Upgraded Alternator Wiring

[Pete1122]

I am upgrading my wiring in preparation for a 120 amp alternator and have a few questions about the wiring. As I see it there are a few options on how to wire my new 4awg wire from the alt.

1. Keep the existing 10awg and run my new 4awg wire in parallel from alt to fuse box to battery.

2. Keep the existing 10awg and run my new 4awg wire directly to the battery with a new fuse/breaker.

3. Keep the existing 10awg and run my new 4awg wire directly to the house side of the isolator.

Which would be the best?

[Bob C]

I would run the new wire in parallel from alt to the isolator. It would give you the best power transfer to both batteries. I would add heavier wires from the isolator to both batteries also. Don't forget to put fuses in line also.

[Pete1122]

What side of the isolator do you recommend I hook the new 4awg too? The truck side or house side?

[zero]

I assume the OEM wiring from Toyota had the output from the 60 amp alternator directly wired to the positive terminal of the battery with a 80 amp fusible-link protecting the circuit. When the RV company stuck an isolator in there, they re-rerouted that alternator output lead to the common terminal on the isolator. No need or benefit to change that for any reason I can come up with. But #1 that 80 amp link is still in that circuit (unless it has been bypassed). Two issues come to mind.

#1 - how long is your total wire run from the alternator output to the positive terminal of the battery?

#2 - do have a V-belt driving the alternator or a flat-ribbed serpentine belt?

In ref. to query #1 - the total length is what counts. New from the factory that total run is often no more then 3 feet. 3 feet of #10 copper wire can easily carry 100 amps @ 14 volts and thus the reason Toyota stuck an 80 amp fuse-link in there. #8 is safer though. If you change things - you need to know the total length of the charge wire from start to finish, and then protect that circuit with a fuse that will melt (or breaker that will blow) before the wire can melt. If you start sticking extra parallel wires in there, it really confuses things. Since you are installing, in theory, an alternator with more potential output, are you wanting to send more charge power to the back of the RV where ever your house battery is? If so that needs to be factored in. E.g. my 1988 Toyota Minicruiser has the OEM 60 amp alternator (capable of 80 amps). From the isolator-relay, running 14 feet to the back of the RV house pair of batteries, I have #2 copper cable with a 100 amp breaker on both ends.

In ref. to query #2 - I'm just curious what belt drive you have. If a four-cylinder with a single V-belt, it cannot turn an alternator to make 120 amps anyway. It will just slip. 120 amps needs dual V-belts or a ribbed and wide flat belt. I don't know what sort of alternator you got but usually - the reason for using something like a 120 amp or 150 amp alternator is not to get those max amps. It's to get the higher output at low engine speeds. A stock 60 amp alternator can charge around 25-30 amps with the engine at idle speed. Certain 120 or 150 amp alternators can charge 50-60 amps at engine idle speed. But I don't know what alternator you have. There are a lot of "hyped up", old-tech Delco 10SIs and 12SIs being sold to stick on tractors, old cars, etc. Mostly because they are simple and cheap. Not because they are any better then what Toyota used, OEM.

[Maineah]

This^ Unless you are running a new wire to the back battery use what you got. I also agree the belt will slip it's border line slipping with the stock load.

[Pete1122]

Thank you for the replies, let me clarify my situation.

The total charge wire from the alternator to the battery is maybe 6 feet, I also have the stock V6 alternator right now. I will be having it rebuilt to run at 60amp idle with a peak of 120 amp.

I have upgraded the wire from the isolator under the hood all the way back to my two 6 volt batteries using #4awg wire, total length of about 14 feet. It is fused, plus I have upgraded all my grounds to #4awg as well. I also have a new 200amp isolator ready.

My goal is to provide more charging power to my rear six volt batteries. I run a ton of electronics with a 1000 watt inverter and find that the alternator cannot keep the house batteries charging. Meaning I use more power than the stock alternator makes.

So with those goals in mind should I connect the new 4awg wire from the alternator directly to the battery? Directly to the isolator? Or directly to the fuse box and use a bigger fuse?

[zero]

I'm not sure I understand all of what you've described, but here's my take. Your total run cannot be just 3 feet if you've got 14 feet of wire running to the back "house" batteries. You should figure what size wire you need for the longest run - and then make all the charge wire the same size Or protect those wires going by the smallest gauge wire used. Then install appropriate sized fuses or breakers.

If you have #4 wire in all the charge wires - it has a ampacity of 90 amps max. Forget the voltage drop for now. The ampacity is the ability of wire to carry current before it starts to melt. #4 copper is 90 amps, more-or-less (not all wire is the same). So #4 wire needs protection from 80 or 90 amp fuses or breakers. 120 amps is too much. If #4 is what you want, it should be #4 from the alternator output terminal, to the relay, to the front battery, and to the rear batteries (or bigger).

Figuring for voltage drop is another story. Proper wiring needs proper voltage drop and ampacity calculated. Ampacity for safety,and drop to keep your stuff working correctly.

I have my doubts that someone is going to rewire a stock V6 so-called "60 amp" Denso, and turn it into a unit that puts out 50 amps at idle speed and 120 amps max. Stock Denso alternators like used on Dodge minivans do that and they are 118 mm alternators. I have two. Quite a bit larger then what's used in your Toyota, as I recall. My 98 Dodge van has a Denso 120 amp alternator and my 2001 Dodge van has a Denso 160 amp alternator. You can buy either of those alternators brand new for $100 if wanted. But I digress.

I wired up my 1988 Toyota Minicruiser to send 60-70 amps to the rear house batteries when needed. Why? Not to just charge batteries. More to power my 3000 watt inverter when I need more AC power then my batteries can supply. If I have a high AC load, I just start the engine and the alternator adds to the DC input to the inverter. Two large house batteries usually can only supply around 1200 watts before the input DC voltage drops below 10.5 volts and the inverter shuts down. Using the alternator is a good work-around.

This is how I did mine. #4 copper AWG wire from the alternator output to the isolation relay. From the relay, #4 back to the front battery, and #2 to the rear house batteries (around a 15 feet run). Several manual-reset 80 amp circuit breakers protecting it all.

Back to inverters - voltage "droop" is a big problem. Modern inverters have automatic trip-chips that shut them down when input voltage drops below 10.5 volts even it just happens for a fraction of a second. I've tested a pair of Trojan T-105 six-volt, deep-cycle batteries hooked in series - running a microwave. 10-11 amps of AC current is about the max those two batteries can handle at initial surge before dropping below 10.5 volts.

[Maineah]

You need two things to get more output from an alternator more field windings and a larger stator no rewiring will do that. That being said you will also need some thing to drive the extra load the puny little 1/2" drive belt will not do it..

[Pete1122]

This helps a ton, thank you. My last thought was which post on the isolator to connect the new 4awg wire too? If I connect it to the aux battery post then those amps will be dedicated to those batteries while the original 10awg charges the truck battery until the isolator closes. Any drawback to this?

[zero]

This helps a ton, thank you. My last thought was which post on the isolator to connect the new 4awg wire too? If I connect it to the aux battery post then those amps will be dedicated to those batteries while the original 10awg charges the truck battery until the isolator closes. Any drawback to this?

I'm assuming you've got 10 gauge wire running from the alternator output to one big post on the isolator relay (TOO SMALL). From that same post a wire runs to the positive terminal of the cranking battery in front. Then on the second big post on the isolator relay - you've got your #4 copper AWG wire running the long run to the back of the RV to the house batteries. This 2nd post is the one that only gets energized when the relay contacts are closed. That can be done automatically with a voltage charge sensor, or by hooking it to the ignition circuit, etc. All that wiring with 10 gauge and 4 gauge in the same flow line must be protected as if it was all 10 gauge wire if just using one circuit-breaker or fuse. Toyota, OEM does not use wire as small as 10 gauge in the alternator circuit. Usually 8 gauge as I recall and protected by an 80 amp fusible-link. Might even be 6 gauge OEM (I don't remember). If you have 10 gauge it's because someone added it and it's too small for any of that charge circuit.

That all said, if you choose to have 3 feet of #10 gauge (to the relay), and then 14 feet of #4 gauge running to the back from that relay - you get a 10% voltage drop @ 100 amps @ 14 volts. If you really want the capability of 100 amps you need bigger wire.

My Minicruiser has #6 wire from the alternator to the isolator relay (about 2 feet run). That has a 1.2% drop. Then my 15 foot run to the back batteries is with #2 AWG wire. That has a 3.6% voltage drop @ 14 volts. So the entire run @ 100 amps had a total drop of 4.8% which is fine. I don't even run 100 amps anyway. I have 80 amp breakers on both ends of the line.

[zero]

I forgot to ask. What was this person going to do to make your 60 amp Denso charge 50 amps at engine idle? Did he say? One trick some installers do is put on a smaller drive pulley which spells disaster when high max amps are needed. Smaller pulley makes the alternator put out more charge at engine-idle speed because it spins faster. But with a smaller pulley, you get even less belt traction and it's more prone to slip when max amps are needed. With the stock setup from Toyota, the alternator spins at 1800 RPM with the engine idling. If you have a V6 with a flat ribbed belt, your stock pulley is probably 2 1/4" with four grooves. That is for a 60 amp rated alternator. The Denso 120-160 amp alternators usually have 2 3/8" pulleys and 6 groove belts.

[Pete1122]

Ok, so just to simplify Here is my thought process, please let me know what I am missing or if my logic is sound.

If I keep my existing 8awg wire from the alternator to the truck side of the isolator, that will continue to draw all the amps/power needed to charge my truck battery and truck accessories. Even if I have an upgraded Alternator it should be fine as the truck will never draw enough amps to overload that wire.

I then run my new 4awg wire from the alternator to the aux battery side of the isolator, this will always be providing power/amps to the aux batteries. This sounds like a great solution to maximize the power of a 120amp alternator. Or I am better off just hooking the 4awg wire to the truck side of the isolator and let the isolator do it's thing?

[zero]

If I keep my existing 8awg wire from the alternator to the truck side of the isolator, that will continue to draw all the amps/power needed to charge my truck battery and truck accessories. Even if I have an upgraded Alternator it should be fine as the truck will never draw enough amps to overload that wire.

I then run my new 4awg wire from the alternator to the aux battery side of the isolator, this will always be providing power/amps to the aux batteries. This sounds like a great solution to maximize the power of a 120amp alternator. Or I am better off just hooking the 4awg wire to the truck side of the isolator and let the isolator do it's thing?

No, faulty logic IF my brain is working right today. Wiring the way you propose defeats all your battery isolation. The aux batteries will be connected to the cranking battery all the time and the isolation relay does nothing. There can be only ONE lead connecting the output post of the alternator to one of the posts on the relay.

[Derek up North]

That's the way I'm understanding it as well.

[MontanaChinook]

If you want more amps flowing to your batteries, you need to replace the wire from the alternator to the isolator, and the wires going from the isolator to the two individual batteries. For the isolator to...isolate, you need to use it correctly. This puts one wire on each post. No doubling up.

If you're sure that the extra energy you somehow plan to get out of a different alternator (which has been debated here at length) won't be too much for your original wiring, then it's ok to leave that wiring from the isolator to the battery you for some reason don't care as much about. So if you don't want to buy any more wire than you absolutely can get away with, then you can skip one. So you need thicker gauge wire going from the alternator to the isolator post. Then thicker gauge wire running from the "chosen" (as in 'the chosen one', the 'golden child' etc ) battery to its individual post on the isolator. The wiring from the "non chosen" battery to its individual post on the isolator can stay the same, if for some reason that's how you want it.

But if you ran thicker wire from the alternator to the isolator, but instead of putting it on the correct isolator post, you're putting it on the "chosen battery" isolator post, you're doing two things. Defeating the purpose of an isolator, and getting no more charge to the chosen battery. Because the wire gauge from the isolator to that battery is still small.

To get the extra amps you allow to flow from the alternator to the isolator, by using thicker wire, you also need thicker wire running from the isolator to the battery(ies).

Or I could be completely off...

[zero]

A 12 volt battery when fully charged is 12.7 to 12.9 volts (depends on make and temp).

To charge that 12 volt battery voltage needs to be substantially higher.

A typical voltage auto/truck regulator is set to charge a 12 volt battery @ 14 volts.

So maintaining 14 volts or close is the target.

That is where figuring voltage drop comes in. A 5% drop is usually the highest wanted in a good system and less is better.

Some examples.

OEM wiring on a Toyota truck with a "60 amp" OEM alternator has around a 3 foot run of #8 copper wire between the alternator and the positive battery post. That has a voltage drop @ 14 volts of 1.7% drop @ 60 amps. So if at that 60 amp rate, voltage reaching the battery is 13.8 volts.

Now - take that stock setup and convert to an RV with an isolator. Most I've seen get that OEM charge-wire extended. If that added wire is 3 feet of 10 gauge wire - that adds another 2.8% drop @ 60 amps. Now there is a total of 1.7% plus the added 2.8% that totals to 4.5%. That drops the battery charge voltage down to 13.3 volts which is getting low.

Now - add even more wire to run to the back batteries? Let's say 14 feet of #4 copper wire. Just that wire in itself @ 60 amps has a 3.2% voltage drop.

Add it all together - from the source at the alternator to the final destination at the rear "house" batteries? That total run consisting of 3 feet of #8 AWG, 3 feet of #10 AWG, and 14 feet of #4 AWG, comes to a total voltage drop of 7.7% That means if trying to send 60 amps to the rear house batteries - that 14 volts leaving the alternator drops down to only 12.9 volts when it gets to the back. 12.9 volts can barely charge a 12 volt battery at all..

Obviously most of the time charge current is going to be less then 10 amps, not 60 amps. At 10 amps - all that wire in total will only have a voltage drop of 1.3%. So @ 10 amps, charge voltage to the rear "house" batteries will be 13.8 volts that will charge a 12 volt battery fine.