3 battery isolator

Kamahele

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My boat has a Volvo Penta MD2020 inboard engine. It has one 12v alternator (60A Max) with a 2 battery isolator (see attachment) charging 2 battery banks (starter + house). I want to replace it with a new 3 battery isolator so I will have 3 battery banks (starter + 2 house circuits). Are there things I should consider before installing a 3 batt isolator for a new house circuit? (all circuits are 12v)
Need to know this soon so I can buy the isolator before heading off to the far-away place where the boat is. Thanks!

Volvo 2 batt isolator.jpg
 

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alldodge

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Not the best method, but all it will take is installing a 3 bank isolator and add the new lead to the 3rd bat. Should install another switch

Three bank w isolator & no combiner.jpg
 

sam am I

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Old school trick .........Add a comp diode (see Dodge's edited drawing below w add-in comp diode) which will raise the alt's output by about 0.5V (14.4V + 0.5V = 14.9V) making up for the drop the iso has to each battery........It won't track 100%, 100% of the time, but......

For example, when the Iso's diode's Vf = 0.7V @ 30A, the comp diode's Vf = 0.5V over temp/current, the difference is 0.2V BUT, when the batt's charge (highest of the three) the current decreases..........As such, the batt's charge is topping off, at this point then Iso diode's approach Vf = 0.5V @ 1A for example. The comp diode's remains Vf = 0.5V as before however, the difference now becomes 0.0V'ish and the batt's top off (14.9V - 0.5V = 14.4V)....works quite well.

To help track temp related Vf's change somewhat, you can also mount the comp diode on the heat sink fins. (this technique is typically used in old school audio amps where the high current bipolar driver xsters need to track the current mirror used to bias them)
image_274125.jpg
 
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sam am I

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citcelsyD....errrr, Dyslectic diode test. Backwards above, apologies all, anode goes to batt+ ;)

image_274125.jpg
 

Kamahele

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Hi AllDodge, Thanks for the quick reply. Nice diagram that I'll take a closer look at tomorrow. But is the"3-Bank Charger" the AC shore power charger?
I assume that the red switch in your diagram is to enable House 1 batt to start the engine if the starter batt is flat. My boat doesn't have such a switch. Only has individual switches to turn off/on the existing starter and house batts.
 

Kamahele

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Hi Sam am I, Whoa, I think your diode trick is above my pay grade. I'll need a lot more help to pull it off. What size and type of diode are you talking about?
 

sam am I

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There's no real current in the sense path, milliamps, so small sig diodes are fine then......

1N4148, 1N914 would work fine as the Vf spec for silicon type of Iso's = 0.5V min. If your Iso is a "schottky" type (I see a few out there), you can use 1N5817, 1N5818, 1N5819
 
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alldodge

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Hi AllDodge, Thanks for the quick reply. Nice diagram that I'll take a closer look at tomorrow. But is the"3-Bank Charger" the AC shore power charger?
I assume that the red switch in your diagram is to enable House 1 batt to start the engine if the starter batt is flat. My boat doesn't have such a switch. Only has individual switches to turn off/on the existing starter and house batts.

The changer is for shore power, IF it is installed.

Not knowing what you have but most have the typical 1, 2 ALL and OFF switch. So yours would be more like this other one. I would suggest using automatic charge relays (ACR) over an isolator, less issues. I just removed my 3 bank isolator and installed an ACR

Three Bat Isolator 2 sw.jpg

Mine like this. When the motor is running the ACR charges the start and house Bat. When connected to shore power all bats are charged. The relay between start and house Bat is controlled by a push button at the helm

Three bank with DVSR.JPG
 

Kamahele

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Would a 70 amp, 1 alternator, 3 battery isolator do the trick? The isolator now is rated at 70 amps - the alternator 60 amps max.
 

Kamahele

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Further to my post: The third battery will be small, to power an LED lighting circuit. Currently the lights flicker too much when other things, like the water pump, are running. I want the lights on a completely separate circuit.
 

alldodge

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The isolator will work, but would be better if it was a bit higher rated.

lights flicker too much when other things, like the water pump, are running.

Instead of putting in another bank to power the lights I would run heavier gauge wire up to buss bars or separate wires feeding them. Your getting voltage drop
 

Kamahele

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AllDodge: I thought of that, but the wire to the water pump (the real culprit here) is pretty heavy already. The (incandescent) lights always flickered, but it go worse when I upgraded to LED bulbs.
 

alldodge

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No I'm thinking of a heavier wire feeding the lights and accessories.

The (incandescent) lights always flickered, but it go worse when I upgraded to LED bulbs.

From your statement you may have some corrosion issues going on because the lights should flicker less because they draw less current. I guess it could also be the pump is just putting to much noise out on the line.
 

Kamahele

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Are there any DVSR's designed for 3 batt systems? Or am I thinking the wrong way about this?
 

alldodge

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The DVSR or ACR (same thing for the most part) are one to one. To use this setup you would use 2 of them. That said, if a bat was drained to some extent the ALT would not be able to totally recharge the bat using your current setup. If the motor run long enough then the recharge would complete. This would be the setup to go that route.

Disregard the extra stuff your not using, and also the 2nd DVSR/ACR could go between the house and 2nd house bat

Three bank with Two DVSR.JPG
 

sam am I

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Agreed that you'd need higher capacity, as such, I'd personally go with at least a 120A unit. Doubling the capacity rating in this case is a safer bet.

Also, reviewing my old school notes and noting there are a few ways to skin a cat, .......Method below (sorry no pretty pictures embedded) also allow you to switch in and out each/both/all house and start batts for backup use to start engine in the event your start batt craps out. Also allowing you to shut off all batts entirely to all sources........

3 batt Iso.jpg


Side noting also that a solid state Isolator (Or'ing diode) as compared to the mechanical relay's in ACR's/DVSR's gains you the ability to NOT have to parallel a charged battery with a dis-charged battery in order to charge it/them.

Each battery can draw (charge at will as needed) what it needs, when it needs with no chance of back charging (dis-charging) one batt into another especially during higher current in-rush/alt voltage sag while paralleling/connecting a fully charged batt to a not so charged/dead'ish batt.......Sorta provides you a Duracell analogy of "never mix good battery's with bad battery's" by nature eh.

ACR and DVSR and the like are also are mechanical contacts (relay's) that arc and burn and wear over time (highest arcing/wearing during high inrush currents during initial paralleling), A diode Iso of course is a solid state device and if spec in correctly (using a 120 Amp model w a 60 Amp alt) will typical run 10's and 20's of years if not more.

All just a personal preference I'd have and have had in the past with your type of set up and that has worked very well for me and what I prefer...........Also, you could bump that alt up too, if inclined BUT, the Iso would have to come up as well.
 
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bruceb58

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I would use 2 ACRs/VSRs myself. They will keep a deeply discharged battery from connecting and you don't have to worry about creating a remote sense wire for the alternator.

I have been using the BlueSea ACRs for many years on multiple boats and never had issues with them. Even used them for an RV and horse trailer. If you don't want the ACR to operate, you can add an override switch to keep it from connecting.

https://www.bluesea.com/products/7610/SI-ACR_Automatic_Charging_Relay_-_12_24V_DC_120A
 

sam am I

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Correction on that house batt. switch, meant to say "Off, On, Combine"............Just a standard "dual circuit" batt switch is all where both house batts will be switched on or off at the same time with the addition feature of being able to combine them together if need be.

Although the lower left Perko "Off, 1, Both, 2" switched into the "Both" will combine the two house batts by default. This Perko switch then can cover the situation where some manu's "dual circuit" switch's don't offer the "combine" function.
3 batt Iso.jpg
 
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Kamahele

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Follow on question: I want to install a third 12V battery (about 50AH) that will exclusively power LED bulbs. There are a number of bulbs that are 2.5W (about 0.2A) each, but on a given evening/night the typical total power draw will be about 5AH.
MY QUESTION: If I'm connected to shore power overnight (at least 12 hours) would a 3A charger be sufficient? Would that put 36AH of power into the battery per 12 hours? Also, if I'm moored somewhere with no SP and not running the motor, say for 3 nights, the total power draw on the battery should then be about 15AH, which shouldn't stress the battery. Am I looking at this the right way?
Thanks!
 

Kamahele

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AllDodge, sam am I, bruce58: thanks for the ideas and diagrams, but I think it's all beyond me and I'm thinking of a simple approach. I will make sure a fuse and switch are installed - existing circuits have them already.
 
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