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Old 03-11-2020, 16:04   #1
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Realistic output from a 55 amp API alternator?

We have a new Moyer Marine late model A4 in our Tartan with the “normal” API alternator. I assume this is internally regulated.

We have the indigo prop, so the engine is spinning a bit quicker than stock. (Love it).

We have a simple 1-2-B switch but have informally designated Battery 1 (new West Marine 80 ah 12V AGM) as engine start and Battery 2 (old Dekka gel) as house.

Boat is a weekend cruiser with maybe a couple of days strung together on the hook. Would like to extend the charging of the house without directly cooking it with the “dumb” alternator if we motor for a few hours a day.

My plan is to put a DC to DC charger from the start AGM to the House Gel. Renogy, Sterling, Victron, etc have a variety of amperage: a 20, 30, 40 and 50 AMP 12V B2B chargers.

30 AMPS seems like the sweet spot to charge to get some realistic charge put in the house bank (80 ah).

Too much? Would that make the alternator run too hot? I don’t think we ever asked more than a quick recharge after 10 seconds of starting from the alternator. Boat is on shore power 99% of the time with a Xantrex 2 bank set to 3 stage charging.

Comments appreciated on anyone who has pushed a similar alternator at a constant charge load. (Not sure if the gasoline torque curve difference plays into this either)

Thanks
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Old 03-11-2020, 16:14   #2
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Re: Realistic output from a 55 amp API alternator?

The AO - alternator output - is determined by both the size and the SOC of the bank being charged.


With one gel cell house bank, you won't tax that alternator at all, even if the SOC of the house bank is 50%.


For comparison purposes: a nominal 400 ah (yes 400) house bank at 50% is between 50-80A.
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Old 03-11-2020, 16:19   #3
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Re: Realistic output from a 55 amp API alternator?

Quote:
Originally Posted by Juice13 View Post
We have a new Moyer Marine late model A4 in our Tartan with the “normal” API alternator. I assume this is internally regulated.

We have the indigo prop, so the engine is spinning a bit quicker than stock. (Love it).

We have a simple 1-2-B switch but have informally designated Battery 1 (new West Marine 80 ah 12V AGM) as engine start and Battery 2 (old Dekka gel) as house.

Boat is a weekend cruiser with maybe a couple of days strung together on the hook. Would like to extend the charging of the house without directly cooking it with the “dumb” alternator if we motor for a few hours a day.

My plan is to put a DC to DC charger from the start AGM to the House Gel. Renogy, Sterling, Victron, etc have a variety of amperage: a 20, 30, 40 and 50 AMP 12V B2B chargers.

30 AMPS seems like the sweet spot to charge to get some realistic charge put in the house bank (80 ah).

Too much? Would that make the alternator run too hot? I don’t think we ever asked more than a quick recharge after 10 seconds of starting from the alternator. Boat is on shore power 99% of the time with a Xantrex 2 bank set to 3 stage charging.

Comments appreciated on anyone who has pushed a similar alternator at a constant charge load. (Not sure if the gasoline torque curve difference plays into this either)

Thanks
I'm doing more or less the same thing, out of a 65A on the original Volvo Penta (100HP) to a larger LFP bank of 280A (I have added on the existing AGM house bank of 400A).

If you connect the DC to DC from the cranking (AGM/flood etc) battery (NOT from the alternator output) to the new LFP bank, you should get a good protection for your alternator. I would suggest to monitor the alternator's temperature temperature with IR thermometer and compare temps at different RPM before and after switching the DC to DC charger.
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Old 04-11-2020, 05:02   #4
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Re: Realistic output from a 55 amp API alternator?

Have run our Bosch 55 amp alternator at 27 amps for 90 mins quite a few times with no problems. Only puts out 35 amps once hot at max revs.
This is using a 14.8v setpoint dumb voltage regulator into a nominal 225amp battery bank. IIRC it runs about 85oC charging at anchor
Doubt it will push 30 amps into an 80amp battery for that long anyway.


My guess is that you will be ok without knowing your alternator.
Note I'm an electrical novice, just our experience.
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Old 04-11-2020, 05:38   #5
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Re: Realistic output from a 55 amp API alternator?

Often misunderstood. The current rating of the alternator is the MAX current it can output without being overloaded. The actual current is determined by the battery capacity and state of charge. A DC-DC charger won't change this. The battery can only draw the Amps it is capable of. So you can safely use any alternator or charger as long as its rating is greater then what the battery draws.

80Ah is a small battery. You should never use more than 50%, so that means you should never need to put more than 40Ah back into it. Likely that would not draw much more than 20A (half the used capacity) and only for a short time.

So, I would expect, regardless of what alternator or DC-DC charger you have, you might see 20A for a little while, and as the battery charges that will quickly drop and mostly be less than 10A.
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Old 04-11-2020, 06:28   #6
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Re: Realistic output from a 55 amp API alternator?

The way I read J13s post is that the engine start battery is the 80A AGM battery. If so that's fine. The older gel is the house battery but isn't described in detail.

Now what is going to happen with the 1/2/both/0ff switch? particularly if someone turns the switch to "both" which will charge both batteries in addition to the B2B charger? The "both" setting may be need if the engine start battery is flat at any point in the future.

There is also the issue of different battery charging voltages needed, in particular I bet the gel house bank has a lower absorption setting, which the B2B could take care of rather neatly.

If the voltages are similar however, then a VSR would also do the same thing and may be a lot cheaper.

As wholebe said, the batteries will self regulate how much charge they absorb, but also a small alternator in a hot engine space after an hour will also reduce the output. Watching it on a battery monitor as you motor across a flat calm sea is just depressing when you start at 40A, but see it drop to 20A after half an hour, and you're down to single figures in a couple of hours. Those last few AH can really take forever.

Suggest J13 replace the gel battery with a larger AGM battery when it fails but ensure the charging regime matches the engine start battery.
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Old 05-11-2020, 10:42   #7
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Re: Realistic output from a 55 amp API alternator?

Quote:
Originally Posted by DeValency View Post
I'm doing more or less the same thing, out of a 65A on the original Volvo Penta (100HP) to a larger LFP bank of 280A (I have added on the existing AGM house bank of 400A).

If you connect the DC to DC from the cranking (AGM/flood etc) battery (NOT from the alternator output) to the new LFP bank, you should get a good protection for your alternator. I would suggest to monitor the alternator's temperature temperature with IR thermometer and compare temps at different RPM before and after switching the DC to DC charger.
Yes, I should have been explicit about the hookup: DC to DC connected FROM "engine" 10ah AGM TO "house" 80ah gel.

My intent was to keep the house separated from the alternator in 99% of situations. ("2" switch setting would be for when even the portable lithium jump starter died).

I do have an IR temp gun, so will use that to check temperature rise. I've also scoped out some bilge blowers if aiming some cooler air on the alternator becomes necessary.
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Old 05-11-2020, 10:49   #8
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Re: Realistic output from a 55 amp API alternator?

Quote:
Originally Posted by Stu Jackson View Post
The AO - alternator output - is determined by both the size and the SOC of the bank being charged.


With one gel cell house bank, you won't tax that alternator at all, even if the SOC of the house bank is 50%.


For comparison purposes: a nominal 400 ah (yes 400) house bank at 50% is between 50-80A.
So what that 80ah gel house battery gets replaced with a 100ah battle born lithium? My intent is to create a house charging system to allow a lithium drop in solution in the future.

I assume the DC to DC would pull a consistent 30amps.

Spoiler alert, i'm think of the Renogy DC to DC with the built in MPPT. Not much more room on the sun shade for more than two 100W panels, so the limitations of the combo charger-controller won't come into play.
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Old 05-11-2020, 11:40   #9
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Re: Realistic output from a 55 amp API alternator?

Quote:
Originally Posted by Juice13 View Post
Yes, I should have been explicit about the hookup: DC to DC connected FROM "engine" 10ah AGM TO "house" 80ah gel.

My intent was to keep the house separated from the alternator in 99% of situations. ("2" switch setting would be for when even the portable lithium jump starter died).

I do have an IR temp gun, so will use that to check temperature rise. I've also scoped out some bilge blowers if aiming some cooler air on the alternator becomes necessary.
So you have a good plan. Hope you can indeed keep the house on an internal (vs shore power of solar) charge at only 1% of the time. Unfortunately, in my case there is a lot of power consumption while sailing; fridges, auto pilot, electronics etc., hence I wanted to almost triple my effective house bank instead of getting into the expense of a new/additional alternator and engine use while I have 300W solar and while changing my sailing pattern to no more than a single overnight sailing.
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