Alternator, Charging and isolator

[Bfenske]

I am in the process of rebuilding two non-functional 50 amp alternators that are being rebuilt to 94 amps.

Currently, the boat setup is that I have a main 8D 12 volt battery for each engine and an auxiliary battery for a backup. Normally, the switches for each engine are set on the one battery for each side and the auxilary does not get a charge from either alternator.

Now that my alternators will be rebuilt at a higher amperage, I was wondering if I should use the extra amperage to charge the auxiliary battery as well as the main batteries while the engine is running and i'm underway.

I have been told to get an isolator to do this . just wanted to know from you guys what the best way to do it would be.

I do not have a generator on the boat but there is a solar trickle charger dedicated for the auxiliary battery. Just thought it would not hurt to charge all three batteries with the extra amperage.

Brian Fenske

[In Memory Walter K]

Brian- The way my batteries are set up is there are 3 batteries. Two for the engines and a "House" battery for the instruments. The alternators charge all the batteries via an isolater which allows the current to go only in one direction...TO the batteries...and will not allow one bad battery to drain any of the others. Your switches can be set up to give you the flexibility to switch batteries in case one fails for any reason. Isolaters come in some basic configurations from one to multiple battery configurations if I am not mistaken. I am sure that within a day or so, you'll get more and more specific input on your question. Walter

[Skipper Dick]

Brian
I have 3 banks of batteries. One for each engine and another bank of two. They are charged via an isolator that has two inputs, one for each engine alternator and three outputs, one for each bank. The isolator keeps the three banks separate from one another. If you would like to see the system, just give me a call and I'll show it to you.

Dick
770-8152

[Bruce]

Altenator output capacity is directly related to case size.

Numerous times lower amp altenators have been upgraded with larger capacity diodes. More output requires more heat transfer. Small cases allow the altenator to run hotter and give a shorter life.

So be carefull how its done.

One other thing is the regulator style. Alternator output in most cases is linear to rpm. Where as a lot of high output alternators won't putout their high current unless they spin up wards of 5000 rpm.

Check the pully ratio's to see what you'll be turning.

Aftermarket regulators allow the altenator to load up at lower rpms to a point.

If your getting an alt shop to build the things, then be there for the test and find out at what rpm they are spinning the altenator for the output tests.

More than once a customer has gotten one of those upgrades only to find out it didn't perform the same as the bench test.

You do lose some voltage across the isolator(diode bank). Under normal circumstances you'll be okay.

On heavy discharges and charges you'll fail to bring the batteries up to their optimum voltage for long life and the plates will begin to sulfate.

This is where an aftermarket adjustable regulator can be talyored to your needs.

Last is dual belts. You start pulling upwards of 80 amps, a single belt will slip and chew.

I said all this to say for your setup under normal circumstances, a stock 65 amp would be okay.

A good many boaters install higher output altenators and never realize the cost/potential or lack of actual battery power.

[Bfenske]

Bruce, I took it to a local alternator rebuild shop and told him the max rpm I would normally run the engines at was 2500 rpm. I didn't think about the extra belt because the originals just have a single belt. So you're saying over 80 amp I'll need two belts? Don't wanna run offshore thinking I improved my battery system and then shred a bunch of belts in the process. He said he could rebuild them to 94 amps for $150 each.

[Bruce]

At those high load outputs yes your belt will stretch and slip. Slipping will heat the belt and make it come apart.

Cranking on the belt to keep it from slipping will cause excess rotor bearing wear due to the high side tension and will shear bracket bolts.

But unless your running an inverter needing to keep the batteries up, I doubt you'll ever see the ouput over 40 amps.

Output is load dependant.
In other words unless you have deep discharged batteries or a high DC load, the altenator won't put it out cause you don't need it.

[Bfenske]

So if the output normally stays in the 40 to 50 amp range even though it can put out a max of 94, the single belt should still be fine? There is a 1500 watt inverter for a microwave that we rarely ever use and that is the only non-DC power on the boat.

[Hookshurt]

Great info! I am FINALLY about to tackle a very similar project and have/had some of the very same questions.

[Bruce]

Yes.

For a 900 watt micro wave you'll draw 75 amps from the batteries feeding the inverter when its on.

How much of a charge the altenator will put out at the same time is dependant on battery amp/hr capacity, condition and type of regulator on the altenator.

A simple formula for figuring out current draw is:

P = I (x) E

P is watts
I is current
E is voltage

To figure your microwave we'll plug in the numbers. (watts ac or dc is the same)

900(p) or (watts) = (I) x 12(e is 12dc)

We are looking for I which is current draw.

To find the missing I (current(amps)) divide 900 by 12 which equals 75.

Now some inverters are a little more efficient than others and won't draw as much current.

And remember high current draw equals, class, anyone, anyone. Bueller, Bueller?

HEAT. Becarefull where the inverter is mounted and make sure it has good cooling air.

One other thing if your not confused enough already. Mounting the inverter in a hot engine room degrades the performance and will lead to higher current draws and shorter life.

Only if your using it, not if its just mounted like eye candy.