Alternator Sensing Return Lead

swifty

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I am rewiring my boat, and it is subject to the Boat Safety Scheme. My electrical book states that the alternator sensing lead should go to battery + terminal. My friend says that the BSS insits that it is wired to the switched side of the battery master switch.

Can you please advise on this situation and any points I should consider.

Thank you.
 

john_morris_uk

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I wired mine straight to the battery terminal via a small in line fuse holder. The current drawn is a few milliamps, but I don't like unprotected wires connected straight to the battery terminal and capable of causing a fire if they chafed and shorted. The fuse solves the problems, and although I cannot speak for the surveyor you use, my previous boat was certified for commercial use and was wired exactly in the way I have described.
 

boatmike

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The switch you have should be wired directly to the battery positive terminal with heavy cable so the voltage drop there should be virtually nil. Wiring to that will ensure that when you switch it off the battery is still isolated and a short in your wiring can't burn out your boat. I don't see any advantage in wiring it direct to the battery. Yes you could fit a fuse but why complicate things?
 

Danny

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[ QUOTE ]
The switch you have should be wired directly to the battery positive terminal with heavy cable so the voltage drop there should be virtually nil. Wiring to that will ensure that when you switch it off the battery is still isolated and a short in your wiring can't burn out your boat. I don't see any advantage in wiring it direct to the battery. Yes you could fit a fuse but why complicate things?

[/ QUOTE ]I suspect there may be another good reason for doing what Boatmike suggests: if you connect direct to the battery then the alternator control circuitry will see the battery voltage even if the switch is accidently turned off. This would mean that the controller would keep upping the alternator output to try to reach 14.8v (or whatever voltage it wants to charge at) but would not see any change! Could cause problems, maybe? Like the alternator would be 'fully on' with absolutely no load on the output?
 

Zephyr362

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Surely a high capacity cable being used without a fuse is a fire risk? If you take if from the isolator(battery side) and fused of course, then it will measure artificially high by whateve the volt drop is down the isolator to battery cable which ought to be minimal. Also if all volts are removed apart from the reference then the regulator may get upset/blown in the same way that any input is connected but no power to the normal power rail ie input greater than supply voltage

Neil
 

swifty

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I think I may not have made myself clear, the sensing line I am talking about is the small bore cable that runs from the alternator to sense the charge at the battery end. It is not a supply cable and carries only a small current.
 
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