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Old 18-07-2017, 22:10   #1
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Join Date: Aug 2009
Location: Kemah, TX
Boat: Whitby 42
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Dead Batteries and Bad Wiring

I recently discovered that my starting and house batteries have all gone bad. They were 6 years old but I suspect a major culprit is the wiring disaster and charging circuit that I inherited with the boat.

I am replacing all of the batteries and want to start the rewire job with the battery / charging circuits to make sure I do not damage the new batteries and they are protected properly. Here is the simplified 12v schematic of the plan so far: AHoy 12V Schematic

I currently have the charger connected on the battery side of the isolation switches. Is this common practice? I like the idea that if I smell burning I can turn off the battery switches and all loads will be disconnected even when on shore power with the charger on.

I plan to add a battery terminal fuse to the house bank. I would imagine these are not recommended for the starter batteries due to starting currents? Are there any recommended protection (slow blow fuse?) for the starter batteries?

Any other comments or corrections are greatly appreciated!!!
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Old 18-07-2017, 22:33   #2
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Re: Dead Batteries and Bad Wiring

Unless the batteries were quality (like Rolls etc) then im surprised you have got six years out of them, these days using regular batteries it's more like three if your lucky before they fade away, so your wiring system may be OK,

Most starting systems do not use fuses or fusible links as starting currents are very high and on a dark night on a lee shore and you need your engine the last thing you want is a fuse to blow!!

Cheers Steve
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Old 18-07-2017, 23:08   #3
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Re: Dead Batteries and Bad Wiring

I like the way you have drawn the diagram, looks professional.

Although you said it was a simplified drawing, some questions:
- Where does the alternator outputs go to? Both to the house bank?
- The earth connections, no lines are drawn; do you have real wires from each of these back to the batteries?
- You have a combine/isolate switch for the engine, not on the genset, only isolate switch. Any reason for that? BTW the way you have drawn that engine 'combine' switch, it is actually a 'switchover' switch, not a switch that combines the engine and house batteries. That is good in my opinion, as you were to combine two battery banks you are likely to combine a dead battery with a good one, the result being two half empty batteries which soon will be both dead.

My comments:
- I have never seen the power supply for a starter motor fused
- Yes, on my boat I have a large isolation switch for the starter motor, but with an 'off' position, so I can work on the engine without fear of sparks
- No, do not run your starter motor leads via a distribution board of some kind: causes too much voltage drop and most blocks or boards are not made to handle up to 500 amps
- Yes, the supply after the '+house batt' could have a fuse of 50 to 200 Amps, depending on your max load, while each circuit has an appropriate fuse separately as you have drawn
- Not sure about your inverter, but ?some/?most of them do have low voltage fuses; if fused, then connect to your '+house batt' stud, if not include the inverter in the max amps of your houseload, and connect after the fuse that connects to the '+house batt' stud.
- The supply to the windlass needs to be appropriately fused, depending on the windlass, 50 to 150 amps, then connect to the 'house batt' stud
- Still on the windlass: there are lots of gear available that have switches and fuses combined in one unit, so you have to buy only x1 piece of equipment, and that way reducing the number of connections and therefore voltage drop.
- Some circuits may need permanent access to the positive battery Voltage, like memory for electronic equipment (radio/tuner), and bilgepumps. They need to draw from your your '+house batt' stud and separately fused.
- When installing alternative charging sources ie solar or wind, these need to be fused and can be connected to the '+ house batt' stud.

Disclaimer, the above is my opinion, and based on what I have. But I am happy to learn from others, more knowledgeable than I am.
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