AQ120b Alternator: Internal or external regulator?

1981 Bayliner Explorer with a AQ120B/270 package. Need to know to wire up a two battery set up and switch. Thanks.

I see your thread title is; AQ120b Alternator: Internal or external regulator?

pugetsounder said:

Need to know to wire up a two battery set up and switch.

Click to expand...

If concerned with field protection, you'll want an MBSS (main battery selector switch) that has the field terminals at the rear.
IMO, this is not a deal breaker if you make certain that NO ONE ever passes through OFF (while the engine is running) when making a change to the selected battery bank.
Most all of these are "make-before-break", so switching between banks (while the engine is running) is not an issue.

As for a two battery set up and MBSS, here is a scenario that is simple yet gets the job done.
(see image below)

Note that only POS and NEG Battery Cables connect to the batteries.
This avoids the Rat's Nest at our batteries.
Each battery bank should have it's own independant Negative connection to the engine.
The O/B Charger leads can use the rear MBSS terminals #1 and #2 for any direct connections.
For any interruptible power, the "common" terminal can power an accessory panel.
All of these circuits should be fused.

Typically, although no rule, #1 will be a dedicated cranking battery, and #2 will be Deep Cycle batteries.

I see more issues with the Perko MBSS (small contacts/weaker spring) in particular the 90* sweep switch.
I'd rather see the Bluesea, Cole Hersee or Guest used.
However, the Perko will work.

RicardoMarine said:

I see your thread title is; AQ120b Alternator: Internal or external regulator?

If concerned with field protection, you'll want an MBSS (main battery selector switch) that has the field terminals at the rear.


I see more issues with the Perko MBSS (small contacts/weaker spring) in particular the 90* sweep switch.
I'd rather see the Bluesea, Cole Hersee or Guest used.
However, the Perko will work.

Click to expand...

I already have the Perko installed on the boat. If it breaks I will replace it with something else. It has the field terminals so I will use them. The PO has hooked up the two battery system so that everything is working off of the 2 batteries. I want to isolate them so that one battery is for starting and the other is for house. I plan on adding a invertor/charger/shore power at some point but that will be next year. I also want to add a float switch as well. Thanks for the replies.

kimcrwbr1, so what you're saying is that with this:

http://www.ebay.com/itm/DUAL-MULTI-...Parts_Accessories&vxp=mtr&hash=item45fca1bb05

You don't need a manual switch? That would be cool!

pugetsounder said:

.......... The PO has hooked up the two battery system so that everything is working off of the 2 batteries. I want to isolate them so that one battery is for starting and the other is for house.

I plan on adding a invertor/charger/shore power at some point but that will be next year.
I also want to add a float switch as well. Thanks for the replies.

kimcrwbr1, so what you're saying is that with this:

You don't need a manual switch? That would be cool!

Click to expand...

pugetsounder, is this you on the BOC?

A working MBSS will separate the two battery banks from one another when selecting #1 or #2.
Only while selecting ALL/BOTH will the two banks be combined.
Something is either wrong with the way that they are cabled, or with the MBSS itself.


IMO, the DUAL MULTI BATTERY SMART ISOLATOR 12V 140AMP KIT is rather small for your potential loads.
The Bluesea ACR or a VSR would be a better choice.... but again, IMO.

Bluesea also offers the auto-combining unit that would meet your No Manual Switch criteria.

Quite honestly, managing your battery banks manually becomes second nature after you get used to it.

Yep that's me. Are you 2850Bounty? If s,o sorry I got you double timing on two boards. I'm new to the BOC but have been around here for a few years bouncing between mercruiser and VP. I figure here is a better place for VP mechanical inquiry and BOC is a good place for inquiries about the boat itself.

Do you think 140AMP wouldn't be enough? I wouldn't be running a lot of components at one time. just a thought. For now since I recently bought the boat I just want the dual battery set up the way it should be. I'll use what I have until I get familiar with it. The more I monkey around with this boat the more familiar I become and that's a good thing. Getting everything working the way it should is first priority, then installing the goodies like shore power inlet, inverter/charger, possibly electric anchor winch since the wife is excited about anchoring out in this boat. All that will come later.

pugetsounder said:

Do you think 140AMP wouldn't be enough? I wouldn't be running a lot of components at one time. just a thought.

Click to expand...

Well, I think that the four open fork type terminals were a give-a-way for me, that the unit may be on the light duty side.
You'll want to cover yourself for the worst case scenario....., such as an emergency cranking load.
140 amps is not very much in terms of starter motor load....... so IMO, it just isn't quite large enough.


The Blue Sea 9112 can handle 450Amps, with a short term cranking capacity of up to 800Amps.
This would be more than enough capacity.

Ah. I was thinking in terms of charging amperage. I found it online for around $200 not bad if it takes the place of the switch and the isolator function. Thanks.

I believe that you'll want to determine just exactly what it is that you want to accomplish.
I realize that you want two battery banks.

• ... are you looking for combining for charging only..... such as an ACR or VSR?
• ... or are looking for a system that will automatically default to the #1 cranking bank during starting?
• ... or both?

Bluesea offers many products that can be used for this. They are high quality, and are somewhat expensive.... but very worth it!


I like KIS.... (keep it simple)

Being rather redundant here........ one of the goals for most people is to keep #1 in "Reserve" (very important, IMO)
This means that you can crank on #1, charge it while warming the engine up, switch to your #2 HLBB, and enjoy the entire day on #2.
(#1 is quietly resting in reserve)

If the #2 HLBB (of Deep Cycle batteries) is large enough, and if your engine is quick firing, you can crank on the #2 HLBB throughout the day without damage to it.
(remember, #1 is there in "reserve" should you need it!)

If this helps you with the thought of not having to switch back and forth, and perhaps forgetting to keep #1 in reserve, this may be your solution.
All in all this would be under the guise of KIS.


Example, but in a twin engine scenario:
My Port engine has it's own dedicated cranking battery (as does my generator).
My Stdb engine battery bank is also my HLBB (of 6 volt D/C Batteries... lots of Amp Hours).
We can crank on a large a bank of Deep Cycles (key word "large") without harming them..... with one caveat, and that is that it helps if the engine is quick firing.
My Stdb engine is quick firing, and with litte Amp Hours used comparitively.
(200+ amps for 3 seconds is nothing in terms of Amp Hours used)

OK.... why I am telling you this?
Because in my scenario (contrasted to yours), my Port side cranking battery is my "Reserve" (#1 in your scenario)!
It is my "reserve" for the Stdb engine via a momentary combining solenoid...... of which you will not actually need, but could use.

So for YOU, you could operate all day long on #2, starting/stopping, using the radio, etc, and be just fine as long as your're not drawing down the #2 HLBB beyond 50% or so.

Again.... #1 is quietly sitting there in the background waiting to be used should the #2 become a bit too low.

Make sense?


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