Bringing back the dual batteries topic

[79highrider]

I read the topic after already turning over in my mind the idea of having two batteries(1 red top, 1 yellow top), one hooked up to the truck and motor and the other for just accesories, (eight 6" off road lights, winch, cb, air compressor. etc.) my question is how do you use one alternator to charge both batteries? or since I have a second stock alternator lieing around do I run two alternators with a custom bracket and different belt?

 

[Seabronc]

OK, my question for you is, do you intend to run your accessories with the engine off and if so, which ones?

Any way, there are a number of ways that you can hook up the batteries if you have more than one. If the engine is running, basically you need an alternator/s that can handle the load. In normal operation the battery is only used to start the vehicle. Once it is started, the alternator supplies the required load. Right after starting, it supplies the load demand of the ignition, lights, radio, etc. plus recharges the battery. If your load demand is larger than the alternator can supply the battery will make up the difference. That is OK for short periods, but running with a load that is larger than the alternator will handle will eventually **** the battery. Having a second battery and a too small alternator will only prolong the agony.

So what I'm saying is number one you need to be able to supply the entire load demand from your alternator. If you had more than one alternator you would have to isolate the two systems for the sake of voltage regulation.

Back to two batteries: The usual hookup and reason for having two batteries is to allow you to run stuff while the engine is off and not have to worry about running the battery so low you can't restart the engine, like camping gear, radios, etc. That hookup is done using a common alternator but isolating the batteries via a relay. Then running axillary stuff from a distribution panel that is hoked to the isolated battery. With the engine running the relay allows charging of both batteries and hooks the batteries up in parallel for the purpose of charging. When the engine is off the two batteries are isolated, and if you have chosen right, you can run your axillary stuff without risking not being able to start the engine.

In any event, what I'm saying is you need to know what the load demand is and how you intend to run it so you can make a sound design decision. That includes wire type and size so you don't set the truck on fire.

Food for thought,

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[79highrider]

Ok so if intend to only run the winch and lights while the truck is running and maybe the lights for brief time periods when it is not, it would be ok to use one optima battery and a bigger alternator that can supply the load, ok now how do I figure out the size of alternator I would need?

 

[Seabronc]

Right now you probably have around a 60 amp alternator and the truck is running fine with that. For any device, divide the watts by 12 and that will give you the amps required, so if you have a light bar with four 150 watt lights, you multiply 150 x 4 = 600 and then divide 600 / 12 = 50 amps, that in itself means you need to consider how the circuits for the lights are designed and what components to use for controlling them. As far as the winch, the manufacturer can tell you what the maximum current is that it requires. Add up the extras and tack that onto the current size of the existing alternator and you should be good. You can play with those figures by deciding what components don't get used together, like nothing else on with a wince running. As long as you know the current requirements of the equipment, you can mentally schedule their usage. In other words, you just don't run everything at the same time. So once you have a real feel for what you would normally want to have running at the same time then the current requirements of those items plus what you presently have, like light, blower fan, radio, etc. determines what you need.

You would probably be pretty safe with a 3G 130 Amp or go strait to a 200 Amp 3G. Just don't get confused into thinking that the alternator is always putting out that much power, that number is it's advertised maximum continuous load. It only puts out what is required by the electrical system. The more stuff you plan on running, the more accurately you need to calculate the load. Also, consider the HP loss to the wheels by loading down your engine with large electrical loads. On my truck, with an electric fan from a 4.6L Lincoln Mark VIII shows a noticeable change in the demand on the engine when it cuts in. I had to add a throttle kicker to my carburetor to keep it from stalling the engine when I was setting at a light and the fan kicked in, (100 amp start current and 33 amps running).

I guess the main thing I'm trying to convey to you is that you can "Shotgun it" or you can "Design it". Shot gunning may work but a careful look at it will save a lot of possible problems, some more serious than others. Design can be a simple as checking on the current and wire size requirements for the winch and and deciding that you already have everything you need for it, or you need to run a larger wire and increase the size of your alternator.

I hope that helps since it is mostly generalizations.

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[79highrider]

Thanks for all of the info you have been a great help, I think I will bounce around prices for a 130amp alternator and put it in when the new motor goes in so I am more prepared for everything than I am now(6 of the lights are piled on my toolbox, and I don't yet have a winch)

for your problem with the HP loss would a 200amp alternator fix that for you? or does it not matter what size the alternator is when it kicks on and draws 100amps

 

[Seabronc]

for your problem with the HP loss would a 200amp alternator fix that for you? or does it not matter what size the alternator is when it kicks on and draws 100amps

The size of the alternator won't help that is a function of load on the engine caused by a sudden electrical demand on the alternator. The more that is required of the alternator the harder it is to turn it over. The problem is that the larger the electrical load the more HP it takes to operate the alternator and your engine has a limited number of horses and at idle it is producing very few of them. I don't have any numbers about HP requirements based on load, but it definitely is noticeable when that fan kicks in. I don't think anyone has done any dynamometer tests to see what the actual losses are based on electrical load, though that would be interesting, to me anyway /emoticons/[email protected] 2x" width="20" height="20" /> . It only takes about a second or two and the fan is back to it's running 33 Amps and is only noticeable when at idle. The kicker pushes the RPMs up from 650 to 850 and that is enough to keep the engine from stalling, but without it the engine almost dies, in fact it probably would if I were not quick on the peddle.

I subscribe to Ford Muscle Magazine http://www.fordmuscle.com , there are a lot of articles you may find interesting and helpful in what you are doing.

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[79highrider]

The size of the alternator won't help that is a function of load on the engine caused by a sudden electrical demand on the alternator. The more that is required of the alternator the harder it is to turn it over. The problem is that the larger the electrical load the more HP it takes to operate the alternator and your engine has a limited number of horses and at idle it is producing very few of them. I don't have any numbers about HP requirements based on load, but it definitely is noticeable when that fan kicks in. I don't think anyone has done any dynamometer tests to see what the actual losses are based on electrical load, though that would be interesting, to me anyway /emoticons/[email protected] 2x" width="20" height="20" /> . It only takes about a second or two and the fan is back to it's running 33 Amps and is only noticeable when at idle. The kicker pushes the RPMs up from 650 to 850 and that is enough to keep the engine from stalling, but without it the engine almost dies, in fact it probably would if I were not quick on the peddle.
I subscribe to Ford Muscle Magazine http://www.fordmuscle.com , there are a lot of articles you may find interesting and helpful in what you are doing.

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It is funny that you said this because last night my brother and I put underdrive pulleys on his 87 5.0 Mustang Gt and with the different pulleys we noticed at idle everything dims and the volt guage drops but off idle everything is fine.

 

[Seabronc]

It is funny that you said this because last night my brother and I put underdrive pulleys on his 87 5.0 Mustang Gt and with the different pulleys we noticed at idle everything dims and the volt guage drops but off idle everything is fine.

That is because the alternator isn't capable of putting out much at idle, especially with under drive pulleys. They usually don't put out until around 1000 RPM, (keeping in mind that the alternator normally turns about 2 times faster than the engine). Here is a graph which I believe originally came from Ford Muscle magazine. If your brother is going to do much night running, you may want to consider putting a different pulley on the alternator.

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