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Old 07-29-2021, 10:20 AM   #1
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Multiple charge inputs to batteries . . .

Back when I was a kid, a teacher told me one time that there is no such thing as a stupid question. Well, I didn't believe that then, and don't believe it now. So here goes what I think is probably a stupid question.


When I run my engines in the slip, I am charging the batteries from both the onboard battery charger, connected to shore power, and also from the alternators of the engines.


Does it hurt to do this? Should I disconnect the battery charger when I run the engines?


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Old 07-29-2021, 10:23 AM   #2
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I have no idea.

I do the same thing.

Until I notice I'm doing it.

Then I cutoff the battery charger.
Hasn't made a difference in 27 years.
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Old 07-29-2021, 10:31 AM   #3
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Multiple chargers typically co-exist just fine. The charger on shore power will see the battery voltage rise due to the alternator and throttle back and vice versa. The charging source with the more "aggressive" algorithm will supply most of the current.
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Old 07-29-2021, 10:36 AM   #4
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Originally Posted by gsholz View Post
Multiple chargers typically co-exist just fine. The charger on shore power will see the battery voltage rise due to the alternator and throttle back and vice versa. The charging source with the more "aggressive" algorithm will supply most of the current.


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Old 07-29-2021, 10:50 AM   #5
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See? Not a stoopid question!
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Old 07-29-2021, 11:18 AM   #6
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See? Not a stoopid question!

Oh, I think it is within the realm of possibility to get good answers to a stupid question.


Anyway, thanks guys. That is good info. I'm not going to worry about it any more then (plenty of other things to worry about these days without obsessing on that).
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Old 07-30-2021, 05:39 AM   #7
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It wont hurt a thing , BUT if you are thinking it might speed up charging a flat batt it wont.

The chargers and alts only measure voltage , and add what they think will be needed to get fully charged.

Split the batts and charge some with shore power and some with the alt , and they wont be fooled by each others voltage.

Many folks think operating a diesel with no load is much ungood for the engine.
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Old 07-30-2021, 10:00 AM   #8
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Many folks think operating a diesel with no load is much ungood for the engine.

I think that also. I always put it in gear, once the engines have warmed up for about a minute, and let it run at about a thousand RPM until the instruments show that both engines are running at normal operating temperature. Then throttle it back down to idle, take it out of gear, and let it idle for a couple of minutes before shutting them off. That sound OK???


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Old 07-31-2021, 05:40 PM   #9
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It wont hurt a thing , BUT if you are thinking it might speed up charging a flat batt it wont.

The chargers and alts only measure voltage , and add what they think will be needed to get fully charged.
I am not expert, but it seems to me that the voltage each charger will see is the voltage of the partially charged battery, not the voltage of the other charger. If the battery is sufficiently discharged and of sufficient capacity that neither charger is capable of suppling more amps than the battery can absorb at the chargers' algorithmic voltage, then both batteries will be supplying some amperage to the battery, and in combination more than either charger alone could provide. That theoretical result is backed up by real-world experience. My house bank charges faster with both of my chargers on. Of course, if the chargers are quite large by comparison to the battery bank and its state of discharge, then I suspect the second charger will not increase the total amps delivered to the batteries.
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Old 07-31-2021, 06:29 PM   #10
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Originally Posted by Kuncicky View Post
Oh, I think it is within the realm of possibility to get good answers to a stupid question.


Anyway, thanks guys. That is good info. I'm not going to worry about it any more then (plenty of other things to worry about these days without obsessing on that).
.
Bill
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Old 07-31-2021, 07:09 PM   #11
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I am not expert, but it seems to me that the voltage each charger will see is the voltage of the partially charged battery, not the voltage of the other charger. If the battery is sufficiently discharged and of sufficient capacity that neither charger is capable of suppling more amps than the battery can absorb at the chargers' algorithmic voltage, then both batteries will be supplying some amperage to the battery, and in combination more than either charger alone could provide. That theoretical result is backed up by real-world experience. My house bank charges faster with both of my chargers on. Of course, if the chargers are quite large by comparison to the battery bank and its state of discharge, then I suspect the second charger will not increase the total amps delivered to the batteries.
This is how I would think it should work but Iím no expert. I have a 675Ah battery bank with a 60A shore charger and a 150A alternator. When I charge the bank with the engine it will easily push out 120A and charge fairly quickly. If I was to add a second 50A shore charger to the system and charge, the bank should accept the 110A until the voltage comes up and then eventually having the second one wonít make any difference but the initial bulk charge should be faster with the two shore chargers connected in parallel?? Iím not sure this is actually correct but would love if the hive mind could chime in, thanks

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Old 07-31-2021, 07:42 PM   #12
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My previous boat had a 240v washdown pump so the genset ran during anchor retrieval, and the charger ran. Plus engines running so alternators operating. And we had sun,so solar too. Didn`t seem to be a problem.
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Old 07-31-2021, 07:56 PM   #13
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This is how I would think it should work but Iím no expert. I have a 675Ah battery bank with a 60A shore charger and a 150A alternator. When I charge the bank with the engine it will easily push out 120A and charge fairly quickly. If I was to add a second 50A shore charger to the system and charge, the bank should accept the 110A until the voltage comes up and then eventually having the second one wonít make any difference but the initial bulk charge should be faster with the two shore chargers connected in parallel?? Iím not sure this is actually correct but would love if the hive mind could chime in, thanks



Rob
That's basically correct, I think. I'm my simple view there is a battery acceptance rate that often limits charging - I think that was FF's point - but if one charger is delivering below that acceptance rate then adding a second charger should increase the charge rate.

I think we said the same thing :-)
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Old 07-31-2021, 08:45 PM   #14
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I have twin alternators, a generator and solar. Once the charge rate tapers at the float mode, whichever source puts out the highest voltage, seems to win the tug-of-war for the load. It's not uncommon on my boat to see one alternator-driven tachometer indication drop off because the other alternator is carrying the float. Adding a house load brings the resting alternator back on line.

Other than that, I never see a problem with competing charge sources.
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Old 07-31-2021, 10:28 PM   #15
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I am not expert, but it seems to me that the voltage each charger will see is the voltage of the partially charged battery, not the voltage of the other charger. If the battery is sufficiently discharged and of sufficient capacity that neither charger is capable of suppling more amps than the battery can absorb at the chargers' algorithmic voltage, then both batteries will be supplying some amperage to the battery, and in combination more than either charger alone could provide. That theoretical result is backed up by real-world experience. My house bank charges faster with both of my chargers on. Of course, if the chargers are quite large by comparison to the battery bank and its state of discharge, then I suspect the second charger will not increase the total amps delivered to the batteries.
Correct

When in bulk mode the charger(s) will each output full current they are capable of producing.
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Old 08-01-2021, 07:32 PM   #16
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Observation:

I have CAT 3208TA. I have green lights on my dash for each engine. I have to raise the RPMs to 1200 to bring the alternators on line.

When I am at a moorage and the shore power has fully charged the batteries and I run the engines with the alternators running. They shut down with full batteries.
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