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Old 13-04-2009, 10:10   #1
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Charging Dead Batteries

My boat has a Yanmar 3QM30, the original alternator fails so I put a new, more powerful, one, but the shop that sell it make a point in emphasis that an alternator will not charge death batteries (that was my situation, my 450 AH bank was discharged by the autopilot, to the extend that the density measure donít go up to red!!!!).

It was true, it never charges that bank, Iím wondering if I put 3 diodes in series with the ground connection of the regulator and a switch to shortcutting them ( with 3 diodes bypassed it will be as original, with one diode between regulator and ground the voltage will rise 0.7 V, with 2 1.5 etc.).

I expect that with this scheme to increase voltage I can charge death batteries, just looking not to obtain excessive current from the alternator.


Any comments, will be too risky???
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Old 13-04-2009, 10:23   #2
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Any comments, will be too risky???
I agree with the above and suggest you change the batteries before they truly become death batteries
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Old 13-04-2009, 11:46   #3
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Replace them. Once you damage the batteries to that extent, they will never function as well as they are supposed to function. Diodes make it so that current flows in only one direction. This has nothing to do with battery damage. No arrangement of diodes is going to fix a damaged battery or make it work better.
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Old 13-04-2009, 12:58   #4
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I think you need to consider the above plus consider having someone help you review the entire electrical system. You really won't want to be replacing batteries all the time if something about your system is killing them. I wouldn't connect any new batteries until you can be sure the charging system is working properly and not the cause of you death batteries. A dead alternator seems like it might be a sign of something else. Charging power needs to be regulated for optimal performance and long battery life. The cost of doing this correctly is quickly recovered in longer battery life and adds the joy of trouble free power.
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Old 13-04-2009, 15:32   #5
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It sounds to me like you don't have enough voltage to excite the alternator or regulate the charger. If you don't have a shore power charger to get your batteries charged then you could try connecting the regulator to the start battery (if you have one) and see if that will get the charger going. Once you have a bit of charge in the house bank you could shut down, reconnect the regulator to the house bank and then continue the charge. After charging the house bank you can test it to see if it needs replacing. You may be able to get a few more years out of the batteries yet, depending on their age and condition.
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Old 21-04-2009, 18:00   #6
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First I would be sure the alternator is working. Be sure the voltage rises when the alternator is charging.
One of the home built manual charge regulators I have on my boat, is 3 diodes, with switches to choose none, 1, 2 or 3 diodes, in the sense wire from the alternator. With very low voltage in the batteries, it shouldn't make a difference, as the regulator already sees low voltage and puts out more charge. They come in handy when you want to charge to a higher voltage than the regulator is designed for.
The other system I made was just a reostat in the field wire. I used this system almost all the time.
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Old 22-06-2009, 06:14   #7
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Thanks !!!

About the diodes, I suppose that the need to manage the full field current, so I like to know what can be the expected field current of a 80 A alternator....
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Old 22-06-2009, 06:31   #8
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Thanks !!!

About the diodes, I suppose that the need to manage the full field current, so I like to know what can be the expected field current of a 80 A alternator....
About 4 - 5 Amps (maybe a little less) @ full field.
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Old 22-06-2009, 08:32   #9
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Originally Posted by Milton Bertin View Post
My boat has a Yanmar 3QM30, the original alternator fails so I put a new, more powerful, one, but the shop that sell it make a point in emphasis that an alternator will not charge death batteries (that was my situation, my 450 AH bank was discharged by the autopilot, to the extend that the density measure donít go up to red!!!!).

It was true, it never charges that bank, Iím wondering if I put 3 diodes in series with the ground connection of the regulator and a switch to shortcutting them ( with 3 diodes bypassed it will be as original, with one diode between regulator and ground the voltage will rise 0.7 V, with 2 1.5 etc.).

I expect that with this scheme to increase voltage I can charge death batteries, just looking not to obtain excessive current from the alternator.


Any comments, will be too risky???

What kind of batteries are discharged? Are they flooded, gel, AGM?
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Old 24-06-2009, 00:22   #10
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I find that sometimes the smart chargers will not charge a battery which is severely discharged.The same goes for an alternator charge. I have brought AGMs back to life by charging them with an automotive "dumb" charger until they are close to 12 volts, charging with the marine charger and then equaliziing them
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Old 25-06-2009, 02:45   #11
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Place a healthy battery in parallel with your dead ones.This will give enough voltage to excite the alternator. Once you have charged for a while, you will be able to remove the healthy battery and there should be enough charge in the old batteries to get the alternator charging. Beware sparks around gassing batteries.
Regards, Richard.
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