2 battaries

[995]

Use two matching batteries.

Screw the isolator(IMO)

Your alternator will work fine to charge two batteries, unless it's already too small.

I've charged literally hundreds of batteries with this 35amp alternator, and a Honda single cylinder motor mounted on a cart.

http://www.prestolite.com/pgs_produ...d=15046&item_series_id=111&refresh=1165027033

It is wrong to use two different batteries in parallel....that doesn't mean that people don't do it, and doesn't mean that it won't work. If you can't afford to buy two batteries, try to get one as close as possible to your original. I definitely would NOT mix types...

All this begs to question why you wnat two batteries though? I'm assuming the answer is voltage drop. If that's the case, go with a bigger alternator and screw the multiple batteries.

 

[995]

I have a 77 chev. and it only had a weak 42 amp alt when I had it tested it actually did around 60. I was amazed when I read the case and saw how few amps it was.

Hot/ cold? What was the voltage drop @ 70 amps? Who or what was measuring 70 amps? How?

There's a lot of variables to consider there, and others that weren't mentioned....

 

[enine]

Isolators are not magic. They will create a voltage drop, and you'll see less power when an isolator is in line. They are not fail-proof, and will break down if overloaded.

73

If your running a newer GM vehicle (and I think toyota copied this now as well) you have to run an extra wire from one battery back to the sense wire on the alternator. The alternator then regulates the voltage based on this sense wire which is past the diode in the isolator so it compensates for the drop. This makes it nice since you don't have the voltage drop and don't have to use a switch or relay. Plus the loads are still isolated even when the engine is running and the batteries are charging so electrical noise from the original battery running the vehicles engine computer and lights doesn't make it past the diodes to your second battery running your electronics like it would with a direct connect switch or relay.

 

[995]

You cannot run an "extra" wire. Only one, otherwise your two batteries are paralleled through that one small conductor.

You can get the reference potential after the isolator, only if you eliminate the stock reference potential.

Perhaps that's what you meant and I took your post wrong...

The problem with this solution is that doing so raises the voltage to your stock battery, and to your vehicle electronics.

This makes it nice since you don't have the voltage drop....

This does not eliminate the voltage drop....it's still there.

This is not unique to Toyotas or GMs...it's the same on any vehicle with an alternator....they all get a reference potential from somewhere....

 

[enine]

You cannot run an "extra" wire. Only one, otherwise your two batteries are paralleled through that one small conductor.

You can get the reference potential after the isolator, only if you eliminate the stock reference potential.

Perhaps that's what you meant and I took your post wrong...

The problem with this solution is that doing so raises the voltage to your stock battery, and to your vehicle electronics.

This makes it nice since you don't have the voltage drop....

This does not eliminate the voltage drop....it's still there.

This is not unique to Toyotas or GMs...it's the same on any vehicle with an alternator....they all get a reference potential from somewhere....

Ok, I didn't word it very well. The stock Gm alternator gets its reference from the battery but putting an isolator between the alternator and battery it looses that reference, you could say that the diode blocks the alternator from reading that voltage. So the isolator eliminates that stock reference potential. So GM added a sense terminal on the alternator and thats where you run what I called the extra wire from the battery. The alternator now reads the voltage at the battery so it compensates for the drop caused by the diode. So say your charge voltage is 14v, you add a diode and you now get 13.3 volts to the battery, but you have a wire from the battery to the alternators sense pin so the alternator see the voltage is too low and raises its output to compensate for it. So you get 14.7v out of the alternator, then the diode drops its .7v and you get the correct 14v at the battery. So to the vehicle the voltage drop is eliminated since you still get the same voltage as you did before the isolator was added.
This is unique to GM and Toyota (unless some other have done the same now too), you can check the isolators instructions and see the different install steps for it.

 

[995]

Ok, I misunderstood your post to a degree....

The first post, where you said, "you have to run an extra wire from one battery back to the sense wire on the alternator".....that's why I replied...I didn't want anyone to inadvertently parallel two batteries through one small conductor....which is how the word "extra" sounds, as in additional.

As far as the voltage to the vehicle and stock battery, you are correct, I misunderstood exactly what you were doing with that isolator, or was just totally lost in the sauce....then again, there's more than one type, I can tell now which you are reffering to....

As far as the tap, it's just external voltage sensing....nothing special about it, a lot of Fords and Mopars do that as well. Some regulators even have a selectable voltage intended for use with a isolator....there's a bunch of them, especially commercial applications....it's not just GM and Toyota. It's not even new...external voltage sensing has been around since a regulator on the fender was the norm.

It's nothing though, and that was my point....any alternator can do the same darn thing by rerouting the reference potential conductor to after the isolator. That's all they're accomplishing with this "feature". There's another way to do it as well....with a forward biased diode rated to handle field current....I'm sure you can surmise....Regardless of the means to acheive the end result, if you use an solid state isolator the voltage drop is still there....you just may not see it.

 

[enine]

Well, but when you say the voltage drop is there, its not there to the vehicle. When most people say to use a relay because they diode isolator causes a voltage drop they make it sound like your battery and therefore vehicle get less voltage when they don't. Your vehicle and battery get the same voltage as they did before. You basically just get a little bit of heat from the isolator. Though if you max out the alternator to where it can't compensate for the extra loss of the circuit I suppose a non isolated system could keep the voltage a little higher but for all practical purposes there is no drop.

Ok, it was someone else in the thread who said"

"Isolators are not magic. They will create a voltage drop, and you'll see less power when an isolator is in line. They are not fail-proof, and will break down if overloaded."

The same thing applies to a piece of wire, every wire has a little bit of resistance and therefore voltage drop and every piece of wire could burn up if you try to pull too much current through it, so could a relay or switch.
But you won't see less power with the isolator in line because the regulator adjusts for it, just like it adjusts for the resistance and drop of the wire coming from the alternator to the battery.
Thats what I was trying to clear up.

 

[995]

When most people say to use a relay because they diode isolator causes a voltage drop they make it sound like your battery and therefore vehicle get less voltage when they don't.

You're correct. That's probably because they do not know how to compensate for the voltage drop. It's understandable I suppose.

You do still loose a little though, as your alternators current rating drops now that the voltage is higher. While this would be signifigantly more than would be expected from any conductor in a vehicle, it's still a negligible loss in available power from the alternator...

Take care.....sorry I misunderstood you the first go-round.

 

[KingCobra_CDX882]

did you ever find out or figure out
how many amps your stock alternator is putting out

 

[enine]

When most people say to use a relay because they diode isolator causes a voltage drop they make it sound like your battery and therefore vehicle get less voltage when they don't.

You're correct. That's probably because they do not know how to compensate for the voltage drop. It's understandable I suppose.

You do still loose a little though, as your alternators current rating drops now that the voltage is higher. While this would be signifigantly more than would be expected from any conductor in a vehicle, it's still a negligible loss in available power from the alternator...

Take care.....sorry I misunderstood you the first go-round.

And thats a max current rating that gets reduced, so my 105A alt is now a maybe 100A, that loss only comes when you max out the alt and it can't keep the voltage up, normal driving you don't have a drop, so a lower max limit would be a more accurate term. But if your running it that loaded anyway you need to think about a bigger one anyway. I'm thinking of how I want to add a second one for when that time comes. I'm trying to figure out a way to get an accurate measurement of my current draw, since you can't really buy 100A Ammeters and wouldn't want to run that into the cab anyway. I've thought of measuring each major circuit but then I need ten Ammeters. I'm thinking now about doing the 1ohm resister in series with each circuit and measuring the slight voltage drop across each and calculating the current then make a simple scanning circuit for one display. There are 10 main circuits in the underhood fuse box and I'm adding some on my second battery. Some of the main circuits are for things that don't run often such as ABS for example.

 

[505NJ]

Well thanks for all the answers it helped me out.

 

[sleepwalker]

if you were running 2 different batteries ...couldn't you disconnect one when not in use? kinda the poor man's way :?:

 

[enine]

if you were running 2 different batteries ...couldn't you disconnect one when not in use? kinda the poor man's way :?:

Wouldn't it be a pain to disconnect/reconnect every time. You would probably wear out the terminals. Thats why some install a switch or relay or isolator to do it automatically.

 

[sleepwalker]

not in my case....battery is in the floor of my truck....remember I said the poor man's way :roll:

 

[enine]

not in my case....battery is in the floor of my truck....remember I said the poor man's way :roll:

hopefully your cab has enough vent holes in it to let out the gasses from charging unless your using an SLA