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Cause the modules in the car need good consistent Voltage.
Never disconnect a battery on a running vehicle.
It also puts a lot of strain on the alternator.
I would not recommend anyone doing this.I don’t see the point in pulling the car battery to test the alternator. The battery contributes zero current to the system while the car is running. If you think about it- the battery is a 12.5v voltage source and the alternator/regulator is putting out over 14v. That means it is impossible for current to flow out of the battery. Just measure the voltageat the terminals of the battery – if you measure anything highet than battery voltage then you know the the alternator is doing all the work so disconnecting the battery is moot.
Thanks for the input guys! I just wanted to know because i have customers coming in store saying thats how they test alternators and i tell them, thats not the correct way but i dont have a reason on why its not correct. but now i do and i Thank you!!!
Although older vehicles might allow this, I would never do it, for fear of voltage fluctuation causing damage to sensitive electronics down the line. When the battery is in line, I would expect a moderating effect on voltage fluctuations caused by the alternator. On the other hand, i suspect it probably is OK to disconnect the alternator before starting the car, and then run for a few minutes, if you are trying to hunt down some problem in the system. I’ve never done that myself, either.
Newer vehicles have sensitive electronics that can be damaged by a voltage spike that could be created by the removal or installation of a battery cable while the engine is running. Think of the battery as a ‘shock absorber’ as it is the alpha and the omega when it comes to the automotive electrical system, take it away and the voltage gets a little crazy because it was never designed to run without a battery. That said I’ve seen some of my fellow techincians do a battery replacement with the car running so that they did not have to worry about resetting the radio and the customer settings, I never saw them have a problem doing this however it kind of scared me a bit.
Quoted From Ironman:
Thanks for the input guys! I just wanted to know because i have customers coming in store saying thats how they test alternators and i tell them, thats not the correct way but i dont have a reason on why its not correct. but now i do and i Thank you!!!
Yup, its all about the voltage spikes as listed. C8-)
Do you work in a parts store?
If the alternator is supplying 14 volts and the battery supplies 12.5 volts to the same electrical system, then the battery becomes a load to the alternator. Disconnecting the battery from the system while the alternator is working, could be like unhitching the horse from the wagon. Any transients from this action are probably suppressed by the regulator. It’s hard to say what the affect would be without putting an oscilloscope or waveform analyzer on the system and also looking at what transients the electronics can tolerate.
I have to agree with Twiggy on this one. Something above 12.5 volts is working for me. And Balon, take it to the autoparts store where the tests are designed for the alternator. And Eric, today’s electronics are running on microwatts, forcing them to use 3.3 volts and lower. Electrostatic discharge (ESD) was the number one killer for the quad NAND gate drivers I used to test for AC Delco. Your own body capacitance is enough to drop 5000 volts on a device gate and destroy it.
Jeff
Quoted From dreamer2355:
Yup, its all about the voltage spikes as listed. C8-)
Do you work in a parts store?
Yes I work at Advance Auto Parts in Jackson, Ga.
Thats cool, nothing wrong with working at a parts storeC8-). Theres an ASE P2 test for part sales people too that i would recommend taking unless you already have it.
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