alternator connection to primary battery bank

[hwd]

I have a 2005 C34 mkII with the Universal M35B engine and original 51 amp internally regulated alternator.  The previous owner replaced the original two 12 v batteries with two pairs of 6 v Trojan golf cart batteries (each pair in series is 12 v 200 amp).  Each pair is connected to the 1-2-both battery switch the same as the original two battery installation.  I plan to reconfigure the system (as explained in many posts on this site) using the four golf cart batteries reconnected in series and parallel (12 v, 400 amp) for use as the primary starting and house battery bank and add an additional  12 volt reserve battery. 

Can someone recommend: 1) what size cable I should use to connect the existing alternator to the primary battery bank and 2) what size and type of fuse I should use at the battery bank?  Also, what type of battery would be best for my reserve battery? - it will be charged with an echo charger connected to the primary battery bank and thus needs to be compatible.   

[KWKloeber]

Universal used both internally and externally sensed alternators, do you have a sense terminal?

kk

[hwd]

Not sure...How can I tell?

[mark_53]

The bigger the better to reduce voltage drop... up to a point. I used a #8.

[KWKloeber]

Not sure...How can I tell?

If there's a post on the alt marked "sense."  or let me know the model # (stamped into the side of the case)  might be a 8Mxxxxxxx and I can look it up.

kk

[hwd]

Thanks...I appreciate your help with this.  I'll check on it when I get back to the boat in a few days.  Will this make a difference in terms of the connection to the primary battery bank?

[KWKloeber]

Thanks...I appreciate your help with this.  I'll check on it when I get back to the boat in a few days.  Will this make a difference in terms of the connection to the primary battery bank?

Well, it depends......  an external sense is better because the regulator compensates for the loss in the charge cable back to the batteries. And internal sense is reading voltage essentially right at the alternator.  So, at least theoretically, you can use a smaller cable size because the regulator compensates for it. But there's other issues too, like the set point of the regulator. The internal regulator on the OEM 51 amp motorola alts are notoriously low by today's  charging standards. Some can be bumped up with an adjustment (potentiometer in the regulator), most cannot.  For an example, I sometimes order Wilson rebuilt alternators (a good brand) -- I call the tech and have him search the database, and ship the alternator that when tested had the highest setpoint.

How do you or will you use her? Day-sailed or extended cruising? Are you tied to a shore charger  (i.e., are the house batteries correctly fully recharged every evening)?

Ken

[KWKloeber]

 An internally sensed alternator will have two wires going to it -- the red alternator out charge cable, and the (usually purple) field excite wire. An externally sensed alternator will have a third wire (to the sense post.).  I say that, but it's also possible that someone may have tied the  sense post directly to the alternator output, thereby defeating that function. -- so there might be only two wires going to and alternator with that sense function not being used.

Unless somebody added a wire to ground the frame of the alternator -- and then you'll have an additional wire. Confusing enough?

It's best to just get the model number and then I can check the database.

Not sure...How can I tell?

If there's a post on the alt marked "sense."  or let me know the model # (stamped into the side of the case)  might be a 8Mxxxxxxx and I can look it up.

kk

[Stu Jackson]

This may help you identify the OEM alternator wiring:

Alternator Regulator Wiring Diagrams - all three http://c34.org/bbs/index.php/topic,4548.0.html

Wire sizing is based on amperage, as you most likely know.  Usually the OEMs were 55 A but the difference between that and 51 A is negligible in the wire sizing.

The larger issue is if you in the future want to install a lager output alternator, consider using larger wires NOW to save you more work later.

The Electrical Systems 101 topic has links to wire sizing charts.

Will this make a difference in terms of the connection to the primary battery bank?

No.

[hwd]

Thanks, guys....I was hoping I could just disconnect the alternator output from the starter and reconnect a cable to the main battery and fuse at the battery...

[hwd]

Ken, In response to your earlier question..I currently am tied up and shore charging each evening..but plan on more extensive cruising (1-2 weeks) in the future
Harry

[Stu Jackson]

Thanks, guys....I was hoping I could just disconnect the alternator output from the starter and reconnect a cable to the main battery and fuse at the battery...

Harry,

From what you said you seem to have read all the switching discussions, right?  Seems you read the OEM 1-2-B Switch Wiring History  http://c34.org/bbs/index.php/topic,4949.msg30101.html#msg30101, too.

The ONE wire between the C post of the switch and the AO/starter serves two purposes.

Your wording is not specific:  i.e., what does "...and reconnect a cable..." mean?

If it means a NEW cable from the AO to the house bank, then sure, you're done.

The existing wire from the C post should go to the starter.

We've all been there in the process of upgrading our electrical systems.  Good luck.

[mark_53]

I would set up your reserve battery to also be your start battery. At least that way your reserve battery gets some use. You can also set up the 1-2-b switch so each battery or both has the capability of starting the engine.
I don't have a fuse on the wire from the alt to house batteries. I suppose if you have a 55 amp alt you should add a 55 amp fuse but from what I've read that 55amp alt will rarely if ever put out 55 amps. Wire fuses are meant to protect the wire so chose a fuse that is rated for a combination of wire length and size.
Here is my wiring diagram.
http://c34.org/bbs/index.php/topic,8790.msg63977.html#msg63977

[Stu Jackson]

1.   You can also set up the 1-2-b switch so each battery or both has the capability of starting the engine.

2.  Wire fuses are meant to protect the wire so chose a fuse that is rated for a combination of wire length and size.

1.  We do not recommend starting on Both.  First, our engines don't need it.  Second, if one bank is in poor health, the very last thing you want to do is to combine it with a healthy one.  If you move the AO off the switch, then the switch becomes only a USE switch.  In the old days, skippers who had the charging run through the switch tried to charge both banks vis the switch.  Those days should be over, by now. 

2.  You size the wire for the amperage and the length.  You size the fuse to protect the wire simply based on the size of the wire. Just to clarify...

[KWKloeber]

Ken, In response to your earlier question..I currently am tied up and shore charging each evening..but plan on more extensive cruising (1-2 weeks) in the future
Harry

Harry so as Stu alludes to, what will your recharge needs be then, so you don't have to re-do wiring again.  When you say 2-week cruise, that means 2- weeks away from a charger?  So you're needing to recharge only with batts?  Or w/ solar also?  I would start with a power budget and that will help define what alt you may want to upgrade to for cruising.  Then we can plan on that for the new cable -- otherwise you may have to re-do this again.

A simple sketch of what you're doing and what you're replacing might help.  How many feet is the cable run?

ken