12.2v start the amps from the alternator are down to 20 even tho the volts are only 14.5 & VR is a 14.8 one.
Feel free to school me up some more if you aren't on the 2qm20 mission
Are you reading current with a battery monitor? They measure current thru the battery with a shunt connected to the battery negative terminal. When the frig comes on the alternator supplies the current to it and current to charge the battery. The frig and battery are then in parallel and the total current supplied by the alternator is the sum of the two currents. The battery monitor will show only the current to the battery and won't change if the alternator can supply all the current the battery will accept and the refrig current. If you turn on the frig and the battery monitor doesn't change, the alternator is supplying the charge current pluss the frig current.
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Old 09-05-2020, 20:26   #23
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Re: Alternator

Sulphated batteries will not accept high charge rates and the voltage rises fairly quickly.
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Old 09-05-2020, 20:29   #24
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Re: Alternator

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Originally Posted by kmacdonald View Post
Are you reading current with a battery monitor? They measure current thru the battery with a shunt connected to the battery negative terminal. When the frig comes on the alternator supplies the current to it and current to charge the battery. The frig and battery are then in parallel and the total current supplied by the alternator is the sum of the two currents. The battery monitor will show only the current to the battery and won't change if the alternator can supply all the current the battery will accept and the refrig current. If you turn on the frig and the battery monitor doesn't change, the alternator is supplying the charge current pluss the frig current.
No, I'm reading current with a clampmeter around the wire ( 35mm2) running from the alternator B+ terminal to the starter solenoid terminal.
Is this the right way to go about it?
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Old 09-05-2020, 20:35   #25
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Re: Alternator

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Originally Posted by Compass790 View Post
No, I'm reading current with a clampmeter around the wire ( 35mm2) running from the alternator B+ terminal to the starter solenoid terminal.
Is this the right way to go about it?
Yep, that'd be the alternator output. Looks like you have a 35 amp alternator. I'd guess that would be about the biggest a manufacturer would put on a 8 HP diesel and that might have been an option.
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Old 09-05-2020, 20:41   #26
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Re: Alternator

Hope you know about this:


Hitachi Alternators 101 http://forums.catalina.sailboatowner...d.php?t=166123 This comes up so often on other boating forums...
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Old 09-05-2020, 20:42   #27
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Re: Alternator

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Sulphated batteries will not accept high charge rates and the voltage rises fairly quickly.
Well Batteries are not in great shape as they were only at 4v ( 6v batteries) when we were given them but I have no capacity tester so dont know what their actual capacity is. The pair that we are currently using on the boat had a resting voltage of 12.5 after their first couple of charges but after a couple of my attempts at equalization & 2 years on the boat the resting voltage is 12.6v so they have improved.

From a voltage of 12.1 the alternator was charging at 28 amps according to the clampmeter for at least 1hr & after 90 mins it was down to 20 amps so I don't know if that tells you anything. Feel free to comment.
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Old 09-05-2020, 20:48   #28
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Re: Alternator

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Yep, that'd be the alternator output. Looks like you have a 35 amp alternator. I'd guess that would be about the biggest a manufacturer would put on a 8 HP diesel and that might have been an option.
No we are actually using a 55 amp Bosch. I took the 35amp Hitachi off as it kept saving itself with the thermal cutout but that didn't get the batteries charged. I will re-install the Hitachi tho for the reason that you say the manufacturer only put a 35 amp alternator on. See post #9
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Old 09-05-2020, 20:51   #29
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Re: Alternator

If the beer stays cold the batteries are good.
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Old 09-05-2020, 20:53   #30
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Re: Alternator

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No we are actually using a 55 amp Bosch. I took the 35amp Hitachi off as it kept saving itself with the thermal cutout but that didn't get the batteries charged. I will re-install the Hitachi tho for the reason that you say the manufacturer only put a 35 amp alternator on. See post #9
What's the voltage at the alternator B+ terminal? What about at the battery terminal?
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