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Posted: Tue Nov 10, 2009 3:33pm

Post Subject: voltage drop

I have a voltage drop of 0.4 volts between the alternator and the batteries (domestic side) The cable is about 0.25 dia. but is very long at about 12 ft. Is a 0.4 drop normal or do I need to upgrade the cable?


Posted: Tue Nov 10, 2009 4:07pm

Post Subject: voltage drop

Dear Peter, first of all your cable size makes no sense to me. The important size is the cross sectional area of the conductor bundle in sq. mm. I also need to know the alternator output if I am to calculate the size for you. If you are saying the cable has an overall diameter of 1/4 inch it sounds far too small for a modern 50 to 70 amp alternator. On charging circuits the lower the voltdrop the better with an absolute maximum of 0.3 volts on each run. Less is better. Before you go changing the cable you need to put the voltmeter across each joint and component in the system because your voltdrop could be caused by a bad connection or faulty contacts in the master switch or split charge diodes. If so then it needs curing before you look at fitting a larger cable. If you have a split charge diode system then they will cause up to 0.7 volt of voltdrop and in this case either change it for a relay OR see if you can convert the alternator to battery sensing (most are machine sensing). If it is already battery sensing then the voltdrop is of less concern because once the bulk phase of charging is over the machine sensing will increase the regulated voltage to compensate. However it will lengthen the bulk charge phase. If you do decide to upgrade the cable run it right back to the master switch and not via the starter lead as is often done. Also remember to check the negative as well! Tony Brooks

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