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4.0 alternator from a jeep grand cherokee
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Scott701
New User
Sep 3, 2012, 8:08 PM
Post #1 of 10
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4.0 alternator from a jeep grand cherokee
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Hello All, I am new to the forum and pretty excited. This place comes highly recommended.
My question is thus: How many revolutions per minute is required for an alternator of this size (one that came from a jeep) to function normally/safely? How many revolutions will produce too much heat for proper function? Is there a limit to what this alternator can handle?
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Sep 4, 2012, 1:44 AM
Post #2 of 10
(2843 views)
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Re: 4.0 alternator from a jeep grand cherokee
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Please clarify what you are up to. Amp rating of alternator is usually stamped on it. As installed in a vehicle engine RPMs and RPM of items on pulley system are not the same by simple pulley size. As installed most alternators should peak out ability by around 1,200 engine RPMs. If/when pumping out it's max rated power for extended periods it will get hot and can ruin it. As installed in a specific vehicle with all things normal it should handle the power needed to run items in that vehicle with engine running and keep up at idle. Double that doesn't get you double.
When battery is drawn below full charge like when starter is cranking by battery alone the system replaces that lost amount quickly such that pretty much battery isn't doing the work but can cover shortfalls of power for X amount of time.
If you are creating something from this alternator to do something non automotive you need to know the load it would need to carry. Even I'd have to calculate how many RPMs alternator pulley turns vs RPMs of crank pulley to know it's RPM which is higher then engine RPM.
Gut feeling is they will tolerate 1/2 of rated load continuously but would overheat at full load and fail,
T
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aabbaachiii
New User
Sep 4, 2012, 2:41 AM
Post #3 of 10
(2838 views)
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Re: 4.0 alternator from a jeep grand cherokee
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the charging system's primary purpose is to provide power for the car's electrical load, for example, ignition, lighting, audio system, accessories, etc., and to recharge your vehicle's battery. The alternator's output capacity is directly proportional to the RPM of the engine and alternator temperature. Charging systems are normally sized by the car manufacturers to provide at least 125% (when operating at high RPM) of the worst-case OEM (Original Equipment Manufacturer) electrical load, so that the car battery can be recharged.
(This post was edited by Discretesignals on Sep 4, 2012, 4:58 AM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Sep 4, 2012, 8:28 AM
Post #4 of 10
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Re: 4.0 alternator from a jeep grand cherokee
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All that was requested was what it can do and that it came from a Jeep. No telling if this is to make up some other generating thing or what yet. Properly mounted, wired it should put out the rated amps without harm and could spike higher for short times but would make itself hot enough to ruin it inside, wires to it and plug-ins. Wild higher RPMs was not the idea of alternators. Generators kept making more commensurate with RPMs they were turning. Alternators reach that much earlier than maxed out engine RPMs.
Let's wait and see what the project is all about,
T
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Scott701
New User
Sep 7, 2012, 6:53 PM
Post #5 of 10
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Re: 4.0 alternator from a jeep grand cherokee
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Thanks for the informative responses! I am not trying to waste anyone's time with nonautomotive problems, but my little project should function a lot like a car. I have this alternator from a friend's old jeep and am trying to rig something up to charge a car battery with my grandma's old stationary bike (saw it on youtube). I am hoping to use the car battery to run the electric motors on my hot tub. If I understand the info about alternators above, RPMs from a serpentine belt are different from those of just a regular crank, and the alternator should be run at about half of the indicated limit with a simple sprocket crank...?
Again, thank you for the timely responses.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Sep 7, 2012, 11:54 PM
Post #6 of 10
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Re: 4.0 alternator from a jeep grand cherokee
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Suspected you were thinking on those lines. Think harder. The power to "pedal power" to charge a battery would have you worn out in a few minutes when perhaps all day would just be enough.
The drag of a hard working alternator will pull down RPMs of an engine. Pedaling is not match,
T
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Sep 9, 2012, 5:45 AM
Post #8 of 10
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Re: 4.0 alternator from a jeep grand cherokee
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Forget the millenniums old urban lore out there. Study Issac Newton. Energy can neither be created nor destroyed, only altered. Every time you take a given source of energy, convert it to another you lose a small to large %. An alternator is not some magic item that created more than it took to turn it in fact it's far less but better than some other ways.
Ex: If you need a fan to turn by pedal power in this case just put a belt system from a pulley on pedal device direct to the fan you get close to 1 to1 efficiency. Create electricity to run the fan with electric power you lose efficiency.
Let's get simpler. You want to beat eggs in a bowl. Manual crank beater is direct. If you want an electric beater (handy, faster etc.) your whole body probably couldn't pedal the electric power needed to run one for just a couple minutes if just done only when needed.
Your hot tub scenario. Making heat out of electricity is a super pig. Making an electric motor pump is a total pig. It would take a team of horses to generate that much power.
In short - no free lunch,
Tom
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nickwarner
Veteran
/ Moderator
Sep 10, 2012, 7:01 PM
Post #10 of 10
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Re: 4.0 alternator from a jeep grand cherokee
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Hot tubs run on 220v electricity. The amperage they draw isn't too high because the voltage is so high. But to drop down to 12v would require hundreds of amps and wouldn't be feasible. If you want to heat a hot tub with something other than electric there are wood heat kits available and natural gas.
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