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Old 20 February 2008, 10:36   #11
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Old 20 February 2008, 11:15   #12
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Quote:
Originally Posted by al40 View Post
Avoid blocking diodes at all costs...
Nonsense. We've had this conversation before.
Your notion of a battery never reaching full charge is not correct and it presumes the battery with the diode is initially fitted in a partially discharged state. Fitting a fully charged battery and then using a diode to preserve the charge works fine. Even if you accept the voltage drop as 0.7v (which it usually isn't) you'll still have a charging voltage of approx 13.7v which is just fine for a float charge. If you're wise, you'll choose to use a schottky diode which has a voltage drop of approx 0.4v.
If you end up with a really difficult situation and you've partially discharged your backup battery too, all you need to do is ensure your switch is in the correct position to recharge it fully.

All these suggestions of switching....what a hassle.

I use a diode with complete success. Both my batteries are now over 4 years old and still work fine and they are starting a big diesel engine not a little outboard motor so I guess they're still delivering satisfactory current.

I have a 1-both-2 switch but I never switch it, it's always on both but I also have an isolating switch for the battery with the diode so it's the equivalent of always being on 1. During my boat build I felt it may be useful to have the switching facility but I've never used it. Some folk say it gives safety but I feel it's very little - after all, do you disconnect your car battery each time you park?

Quote:
Originally Posted by BruceB
All the boat is to be run off the one battery with the second only for backup.
I think that is the way to go. Personally, I'd use two simple single switches; one as a main switch and one in series with the negative connection of the backup battery with the diode wired across it. Dead simple, no switching necessary in normal use and both batteries remain charged.
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Old 20 February 2008, 12:01   #13
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Tis funny how an innocent question can open up so much controversy :-)..............
I had a Hella Contour switch as an isolator on my last boat which looks identical to the BEP ones mentioned.
Is this what you mean about the isolator wiring and if so would it not be easier to wire it in parallel with the switch #2 so charging takes place all the time?
Or could the same wiring be used with a single dual battery switch as a neater way of setting up switches?
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Old 20 February 2008, 12:28   #14
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Much like the second one but I used a switch in the negative line. I did this because negative isolators are available which are in the form of a negative battery terminal which is 'jumped' by tightening a thumb screw. This made it very easy and the diode can have an 8mm terminal on each end which is simply put behind the clamp bolt nuts.

Here's one.

The end with the bolt through it is a taper which your standard battery cable fits onto. I'm sure you could adapt that fuse holder to house the diode to make it a neat solution.
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Old 20 February 2008, 12:34   #15
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So you don't have two isolators as such. You have one main battery isolator and this discarnect unit connected to your neg terminal on the second battery with the fuse holder removed and a diode across it. When required on a flat battery you screw in the black plug to bridge the diode for use? as shown in first drawing.
Does that make any sense and is it correct?

Or how about the scond drawing with one switch for dual batteries but with a diode joining both? The disadvantage I can see with this is that when switched to battery two the first battery doesn't get charged and if the second battery is flat the first will try and equalise the charge between them, more thinking required I think
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Old 20 February 2008, 12:35   #16
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Quote:
Originally Posted by BruceB View Post
So you don't have two isolators as such. You have one main battery isolator and this discarnect unit connected to your neg terminal on the second battery with the fuse holder removed and a diode across it. When required on a flat battery you screw in the black plug to bridge the diode for use?
Does that make any sense and is it correct?
Spot on.
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Old 20 February 2008, 13:12   #17
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I think I have found the answer an BEP 714-100A this has one switch which can be set to battery 1 or 2. Whatever battery is selected as the running battery the other is charged through a VSR. It isn't even particularly dear as 69!
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Old 20 February 2008, 13:25   #18
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Buce
You won't go wrong with that, its a very good quality unit and they also sell a very neat mounting plate for it.
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Old 20 February 2008, 13:52   #19
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It isn't even particularly dear as 69!
Bruce, you live in a different world to me...
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Old 20 February 2008, 14:01   #20
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It isn't dear compared to a lot of other bits needed and when you have gone through several "cheaper" isolators at 15-20 each it makes sense to buy one good quality one that will last.
The hella one I had was still going strong after several years use, the cheaper ones at about half the 35 cost of this lasted one season only before developing high resistance and having to be changed.
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