DC wiring - starter to alternator

[SeaBreeze]

What is the purpose of the wire running between the starter and alternator B terminal?
Rick

[leigh weiss]

What I did for this issue is summarized below:
I installed a heavy short direct alternator to starter wire to reduce the voltage drop of the charging circuit. A loss of one ore two tenths volt will change the max full charge state of the batteries. Using the existing thick heavy gauge wire from the batteries to the starter would provide a low loss path for the recharging of the batteries. The sense wire that energizes the regulator must also be as close to the actual battery voltage value to ensure proper charging (for both an internal or external voltage regulator.) If your charging circuit has the power for the alternator field coming from the the ignition switch which is fed from the wire harness ( with long length leads of about 12 feet, and perhaps some connections which are prone to corrosion) the alternator performance might be degraded. I have opted for an external three stage regulator to provide optimal recharging of the battery bank and have upgraded both the B+ and ground wires feeding the instrument panel and ignition switch. Make sure a separate ground wire on the alternator is also installed, of proper size and free of corrosion. I hope this helps.

[SeaBreeze]

Leigh, thank you for your very detailed reply. However, I am still looking for feedback on the real purpose of the wire connecting starter to alternator. Perhaps I should expand my question. When starting the engine, does the wire between the starter and alternator serve any purpose? When the engine is running and the alternator is operating is there any reason the charge circuit from the alternator B terminal to the battery + terminal has to connect to the starter instead of to the battery directly? Is the connection from starter to alternator just a way for the manufacture to reduce wiring cost by not runnung a seperate wire from the alternator to the battery? Running a wire from alternator seems to just allow for one long wiring run to the battery from the engine compartment for the dual purpose of starting and charging instead of two seperate wires, one for each function. I am asking, because I would like to eliminate the wire running from the starter to the alternator and instead run a wire directly from the alternator B terminal to the battery + terminal. But, before I do this I need to understand if the existing alternator to starter wire performs some function that I am unaware of other than providing a charge current path to the battery. I am asking for help with this because I want to install a device between the alternator and battery but the device can not be subjected to the high inrush current caused when the starter is engaged. So I need to isolate the charge circuit from the start circuit. Leigh, I had not thought of the impact your other ideas would have on charge performance. I plan to implement them. Thank you..

[wolfe10]

The alternator B+ to starter with large-gauge wire/cable from starter to battery is a simple, cleaner way to achieve two things:

Reduce number of wires.
Reduce voltage drop between alternator and battery.

With an external regulator my recommendation is to run the sense wire directly to the battery. That way the alternator voltage is raised to compensate for any voltage drop in the wiring.

And, absolutely, insure that the wire from B+ to starter is large enough-- particularly if you have upgraded the alternator. Said another way, the voltage drop in a wire carrying 110 amps is quite different than when carrying 60 amps. And, voltage drop turned into heat just means the alternator is working harder, meaning you are burning fuel and spending HP unnecessarily.

Can you run a separate wire B+ to battery-- sure, but can't see the benefit.

[SeaBreeze]

Brett, thank you for your post. As to your question, why? Google Sterling Products and look at their AB1280 or AB12130 alternator to battery charger. By installing it between the alternator output and battery it will impose a bulk, absorb, float charge profile on the house battery when the alternator is charging. No modification of OEM alternator/internal regulator required. Wire and fuse size as appropriate between devices of course. Only one wire in and one wire out of the AB1280 for basic 3 step house bank charging. Add 2 two pairs of wire for temperature compensation at battery and alternator. Add another cable from the device to start battery for a seperate charge profile for the start battery. I read about this in Nigel Calder's latest edition of his Boatowners Mechanical and Electrical Manual. For me the benefits are - max output of OEM alternator for house bank charging without modifications to the alternator/internal regulator, 3 step charge profile, equalization profile possible if supervised, conventional charge profile for start battery. Cost is reasonable vs new alternator with seperate smart regulator or modifying OEM alternator to work with external regulator. Fewer wiring changes vs new alternator/ external regulator. My OEM 50 amp alternator is adequate to recharge my 400 ah house bank if I use a bulk,absorb, float profile. This device will give me that with little wiring work and no modifications of existing equipment. To make it work I needed to know I could remove the existing wire between the starter and alternator without causing an unforseen problem. Thank you for helping clarify that its OK as long as I provide the alternate path I describe to get current from the alternator to the battery. For me, 100 ah per day is about max energy useage. While cruising the ICW and coast running the engine 2+ hours each day we move is normal. We don't often sit at anchor more than a day or two when cruising so this seems like a good approach for us. We can also charge batteries using our gen set if on a rare occasion we don't move frequently.