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Old 04-30-2012, 05:18 PM   #1
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Default 2800 scr battery switch

Hi all,
I'm Gary and new to the Forum. After 15 yr. hiatus just got back in the boat business with a "98 2800 SCR, single 260 HP 350 EFI Merc, that we keep on Grand Lake O' the Cherokees in Oklahoma. My question is about the normal operating position of the battery switch. My marina mechanic tells me to always leave it in the "BOTH" position but this is in contradiction to what I have read in this Forum and in the only manual I have ( which is actually for the 2000 model ). If it needs to be on "1" or "2", how do I determine which is the starter battery and which is for "house" and what positions should it be in when hooked to shore power and when underway?
Thanks.
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Old 04-30-2012, 05:46 PM   #2
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never use BOTH...... only in an emergency start.... per the mercruiser manual.

both sends 80amp to the starter with to good batteries and will fry it you crank to much. also there is an 80amp fusible link that holds the juice back from frying your wiring harness.... the 80amp fuse sits on the top post of the battery, and will melt if ran on both before it fails.

using 1 or 2 makes no diffrence, only one is in use at a time, but both are always charged or being charged by the alternator or battery charger.

some guys put a deep cell RV battery for the 2nd slot, for boon docking power, they store more juice but have less top end cranking amps.

a boat battery has thicker plates then a car battery to deal with the rocking motition, if a lead plate comes lose the car battery shorts.
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Old 04-30-2012, 06:08 PM   #3
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hmmm--2nd year on our 1998 2800SCR.

Really like the boat. We run in the "both" position, then in the evening at camp, switch to #2(house)

In the morning return to both for start-up. Guess I'm doing it wrong but have not experienced any problem this way.
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Old 04-30-2012, 06:14 PM   #4
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stick your head upside down, and check to see which one is sitting on the hot post of your starter.....

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Old 04-30-2012, 06:45 PM   #5
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Thanks pascavone. That is the picture of yours I saw yesterday that prompted my question. Might the switch being in "BOTH" position be the reason I can't turn my refer off or is that a whole 'nother topic?
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Old 04-30-2012, 07:01 PM   #6
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Welcome to the site Gary. Unfortunately your mechanic is an....um....is poorly informed.

The first thing you need to understand is this: when the switch is in "Both" what the boat sees is just one battery. One, really large capacity, 12V battery (assuming the boat is wired as typical). When in Both the voltage does not change, nor does the amperage, but the amount of stored power does. Think about this: If battery 1 is fully charged at 12V and can deliver 800 cranking amps for 20 seconds but battery 2 is discharged down to 8V and can now only deliver 400 cranking amps for 10 seconds , then turning the switch to Both will result in one battery with 10V and 600 cranking amps for 15 seconds. Put another way, if B1 is fully charged, and B2 is dead, if you switch to Both then power will flow from B1 to B2 until both are equalized.

If your batteries are identical, say traditional wet cell batteries, and are the same size/rating then either one can be your "starting" battery, but it might depend on how the boat is wired. Sometimes people will wire some of the house loads to only the house battery.

I'd say first thing to do is switch to battery 1 and see if everything on the boat works; drive trim, trim tabs, all lights, all electronics, everything. If it all works on B1 switch to B2 and do the same thing. If the results are the same, and the batts are the same, you can call either one starting or house. Just be consistent. People typically say B1 is the starting battery, B2 is house.

Maybe you have one battery that is of a different type or size, like a gel or AGM that is larger than the other. In that case people tend to use the larger battery as their house battery because they get more run time out of them.

Once you've determined which will be your starting and house batt's, and assuming both are fully charged, the general practice is to use the starting battery to start and run the engine to where ever you're going. Once you've dropped the hook or tied up switch to the house batt. This way if you drain the house batt by using lights/radio/etc then the starting battery is still charged and able to start the engine to get you back to port. Conversely, if you leave it in position 1 and kill the starting battery then you can switch to B2 and start the engine. If you leave it in "Both" you run the risk of draining both the batteries to the point that they no longer have enough juice together to re-start the engine.

As Pas said, Both is kind of an emergency situation, where both batteries are drained to the point that neither will start the engine alone, but when combined (Both) they have just enough to re-start.

You'll also need to determine how your charging system works. Is there an on-board charger that works when you're connected to shore power? Is there a battery isolator/combiner installed? If there is no isolator/combiner installed, with the engine running and the switch on B1 the alternator will charge B1 only and if switched to B2 it'll only charge B2. If, once you have the engine running, you switch to Both the Alt will try to charge both batteries. If both batteries are depleated charging both at the same time puts an enormous load on the Atl and, if the engine is EFI'd, might even cause running problems (the engine computer needs a minimum voltage to run). You can try but if the engine changes note then go back to just one battery.

Lots of variables. Have a look at what you have installed.

Dan
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Old 04-30-2012, 07:05 PM   #7
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Dreamliner, welcome aboard!!! I've got a '97 2800SCR and we love it. Besides problems with both starting and I believe charging with the altenator, just from a safety perspective, I wouldn't want to use BOTH. If something happens and both are drained down, you're in trouble. Live on the house battery and start and run on the starting battery. This way, you can always start and run the boat. Even if I'm on the hook and the house battery is dead, then it's dead. I won't switch to the starting battery. I keep a few small LED lanterns and lights on the boat.

As for the refer, that is a dual volt model. If the switch inside is turned on, it is on. If the breaker for the fridge is shutoff (or the main for the side is turned off, or the shore power is disconnected...) the fridge will automatically switch to 12V mode and continue running off of the battery. You can shut if off by the switch inside. It should be up near the freezer along with a temp dial. If running off of battery, I would turn teh temp down to 2.5-3 to save amp hours on the batt. even at three the freezer starts slowly defrosting, but the fridge itself is alright.
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Old 04-30-2012, 07:08 PM   #8
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It looks like SS beat me to the <somewhat> simultaneous post. ^^ I like his answer better anyway.
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Old 04-30-2012, 07:33 PM   #9
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Let me just say first, you guys are great. I usually hesitate to get involved in forums but you all are the most helpful and friendliest I think I've ever encountered. As for the refer, found the switch but it won't turn off. The green light stays on whether the switch is up and off or down and on, shore power on or off.
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Old 04-30-2012, 08:12 PM   #10
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Welcome to the zoo dreamliner...just curious..what's the name from??.ie dreamliner??..I work on it at boeing....


keep comin here...eventually you'll get sick of us!!!

just kidding.....we tend to grow on people...sorta like moss...


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