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Old 28-10-2023, 17:07   #1
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Old alternator with new AGM?

Hello

I need to replace batteries in a sailboat.

Can I use AGM with a 51 amp, 40-year old alternator design? (That’s the age of the diesel motor, and the alternator looks to be original.)

Will I damage the batteries? Do I need any type of external regulator?

David
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Old 28-10-2023, 17:18   #2
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Re: Old alternator with new AGM?

It will be awfully slow charging imo. You don't say how big the battery bank is but I doubt that an older 50 amp alternator puts out anywhere close to 50 amps, especially after it heats up. My old H38 had a 50 amp alternator and I was lucky to get 30 out of it, and not for long.
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Old 28-10-2023, 17:21   #3
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Re: Old alternator with new AGM?

Hello

The battery bank has one 8D battery and one Group 27 battery.

My question is not how fast it will charge, it is if such an old alternator will work (ie not damage) new AGM battery technology.

David
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Old 28-10-2023, 17:30   #4
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Re: Old alternator with new AGM?

Quote:
Originally Posted by massnspace View Post
My question is not how fast it will charge, it is if such an old alternator will work (ie not damage) new AGM battery technology.

David
Of course it will work, (assuming the regulator is operative within spec,) but with its internal regulation it will take forever and a day to charge-up a depleted 8D.
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Old 28-10-2023, 17:32   #5
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Re: Old alternator with new AGM?

It alternator will be fine. If it charged that size battery before will now.

The better question is whether it will properly charge the new battery. What voltage and for how long does the alternator put out?
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Old 28-10-2023, 17:47   #6
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Re: Old alternator with new AGM?

Are you at a dock plugged in or mooring only?? AGM batteries need to be brought to 100% after depleted or you will quickly kill them.
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Old 28-10-2023, 18:30   #7
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Re: Old alternator with new AGM?

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Are you at a dock plugged in or mooring only?? AGM batteries need to be brought to 100% after depleted or you will quickly kill them.
A combination.

What is your definition of “depleted”? Are you saying that drawing them down to say, 60%, up to 80%, down to 50%, back up to 85%, etc. will damage them???
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Old 28-10-2023, 22:19   #8
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Re: Old alternator with new AGM?

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A combination.

What is your definition of “depleted”? Are you saying that drawing them down to say, 60%, up to 80%, down to 50%, back up to 85%, etc. will damage them???
Yes……. They love 100%. It is what I e been told. Ok for every now and then but as a habit not good. Friend of mine replaces her agm batteries every 2 years because she is on a mooring and never gets them up…… premature death
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Old 29-10-2023, 04:41   #9
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Re: Old alternator with new AGM?

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Are you saying that drawing them down to say, 60%, up to 80%, down to 50%, back up to 85%, etc. will damage them???
Reading suggests doing that -- without periodic recharges to 100% inserted in there -- can eventually damage AGMs. Look for threads about AGMs and "partial state of charge" or PSOC.

If you are more often at marinas with shore power and a battery charger, maybe less of an issue, maybe even no issue at all.

Why have you thought to consider AGMs? What issue might they address?

An 8D and a G27 in the same battery bank is unusual. Common rule of thumb has been batteries in a bank usually do best if all are same brand/size/type/chemistry/age/etc.

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Old 29-10-2023, 05:04   #10
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Re: Old alternator with new AGM?

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Originally Posted by massnspace View Post
A combination.

What is your definition of “depleted”? Are you saying that drawing them down to say, 60%, up to 80%, down to 50%, back up to 85%, etc. will damage them???
Don't get over worried. All they mean is that the AGM batteries not recharged to 100% every day wouldn't last as long as the ones that do. It really has little to do with your alternator question because it would take 5 hours at least for it to fully charge to 100% (assuming it regulated correctly). But if you are at the dock with shore power running the battery charger it still will take the same 5 hours, but it provably will do it (depends on the charger).

Batteries are consumable components on a boat. You use them, kill them, and replace them. They seem expense when you go to get new ones, but nothing on the boat is as cheap when measured based on the use it gives.
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Old 29-10-2023, 06:25   #11
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Re: Old alternator with new AGM?

The best thing you can do if your boat spends much time on a mooring is to install some solar. My boat lives on a mooring all season and I have 50 watts with a PWM controller. Batteries are always floating at 13.5 volts when I get onboard to go sailing. I get 4-5 years out of basic flooded batteries. They could go longer but I change then at that point just because it's cheap insurance.
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Old 29-10-2023, 06:50   #12
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Re: Old alternator with new AGM?

Yes solar is really the only practical way to get a batteries fully charged if not on SP IMO
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Old 29-10-2023, 07:22   #13
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Re: Old alternator with new AGM?

AGM are expensive and allegedly intolerant of partial SOC (i.e, being below 100%). They have two strong positive characteristics.
First, they can deliver very high power for their size. But given your charging options, I suspect you are not running high power loads.
Second, and often useful, they can take very high charge rates. I have 270A of alternators, feeding an 800Ah AGM battery bank. I can easily feed full available alternator power (it quickly drops to about 150A as things warm up) until about 85% , and still put 40A in when at 95%. Flooded batteries would not take that much charge current. With your alternator though, even flooded batteries will take all you have the ability to give.
In your use case, you get none of the benefits of AGM, and you get all of the downside. You would be much better served with cheaper and more durable flooded batteries.
Of course, with flooded batteries you do need to monitor your water. At low charge rates that is a fairly insignificant problem.
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Old 29-10-2023, 09:37   #14
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Re: Old alternator with new AGM?

If your 40 year old alternator has in internal voltage regulator it will not charge your AGMs properly.

Internal regulators are designed to put the starting load back into a battery and then provide the power that a car uses while running. They are not for boats where the battery is being drawn down when the engine is not running. Each battery type has it's own charging regimen. Internal regulators are not capable of doing this. Also, a so called 50 amp OEM alternator only outputs 50 amps for a short interval, they are not up to the task of bulk charging a large battery bank.

So my answer to this question is yes, you need a different alternator and also need an external voltage regulator that can be set to your battery type.
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Old 29-10-2023, 10:53   #15
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Re: Old alternator with new AGM?

The alternator will be fine with the new battery. Spend the money on a small solar panel to maintain the batteries when the boat is sitting around.
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