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Old 10-26-2021, 10:59 AM   #1
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Power Management

I'm still learning about my new boat but I have power management issues. The boat was built with a 24vdc system owing to the type of engine, Scania.
The builder provided me with 2 x 8D batteries for starting and 2 x 31 series for the house. He also provided me with one 60 amp, Minn Kota battery charger with four banks. One bank of the charger is currently connected to each of the batteries therefore each battery is receiving a 15 amp charge.

The engine start batteries have not been an issue but both are by DEKA, model 908DFT and are rated at 1425 CCA. I am trying to determine what the max charge to these 8D's is. The 30 amps that the charger is supplying seems to be adequate, however, I am thinking about installing a second charger so that I have one for the house bank and one for the engine bank.

The house batteries have amp hour ratings of 100 and 105; they are not from the same manufacturer.

According to Minn Kota the maximum charge to the group 31 batteries should be 0.1C or 10% of the Ah rating as they are lead acid type; therefore 10 amps? But I'm currently charging with 15 amps. Some other literature indicates that the max charging for most group 31 batteries is 20 amps.

The problem: while running on generator at night, with the typical stuff that I want to have on, I am consuming 30 amps. The charge on my batteries is running down as the charger/batteries cannot keep up with the demand.

It seems to me that the solution to this problem is either the addition of more house batteries or switching from the group 31's to something larger; either 4D or 8D and perhaps installing another charger. I do have the room to add more or larger batteries.

I'm looking for some help to calculate the proper size and number of batteries to have on board for a 30 amp load over 12 hours; I think that would be the max. I also need to determine the appropriate size charger for the battery configuration.

I also want to understand how to properly use the Blue Seas Automatic Charging Relay that is installed. I have a three position rocker switch with off, auto and crossover positions. I understand that the crossover connects both banks of batteries. I believe that this relay controls the charging from the alternator to the batteries but have not been sure if it is charging both the house and the engine batteries when in the auto mode or only one bank. Is the crossover position meant to be used to charge both banks of batteries or to draw from both banks?

I traced out the wiring and found the following:
- the positive terminal of the house batteries runs into the cutoff switch and then to the ACR. [ +batt----switch----ACR]
-the positive terminal from the engine batteries runs into the ACR in one direction and to the cutoff switch in the other direction.
[ switch----+batt----ACR]

What is the best reference book for DC electrical systems?

thank you in advance for the assist
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Old 10-26-2021, 11:59 AM   #2
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A lot of questions there...

First, if you are consuming 30 amps from the house bank, and the charger (with the generator running) can only supply 15, then you are exceeding the amount of DC current available to the house bank and the batteries will run down whether you are on shore power or on generator.

Your only solution is a higher capacity charger for the house bank. I would not worry about the max charge current of the house bank, they are basically self limiting. You should get a 45-60 amp 'intelligent' charger that will be able to supply enough amps to charge, and also to provide power for the 30 amps of loads you want to run on top of that. Then you can run on the generator or shore power with those loads on indefinitely if desired.

Two group 31 batteries is only 200 amps total, and 100 amps (50%) available in normal usage, and that's rated at '20 hours'. Your 20 hour discharge rate for that size bank is 10 amps. Your 30 amp load will only go 3.3 hours theoretically but it will be less than that due to the draw being 3x higher than the 20 hour rating, you get less power out of a battery when you draw at higher rates. To go 12 hours at 30 amps you need 720 amps of storage (lead acid), so 7 identical 12V deep cycle group 31s would do the job. Or, 8 x GC-2 6V wired series/parallel would also work. I'm not aware of any truly deep cycle 8D or 4D batteries, they are all 'combo' batteries.

I don't like ACRs, they are a terrible way to get batteries charged. Alternators are already poor at charging a house bank, throwing them in parallel with your start battery makes it even worse. I'd remove it and install a DC-DC charger to charge a second bank. If you want good charging of a house bank from an alternator, that's a whole other thread that's been discussed here many times.
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Old 10-26-2021, 12:05 PM   #3
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I missed that you have a 24V system.

So to sizing, you'd need more batteries to get things to 24V, and since you quoted your load in amps, the amp rating requirements stay the same.

So to do a 24V, 30A load for 12 hours you'd need 14 12V deep cycle batteries in series/parallel (24V @ ~700 Ah). It would be 16 GC-2 (24V @ ~800 Ah) wired in series/parallel.
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Old 10-26-2021, 12:31 PM   #4
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thanks for the reply.

My initial thought was to use the existing 4 bank, 60 amp charger for just the house batteries but then I saw the quoted limitations on max charging. Your comment makes sense since most of the chargers are 'smart' like the one I have and stop charging when a battery is fully charged.

No one seemed to know if it was possible or prudent to connect two of the charger banks to one battery. I did find that answer in the MK manual. They say that you can. So, in theory I can connect the four charger banks to the existing two batteries, which equates to 30 amps per battery. Since I have room for two more group 31 batteries I could add those to the house bank and continue using the MK charger to get 60 amps across four batteries. Then I would add another charger for the engine batteries. I suppose a 50 amp charger would be more than adequate since they are getting 30 amps off of the MK now?

Your solution would call for hauling around a lot of batteries. I use this boat for a lot of overnight fishing so that one of my big draws are the LED flood lights. I realized that each of these floods draws about 7.5 amps so that accounts for 15 amps. I suppose if I limit my use of these floods to when I'm actually fighting or landing a fish that would ease my problem. I'll have to also look into other ways to power them.

I also have a sub-panel for 12vdc because you can't get everything in 24 vdc. The builder installed an inverter to power the 12 vdc panel from 24 vdc supply.
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Old 10-26-2021, 12:33 PM   #5
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How is the ACR working? Is it automatically charging either the house or the engine batteries off of the alternator or both? How should that crossover switch be used?
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Old 10-26-2021, 12:44 PM   #6
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My favorite book for boat electrical stuff is Boatowner's Mechanical and Electrical Manual by Nigel Calder.


You need a much bigger battery bank and a much bigger charger, sbman nailed it.



sbman, What don't you like about ACRs? I think they are great and replaced my battery isolator with one last Winter. My little 51 amp OEM CAT alternators do a great job of charging my house batteries now, with the ACR; they really struggled with the isolator due to the .7V drop.
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Old 10-26-2021, 12:49 PM   #7
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Originally Posted by stow it View Post
thanks for the reply.

My initial thought was to use the existing 4 bank, 60 amp charger for just the house batteries but then I saw the quoted limitations on max charging. Your comment makes sense since most of the chargers are 'smart' like the one I have and stop charging when a battery is fully charged.

No one seemed to know if it was possible or prudent to connect two of the charger banks to one battery. I did find that answer in the MK manual. They say that you can. So, in theory I can connect the four charger banks to the existing two batteries, which equates to 30 amps per battery. Since I have room for two more group 31 batteries I could add those to the house bank and continue using the MK charger to get 60 amps across four batteries. Then I would add another charger for the engine batteries. I suppose a 50 amp charger would be more than adequate since they are getting 30 amps off of the MK now?

Your solution would call for hauling around a lot of batteries. I use this boat for a lot of overnight fishing so that one of my big draws are the LED flood lights. I realized that each of these floods draws about 7.5 amps so that accounts for 15 amps. I suppose if I limit my use of these floods to when I'm actually fighting or landing a fish that would ease my problem. I'll have to also look into other ways to power them.

I also have a sub-panel for 12vdc because you can't get everything in 24 vdc. The builder installed an inverter to power the 12 vdc panel from 24 vdc supply.
Yes, 24volts @ 30 amps is 720 watts. You want to do that for 12 hours, that's 8,640WHr of power, nearly 9000 watts. You need a bank double that size in lead acid to handle it per the typical 50% discharge rule with lead acid batteries and that's a lot of batteries, there isn't really any other solution. Reduce load, reduce time, or find another power source that isn't a battery.

The biggest thing is to get more amps from your charger however you do it (new charger or reconfiguration of the existing one) so it exceeds your loads, that way you can always run as long as you want with the generator powering things.

ACR's 'join' when charging voltage is available, usually greater than 13 volts or so. There are multiple models of ACR from blue seas with different specs and abilities, you'd have to look up the manual for your specific model.

I still would not use an ACR except in a very narrow use cases that rarely applies to boats but many people seem to install them even though their use case does not fit their abilities.

Are you fishing overnight with your main engine shut down?
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Old 10-26-2021, 12:56 PM   #8
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Originally Posted by Mischief Managed View Post
My favorite book for boat electrical stuff is Boatowner's Mechanical and Electrical Manual by Nigel Calder.


You need a much bigger battery bank and a much bigger charger, sbman nailed it.



sbman, What don't you like about ACRs? I think they are great and replaced my battery isolator with one last Winter. My little 51 amp OEM CAT alternators do a great job of charging my house batteries now, with the ACR; they really struggled with the isolator due to the .7V drop.
ACRs are better than diode isolators for sure. Alternators already struggle to charge a house bank, they really are not designed to do that out of the box, are configured to provide high initial charge rates and then taper off after a few minutes to fit the typical profile of a start battery.

When you combine two banks with an ACR, the banks are of different capacities, different levels of charge, and different states of health. Paralleling them does provide charging current, but it's not a proper multi stage charge. If it's just an occasional thing it's not a big deal, but if the alternator is a primary method of charging it is far from ideal.

It all depends on your use case. If your usage is mainly at docks and you just want power to your house bank underway an ACR will be OK. The house bank will get a good charge when you reach the next dock. If your usage is anchoring out often and wanting your alternator to charge your house bank, that's not a great use case.
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Old 10-26-2021, 02:04 PM   #9
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Quote:
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ACRs are better than diode isolators for sure. Alternators already struggle to charge a house bank, they really are not designed to do that out of the box, are configured to provide high initial charge rates and then taper off after a few minutes to fit the typical profile of a start battery.

When you combine two banks with an ACR, the banks are of different capacities, different levels of charge, and different states of health. Paralleling them does provide charging current, but it's not a proper multi stage charge. If it's just an occasional thing it's not a big deal, but if the alternator is a primary method of charging it is far from ideal.

It all depends on your use case. If your usage is mainly at docks and you just want power to your house bank underway an ACR will be OK. The house bank will get a good charge when you reach the next dock. If your usage is anchoring out often and wanting your alternator to charge your house bank, that's not a great use case.

I understand your reservations now. For some reason, my alternators are both set to only 13.8ish volts output and are not adjustable. By the time the voltage reaches my house bank it's down to 13.7 and that seems harmless to my battery banks. The low voltage also limits the charge current to about 70 amps max, despite having two "51 amp" alternators. It's certainly not enough to bring the 615AH house bank back from 50% SOC in a couple of hours, but we usually have better than 65% SOC after a night on the hook and a 4 or 5 hour trip the next morning brings the batteries to or near 100% SOC. If we need extra charging after moving from anchorage to anchorage, we just use the genset and bring the batteries up to 90% SOC or better before bed time.
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Old 10-26-2021, 02:16 PM   #10
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I understand your reservations now. For some reason, my alternators are both set to only 13.8ish volts output and are not adjustable. By the time the voltage reaches my house bank it's down to 13.7 and that seems harmless to my battery banks. The low voltage also limits the charge current to about 70 amps max, despite having two "51 amp" alternators. It's certainly not enough to bring the 615AH house bank back from 50% SOC in a couple of hours, but we usually have better than 65% SOC after a night on the hook and a 4 or 5 hour trip the next morning brings the batteries to or near 100% SOC. If we need extra charging after moving from anchorage to anchorage, we just use the genset and bring the batteries up to 90% SOC or better before bed time.
Yes that is typical for a stock alternator. When wired up to a start battery it prevents having to worry about maintenance of the battery (no one wants to add water to a battery... or worry about the particular chemistry used or the optimal charge profile) and so what if they only last a 2-4 years due to sulfation from never getting a full charge, no one will even notice the loss of capacity until it just won't crank the starter anymore.

For a large expensive house bank those compromises aren't great and a robust charge algorithm should be applied as often as possible. Occasionally used to keep the house bank up while motoring, that's fine. If you installed a couple of DC-DC chargers, you'd likely get your house bank up to 100% SOC after a few hours of motoring with your existing alternators but they run $300-$500 each so it's quite a bit more expensive than an automatic relay.
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Old 10-26-2021, 03:23 PM   #11
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I'm not fully bought in to this "isolators are bad" story. They have zero arcing, zero moving parts, unlike most ACR's. i use them with remote alternator sense voltage wires, and all is well. At high loads, the alt outputs clears 15.xV at the B + term. but the house Li bank is at 14.3V. Yes, there is some efficiency loss, sometimes warming the heatsink a bit.
In addition, they offer automatic, redundant charging on either, or both engines. My last cruise I had to run with one alternator, and it accommodated everything the solar could not. With no switching/rewiring.
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Old 10-26-2021, 03:43 PM   #12
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I'm not fully bought in to this "isolators are bad" story. They have zero arcing, zero moving parts, unlike most ACR's. i use them with remote alternator sense voltage wires, and all is well. At high loads, the alt outputs clears 15.xV at the B + term. but the house Li bank is at 14.3V. Yes, there is some efficiency loss, sometimes warming the heatsink a bit.
In addition, they offer automatic, redundant charging on either, or both engines. My last cruise I had to run with one alternator, and it accommodated everything the solar could not. With no switching/rewiring.
That's not a bad setup, using the remote sense to overcome the diodes voltage drop but you don't get that with a stock alternator, you'd need an externally regulated alternator with a remote sense option. With standard alternators and stock regulators, you'll never get a full charge through a diode isolator.
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Old 10-26-2021, 04:17 PM   #13
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Power Management

Ageed, that a common one wire alternator won’t allow remote sense.
More of an issue on boats with longer wiring runs. Fortunately, my Volvo furnished stock alternators have a 5 wire connection, with the option to do local or remote V sensing.

It also allows a bit of a trick to prevent starter actuation when the engine is already running.
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Old 10-27-2021, 05:23 AM   #14
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"The builder provided me with 2 x 8D batteries for starting and 2 x 31 series for the house."

Sounds to me the boat assembler got it backwards.

2 gp 31 make a great engine starting set and a pair of 8D is a good house setup.

What are the DC loads that require 30A of 24V ?
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Old 10-27-2021, 08:12 AM   #15
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thanks for all of the replies
I think we have a workable plan going forward:
1. either add two more group 31 batts in parallel or replace the existing 31's with four x 6 volt golf cart batteries.
2. replace the 60 amp, 4 bank, 12v charger with a 50 amp, 24v charger
3. either use the existing 4 bank x 60 amp charger to continue charging the engine batts or
4. install a Victron Energy Skylla-TG 24-Volt 50 amp Battery Charger, 1+1 Isolated Outputs. this Victron has two outputs; one at 50 amps and one at 4 amps that can be used to keep the engine batts topped off
5. While overnighting on the fishing grounds I am normally running the 5K genset so I don't need enough battery capacity as a stand alone.
6. four group 31's would give me about 200 Ah at 24vdc while four 6vdc golf cart batteries should give me about 225 Ah.
7. charging either option of batteries with the 24v 50 amp charger should keep up with my 30 amp demand
8. it has been recommended to me to use a C factor of .25 versus .10 for max charging of lead acid batts so that with either option of batts I should be safe.
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Old 10-27-2021, 08:41 AM   #16
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Have you considered AC powered deck lights? Take the batteries out of the equation.
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Old 10-27-2021, 08:47 AM   #17
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"The builder provided me with 2 x 8D batteries for starting and 2 x 31 series for the house."

Sounds to me the boat assembler got it backwards.

2 gp 31 make a great engine starting set and a pair of 8D is a good house setup.
That was my thought as well. A group 31 started a Perkins 6-354 easily on a prior boat.

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Old 10-27-2021, 09:04 AM   #18
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Already spent the money on the LED flood lights and they won't run on 110vac
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Old 10-27-2021, 09:12 AM   #19
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The OP and I have been communicating privately. His engine is a Scania 13 liter with 24V alternator and starter. And most house DC loads are 24V. So he needs pretty big batteries to start that beast of an engine, thus the two 8Ds in series.

His routine house 24V DC loads are about 30 amps. He runs the generator all night when he is fishing under the lights which represent most of his DC loads. A 50A, 24V Victron charger will keep up with those loads while his generator is running and provide some slack for battery recharging. The one he is looking at has a separate 4A output that is isolated and can keep his starting batteries charged.

He is going to double his house battery bank to keep it within my recommended 0.25C so he doesn't cook his batteries when there is no significant house load, only battery charging.

That is the basis of his proposed solution.

David
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Old 10-27-2021, 09:12 AM   #20
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I would check to see if the gp 31 are start batts or deep cycle.

Most gp 31 but not all are used for starting , and starts deep cycled have a very short service life.

I would use the gp 31 as starts as they are lighter , less lead , and work very well as starts.
Most large engine trucks only need a pair of gp 31 to operate in winter cold.

I would keep your existing charger and charge the gp 31 with it , and purchase a larger 24 v charger for the house set.

If you are using 30a of 24v while charging a larger 75a charge might be needed to feed the house and have enough voltage left to charge the house at the same time..

8D deep cycle make good house batts, tho a PIA to lift .8D starts do not do well as house batts.
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