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Old 28-08-2011, 18:37   #1
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60 amp Alternator Charging Question

Good day, I have a B361 with the Yanmar 3GM30 engine, with the 60amp alternator. Quick electrical question. I have 2x 6volt Energizer golf cart batteries with 225 amp hours and two x Trojan T-145 6 volt batteries with 260Ah, total of 485Ah.

When on the hook for a couple of days and need to run the engine to recharge, do I calculate that the batteries will absorb 60 amps from the alternator per hour? Example, if I've consumed 240Ah over a day or two, I would need to run the engine for four hours to replenish 240Ah?

Is it that simple to calculate?
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Old 28-08-2011, 19:06   #2
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Re: 60 amp Alternator Charging Question

Ahoy, no, you need to put back in about 30% more to get the removed amphrs back in.
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Old 28-08-2011, 19:15   #3
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Re: 60 amp Alternator Charging Question

Indeed, it is a bit more complicated than that....for at least three reasons:

1. Already mentioned above, is the inefficiency of the charging process itself; there are losses which depend on a host of things...battery type, condition, temperature, wiring, etc.....which usually amount to somewhere between 10 and 25 or even 30% loss; and

2. Your 60A alternator isn't going to put out 60A continuously. As it heats up, it will put out less current. How much current, again, depends on a bunch of things, but especially on the state of charge of the batteries (SOC), their condition and type, and whether your alternator is internally or externally regulated; and

3. As a battery reaches a SOC above about 80%, its acceptance of charging current will go way down, and it will take quite a long time for it to reach full charge.

In practice, most house batteries on cruising boats tend to live somewhere between 50% and 80% SOC since it takes so long to get that last 20% or so back into the batteries, unless you have solar or wind charging power or unless you happen to be motoring for several hours.

Bill
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Old 28-08-2011, 19:28   #4
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Re: 60 amp Alternator Charging Question

A 60A alternator will typically only supply 40-50A at the battery end at cruising revs.

If you are charging by running the engine at 1500 rpm, at anchor, the output is likely to be only about 30A

The batteries also have to accept the charge. A 485AHr (make sure you are calculating this correctly if you are adding voltage do not add amp hours) battery bank will accept 30A or greater until its about 75% full. For the last 25% the charge rate will taper down even if the alternator is capable of supplying more current.
If charging with an alternator most boats cycle their batteries between 50 and 80%, because a 100% charge would be very slow.
If you need to replace 240Ahrs you will need to run your engine for about 8hrs at 1500rpm at anchor or perhaps 6hrs if motoring at 2500rpm. This is providing you stop charging below 80%. The last 20% will be slower.
It is therefore complicated to estimate the amount of AHrs that the alternator is adding, so a battery monitor that will count AHrs in and out and show you when the charge current is dropping, is just about essential.
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Old 29-08-2011, 12:55   #5
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Re: 60 amp Alternator Charging Question

Your first thoughts are right. And if you stay to basic units your calculation is right. Just multiplies the current (in Ampere) with the time (in Hour) and you have how much you have charge your batteries. But then comes the reality, as mentioned in earlier posts. The only thing that’s the same in theory and practice is the time, one hour is one hour. The generator don’t gives 60 A, for more than a short time at max (read thread about overheated alternator). The battery isn’t perfect so they need more than 100%. A battery monitor sense the current, compensates for the lost in the batteries and measure the time and that way calculate the Ah the batteries got.
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Old 29-08-2011, 13:21   #6
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Re: 60 amp Alternator Charging Question

Quote:
Originally Posted by Luken7 View Post
Good day, I have a B361 with the Yanmar 3GM30 engine, with the 60amp alternator. Quick electrical question. I have 2x 6volt Energizer golf cart batteries with 225 amp hours and two x Trojan T-145 6 volt batteries with 260Ah, total of 485Ah.

When on the hook for a couple of days and need to run the engine to recharge, do I calculate that the batteries will absorb 60 amps from the alternator per hour? Example, if I've consumed 240Ah over a day or two, I would need to run the engine for four hours to replenish 240Ah?

Is it that simple to calculate?
Are you sure it's a 60 amp alternator? Yanmar use Hitachi 50amp or optional 80amp on all their motors, which of these do you have?

Just curious!
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Old 29-08-2011, 14:07   #7
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Re: 60 amp Alternator Charging Question

As Bill and others noted. Especially if you have an automotive type alternator with an internal regulator which is actually designed to "not overcharge" a starting battery during long hours on the engine. An external regulator designed to work with deep cycle batteries will put more current into them faster, and bring them closer to a full charge MUCH faster.
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Old 29-08-2011, 15:19   #8
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Re: 60 amp Alternator Charging Question

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Originally Posted by Lars_L View Post
... But then comes the reality, as mentioned in earlier posts. The only thing that’s the same in theory and practice ...
WRONG!
The "theory" fully explains the "real" differences due to various inefficiencies. One merely needs to employ the full formula (more complete theory).
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Old 29-08-2011, 19:46   #9
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Quote:
Originally Posted by Jamel

Are you sure it's a 60 amp alternator? Yanmar use Hitachi 50amp or optional 80amp on all their motors, which of these do you have?

Just curious!
I just looked at my Yanmar owners manual again, it shows 60amp as standard, 80 as optional
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