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Old 14-05-2010, 10:59   #1
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Battery Isolation Question

Nothing special, but I do have a question about battery chargers. We have two battery banks, a starter bank and house bank. The starter bank has 3 reg marine deep cycle batteries, and house bank has three 8D's. When we anchor out, I usually turn off the starter battery circuit and of course just run the house batteries. As an additional measure (to make sure I have back-up starting capacity), I cut out one of the big 8D house batteries....just in case. We also have a wind generator and solar panels, that are wired through charge controllers and/or switches to the main house circuit....but I don't think these are part of the question.

Question is...the way we have it wired (two banks and a charger) even though the starter battery is out of the circuit, it still remains in the charging circuit...the same holds true with the 8D when I isolate it. Now the good part...even with batteries isolated from the main circuit they are still connected through the charging circuit...wouldn't this mean that isolated and turned off batteries are, in actuality, still in the main circuit through the charging system? If so doesn't this negate the isolation system because the batteries are still part of a circuit although not part of the usage circuit.

BTW we have a Professional Mariner charger, and Perko switch isolators (no affilitation)... Do these chargers have some kind of diode protection circuit to prevent using battery power that is not in the main circuit but still in the charge circuit? Whew...I think I understood what I said.

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Old 14-05-2010, 11:40   #2
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I would think yes. The wind generator and solar panel(s) could be wired directly to the battery bank, via blocking diodes and the grounds brought back to the charging items. You could still isolate the batteries you want (probably by switching off the + lead) yet still allow the batteries to charge.

It would depend on how the batteries are connected and switched, though. I'm not sure I'd have 3x start batteries and an 8D in the loop (maybe 1x big start battery and an 8D), but depending on how you've got it wired, I'd say you could either install an aftermarket diode isolation bank for the batteries or rely on the solar/wind regulators.

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Old 14-05-2010, 13:23   #3
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Your system sound a bit wasteful. If you keep your house bank above 50%, which you should do anyway, it will start your main engine in the event of a failure of the start batteries. To keep one of your house batteries as a third start option is shortening the life of the house bank.
The start battery normally uses very little AHrs and therefore is not normally hooked up to wind and solar. (If anchored for a long time and concerned using the “coupling switch” when the house bank voltage is high will give the start batteries a top up charge)
The start battery can, however, be hooked up to any charge source and if done through a split diode or smart relay it will only charge, not discharge, the start bank.
If you are unsure of the wiring use a multimeter directly on the battery terminals. During periods of discharge there should be a noticeable difference between the start and house banks. (say 12.6v for start and 12.2V for house after reasonable discharge)
If the voltages are the same, during discharge, you have no isolation of the different banks.
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Old 14-05-2010, 13:52   #4
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I second the comment on you wasting one 8D battery. It should really be connected to the house bank to give you the most battery power possible. That way your batteries will last longer, as noelex 77 has said.

It also sounds like you are going a bit overboard on the start batteries. You might think about connecting one or two of the start batteries into the house bank.
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Old 14-05-2010, 18:00   #5
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I agree with the comments above, it seems like you are only using 1/3 of your batteries for the house and 2/3 for starting. IMHO that seems a little backwards...
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Old 16-05-2010, 17:59   #6

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Yes, you are wasting the 8D that you isolate and abusing the others. They are being discharged more than they need to be, and when recombined they will be charging unequally as well. That's just not good.

As to whether anything is or isn't isolated'd need to draw up the schematic and diagram the whole system to be SURE of any of that. Sometimes you do that and wind up saying "Gee, how'd that get in there?" Words usually can't replace a good schematic.

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