Is it ever dangerous to use a larger system, such as a car or truck, to jump start a small one, such as a tractor or portable engine, if the two both have the same nominal voltage such as 12 V?
I know that in principle it is fine to do this. The voltage must match and the source must be able to deliver at least the current drawn by the recipient.
But do they ever do something surprising like size the starter motor on the basis of an assumed internal resistance in the starter battery?
By way of analogy, I thought I understood how transformers work, and if you have a 120 V to 60 V step down transformer, you can use it as a 12 V to 24 V step up transformer by using the secondary as a primary. But, no, it’s more complicated, because they make the winding ratio different than 2:1 to compensate resistive losses. Thus I have learned to check such things if I have the least suspicion and mistakes are expensive.
I wouldn’t start the larger engine. The alternator might produce to much for the smaller system on the lawn tractor.
If you just jump off the car’s battery then I can’t see any reason for problems. It’s no different than using a battery charger. They work on cars, trucks, lawn tractors, anything that has a standard 12V battery.
Waaaaa? Huh?
An alternator does not run at full charging current all the time (this is the mistake the browns gas fools make). Alternators are a demand driven device. They produce just enough power to run the car and charge the battery.
My car has a 180A alternator. If I were to measure its output at idle it might be 16A or so. That may sound like a lot but the computer network and lights draw 15A all by themselves.
There is a very slight risk. If you were to hook up the jumpers start the car and leave them hooked for a long time trying to charge a very dead lawn tractor, the batt. in the lawn tractor might not like that. But this is not just giving a quick jump but rather an extended hook up.
I think the alternator will produce more voltage the faster it spins, and will produce voltage sufficient to charge the battery. The starter motor might be surprised by voltage higher than 12 V. I am sure a car system could deliver enough amps to cook a little battery or starter, if they would draw that much – but they wouldn’t, unless the voltage was surprisingly high.
Good point back there, Ace! I think I should not have the larger machine running when I use it to jump the smaller one. Thanks - I like to think that would have occurred to me, but I am not sure. What’s that burning smell???
current doesn’t “hit” anything. the tractor would draw the current it needs from the other vehicle.
the ones selling those hokey hydrogen generators for cars who act like alternators are always producing full output and apparently just beaming the excess power into outer space.
that’s why the car/truck has a voltage regulator. It varies the field current in the alternator to keep the system at a nominal 13.8 VDC. besides, the battery in the smaller equipment itself can deliver enough current to cook its starter.
I don’t see why you think it’ll hurt anything. If the smaller device has a lead-acid 12 V battery, then it operates generally the same way as the one in a car/truck. You aren’t going to “zap” the smaller thing just because you’re drawing from a larger vehicle. The only exception is if you have a simultaneous failure of the voltage regulator in the truck while trying to jump the smaller thing; but even then they rarely fail in a way which would full-field the alternator. Usually the alternator just stops charging.
All systems that use a “12V” electrical system, have a normal operating voltage of between 13.5-14.5 Volts when running. It doesn’t matter if we are talking a lawn tractor, a Jet Ski, or a great big SUV. Same voltage range. The voltage and amperage are limited by the Voltage regulator.
There is functionally no difference between hooking an SUV to jump start another SUV or a lawn tractor.
The browns gas crack was about a melon head (who was an EE and should have known better) that insisted that the 180A alternator in his car put out 180A all the time (no fucking way it does. If it did, it would burn up or explode the battery in just a few minutes)
I tried to explain that this isn’t how the system works, and that alternators are a demand driven device, but he would not hear it, I even offered to hook up the test equipment and show him just how much the alternator put out, but he would not hear it, and he got every condescending, rude, and dismissive of my knowledge. So about the third time he spouted off with I’M AN ENGINEER!* I told him he should not say that so loud, because if his college head the him they would revoke his degree as he obviously did not learn a damn thing in school and he must have cheated.
A few months later he was back and no longer bragging about how he was going to get super gas mileage once his installation complete. Guess he finally read a design and function section of an auto repair manual.
Noting is going to hit the lawn tractor. Hooking up the cable will cause a slight rise in output from the alternator as the system tries to charge the lawn tractor battery. When you try to start the lawn tractor, the demand from the SUV will increase due to the starter wanting more power. At first the SUV battery will supply this increased power until the alternator ramps up its output.
*FTR
I’m an engineer. = Smart person that designs things
I’M AN ENGINEER! = Pompous windbag that is full of shit and doesn’t know what they are talking about.
My car has the dead battery at 10 Volts and Rick’s car with the good battery boosted by the alternator is at 14 Volts. The 5Ω resistor is the total resistance of the batteries plus jumper cables (in reality it should be much lower than 5Ω).
Rick has a deadly alternator capable of delivering 180Amps. Will it fry my battery?
Well, no.
There is just a 4 Volt potential difference (14-10) over a 5 ohm resistor. Using Ohm’s law, there is just a 0.8 Amp current flowing through the cables. Then, as my dead battery charges up, the voltage increases from 10 Volts to 11. Now the potential difference is 3 Volts over 5Ω = 0.6 Amps. This will go on until after a long time my battery will reach almost 14 Volts and the current flowing will be almost zero.
personally we have jump started the car from the tractor , and we have jump started the tractor from the car , we have done this many a time and no problems at all … and the tractor batteries are bigger cranking amps than on a car battery, (not a lawn tractor, its a real tractor lol )
but we dont leave the jumper leads on , as soon as it is running they are removed ,
i was told to do this because of the damage it can cause either the computer in the car , or the computer in the tractor , is this not an issue ?