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Old 04-12-2016, 07:05   #1
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Join Date: Oct 2014
Location: Panama City Beach, FL
Boat: Beneteau 343
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Wiring question for inverter with 4 6v batts

My 12 volt house has 4 golf cart batts with about 450 AH . I plan to use 0 gauge (or close to it) and install a dedicated switch along with proper fusing between the inverter and power source. My question is a "what would you do situation"; maybe a math problem in disguise.

Because of my install particulars and where I can put the inverter in perhaps an ideal place, if I accept use of 6-7' of 0 gauge I can wire to my big switches (+&-) and all is good. I can get by with less than half that length (2-3') if I wire to one of my pairs that is connected in series. Would a 2000W inverter pulling from the single pair of series batts and not the paralleled house bank do harm that makes my voltage drop efforts misguided?

Perhaps,a follow-on question is that I can get by with 3' of wire and connect to big switches if I install the inverter within 18" of my auto pilot's compass. this seems like a bad idea to me as I can see I will want popcorn while relaxing under autopilot. How close can I get to the AP electronic compass? I guess I can move the AP compass but given current wiring not sure it would be easy to get much further away without significant work.
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Old 04-12-2016, 07:36   #2
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Re: Wiring question for inverter with 4 6v batts

Your batteries should be wired such that the positive lead going to the switch is connected to the positive of one pair of series batts while the negative lead going to the negative bus bar is connected to the negative post of the second pair of series batts. In other words, you don't want the two banks of (6+6) and (6+6) to be paralleled and have both leads coming from a single (6+6).

Yes, you can have the inverter wired (with overcurrent protection) directly to the battery bank provided you connect the positive and negative leads to different banks of (6+6).

Note that the inverter will take easily as much as 2amps or more from the batteries even if your AC breakers are off. So if you forget to turn the inverter itself off, you may run your battery bank down quite quickly.
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Old 04-12-2016, 13:07   #3
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Re: Wiring question for inverter with 4 6v batts

If I am understanding your first question correctly, you can indeed connect the inverter to one set of batteries in the parallel/ series set (assuming proper fusing/ switching). And yes the closest battery will bear the brunt of the current supply when the inverter is operating (loaded).

I have a similar situation with 6 GC batteries (4 midships and 2 in the fwd bilge). My bow thruster and windlass are tapped directly from the fwd set and when the bow thruster is operating the current divides 200 amps from the fwd bank and 100 amps from the main bank. Nothing pre calculated in this regard, just how the wire resistances and battery internal resistances work out. I have been running this configuration for 11 years (5 on the bow thruster) with no obvious harm to the electrical system or batteries.

This absolute idea that everything has to have perfect symmetry and balance is a carry over from the piping and fluid flow industry. The fluid flow analogy holds during the actual load current flow phase, but once the load current ceases the slight remaining voltage imbalance will cause current to flow and equalize the battery voltages and charges. In theory this might affect battery life but not in any real world (normal cruising load/ duty cycle) significant way.

I say this as not somebody that just jumped off the Electrical Engineering Turnip Truck, but with 44 years experience designing power distribution systems (30 for US Navy and 14 in commercial practice).

As to the AP heading sensor location you will just have to try and see if the magnetic field causes the boat to veer off course while the corn is popping. You will need to give it some time as the new autopilots use rate sensors (integrated) to provide short time heading, so the interference (if it exists ) may take 60 seconds or so to show up. If required, moving the heading sensor should not be a big deal. If it is an older analog fluxgate you can extend the cable (use a terminal strip) or if it is a newer NMEA 2000 set up you will need a new network cable.

Boating is full of all sorts of compromises, this falls into the just noise category.
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