12VDC with Dual Alternators

[DBCox]

Hello everyone,

I am working on an automotive project where I would like to run 2 alternators. The first alternator will be the stock one and drive all of the stock/factory electronics. The second will supply power to a lot of aux electronics.

There are a lot of reasons I want/need to run 2 alts and 2 batteries (basically 2 separate systems) that I don't have time to explain at the moment, so just take my word for it for now. I can explain more if necessary, but it boils down to the aux electronics be very voltage sensitive and current hungry, so they need their own source.

The engine block is already drilled and tapped for a 2nd alt, so I think all I will need to do is install the new alt and battery, ground both to the chassis, run a power wire from the alt to the aux battery and then connect my aux electronics to the battery (using proper connections and fuses of course). My only concern is having a common ground for both systems. Other than that, everything is separate. Is this going to cause a problem? I will have to double check, but I am pretty sure the 2nd alt should have an internal regulator. If not, i will have to include that. Am I missing anything else? Should this work w/o frying something?

Thanks

 

[VEBill]

Common grounds aren't usually a problem. After all - there's only one Earth.

Beside, I think that you'd run into problem trying to find an ungrounded alternator (and other parts of the system).

Stand by for other opinions.

Car stereo - or something more interesting?

 

[waross]

Hi blakraptor, "Am I missing anything else?"
Possibly.
You may want to run a dedicated ground cable from the alternator to the second battery rather than depending on the chassis ground. Connect the cable to either the alternator body or alternator supports or to the engine block close to the alternator. I have seen cars with a very light ground jumper from the engine block to the chassis. I wouldn't want to depend on that jumper for heavy current. From the sound of the load you're anticipating, you may be well to use a two cable system from the alternator to the battery and from the battery to the load, even though there is a common ground at the engine block.
Hi VE1BLL;
No problem with your post.
respectfully

 

[VEBill]

The alternator probably peaks at 100A (maybe more if it is special). If there are heavier peak currents (presumably low duty cycle), they'd be between the battery and the load. Dual cabling certainly makes sense there, but wouldn't hurt elsewhere. Good idea.

OP can also perhaps put the 2nd battery very near the load (taking into account all the safety precautions, including hydrogen).

Strangely, my car (front engine, RWD) comes stock with the battery in the trunk just behind the right rear wheel. I think they put it there for traction. It's also ideal for a '2000-watt' trunk mounted stereo - if I was still interested in such things.

 

[itsmoked]

Nice point waross. I never have a ground cable problem with my diesel, [2] zeroAWG welding cables.

You should include a heavy duty DC fuse rated at your alternator's output. Shorts in these large wires terminated at large energy reservoirs like car batteries pretty much guarantee full involvement car fires that are particularly hazardous to the garage they happen to be parked in when things go sour..

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[Warpspeed]

A common ground should be no problem, the two systems will still be fully independent. But there are other things to think about.

First you will need to fit a second "alternator" dash warning lamp between the new alternator field terminal and the accessory +12v output of the ignition switch. Otherwise the new alternator will probably not begin to charge when the engine starts.

 

[waross]

Actually I had a little trouble with my diesel. The previous owner went through a battery about every year. I started to have battery problems. I bought a new battery but didn't get it installed. One day the truck wouldn't start. While I was hooking up the jumpers I noticed that the ground cable was really hot. I took a closer look and found that someone (probably the mechanic who sold a new battery every year) had done a very profesional job of installing a cable that was about 4 sizes too small and about 3 feet too long. Instead of jumping to the other battery, I connected both jumper cables in parallel with the ground cable (From the battery to the block) and it fired right up.
Despite the heavy cable from the battery to the block, there is a smaller strap on many cars connecting the body to the engine block. Try connecting jumper cables from bumper to bumper instead of from block to block. If you have the small strap, you will find it by the smoke!!
P.S. My little diesel is 2.7 L, not much power but great mileage.
respectfully

 

[itsmoked]

I pitched the wormy little factory ground cable that daisy chained the batteries to the abysmal side posts that got hot every time I started it. I switched to top posts and that stopped. I then drilled a 5/8" hole in the massive A/C compressor bracket and ran one ground cable there and the other to the original mount. Then I ran the two hots directly to the starter stud. Batteries last about 6-1/2 years. Can't say the same for the starters. About 4-1/2 years.


blakrapter; Do heed warp's comment, the dash "idiot light" is often an integral part of the system. No output? Better start checking that.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[zeitghost]

The dash warning light seems to supply the initial magnetising current for the field coil.

If the bulb blows, then the alternator doesn't start charging.

Something similar happened to me when there was an internal break in the wire to the warning light: no warning light with the ignition on but the engine not running, which I didn't notice... until the battery was so flat that it wouldn't turn the engine over any more... fortunately just outside my garage )

You'll need a switch or relay to disconnect the warning light when the engine isn't running or that'll discharge the 2nd battery for you )

 

[DBCox]

Thanks for all of the great advice everyone! It sounds like it is going to be a pretty easy install. I did not think about the warning light though, good call! I had planned to put an extra voltage gauge inside, so I'll put a light next to it. I suppose the lead for this light will be obvious when I get my hands on the alt...

zeitghost, could you clarify what you said about the switch/relay? I am not following why I would need that if its tied into the ignition switch.

Thanks!

 

[waross]

The warning light and connection to the ignitiopn switch are specific to some alternators only. If you need to energize the altenator I suggest a normally open oil pressure switch. These are commonly used on many diesel engines. When the engine starts, oil pressure closes the switch and sends excitation power to the alternator. That will keep your systems completely isolated apart from the common ground.
There are some one wire alternators available that do not need any external source of power to start working.
respectfully

 

[itsmoked]

Slick idea waross!

blakrapter; Yes do keep in mind a lot of alternators need that bulb doohickey. (%50?)

But the rest do not! Or need something else different. That was a suggestion/reminder for you to actually find out what your specific alternator requires.

Remember a whole lot of vehicle come with ammeters and NO idiot light.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DBCox]

Thanks for the clarification waross and itsmoked. I will have to check my specific alt. It will be a stock GM 145 amp that comes with the 2500 HDs equipped with a duramax. I will put whatever it requires. Hopefully it needs a guage and not a dummy light. I don't care for dummy lights, they are too hard to diagnose...

 

[itsmoked]

I actually prefer voltmeters.. They seem more useful than ammeters and you aren't having to run some enormous wire to the meter and then out to the loads. You'll notice the auto makers figured this out about ten years ago.

If you must use an ammeter check these out, I like them.

http://www.cd4power.com/data/meters/dpm_pdb1.pdf

You could use a shunt one and avoid the big wires to the dash.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[zeitghost]

"zeitghost, could you clarify what you said about the switch/relay? I am not following why I would need that if its tied into the ignition switch."

If it's tied to the ignition switch you don't need an extra switch...)

 

[Warpspeed]

I originally though of a relay, or diode isolation may be needed, but the simplest way that works is always the best.

Try it first, it may not require anything extra if the alternator can self energise. If it doesn't self start, try wiring a second dash "idiot" light back to your ignition switch. I can see no reason why that should not work perfectly. It is pretty basic.

 

[OperaHouse]

The "idiot light" is required to enable the internal alternator regulator. In fact, the auto manufacturers do worry about the bulb burning out or vibrating open. The lamp in the cab has about a 300 ohm resistor in parallel with the lamp just so the lamp isn't a problem. One possible suggestion is to place a diode in series with the power (not lamp)to the regulator. This will cause the the regulator to think the voltage is lower than it is. This will favor the higher amperage alternator to put out more power. Maybe a switch across this diode. This is quite usefull when powering monster amps and you need more power at idle.