Originally Posted By: JHZR2
You sure? IIRC, the value provided by the reference I linked to in this thread (cable manufacturer) indicated ~0.8uH/ft. A 20' set of cables would be 40' of cable length... So more like 32 uH.
Is that for a single cable, or a pair laid side-by-side? Because the fact that most jumper cables are close-coupled significantly reduces the inductance of the circuit compared to two widely-separated cables.
But let's assume that it is 32 uH and do some back-of-the napkin math. How fast is the circuit actually broken by disconnecting a cable? What is the relative voltage difference at the time of disconnection?
I would argue that the circuit actually "breaks" relatively slowly when you un-clamp a cable, because the teeth retract from where they've sunk into the terminal and the resistance ramps up over maybe a millisecond as they do so. Its not like snapping a relay open, which itself is on the order of 10s to 100s of microseconds. I'll be conservative and call it 500 uS. Now the next thing to consider is the current flow. The worst case would be if the recipient car's alternator is actually not working AT ALL, even after that car starts, so that the donor car is both running the recipient car's electronics (maybe 10A) and chargning its battery (full alternator output initially, but even a very, very dead battery will quickly build enough surface charge and internal resistance to drop that down to about 20a. So let's be generous again and say that 50 amps are flowing at the time of disconnection:
So from that, we can guesstimate that dI/dT is on the order of 50A/500uS.
So the voltage "spike" due to the cable inductance will be (50/500e-6)*32e-6. By my trusty HP15c, that gives me a voltage spike due to the cable inductance of 3.2 volts.
That's not going to damage ANYTHING. Even if some of our guesstimates are off by a factor of 2, its still not an issue.
So I'm back to my first claim: the biggest risks in jump-starting are a) operator error, and b) the recipient car having a full-field charging system fault that makes it's system voltage surge high when it first starts.
You sure? IIRC, the value provided by the reference I linked to in this thread (cable manufacturer) indicated ~0.8uH/ft. A 20' set of cables would be 40' of cable length... So more like 32 uH.
Is that for a single cable, or a pair laid side-by-side? Because the fact that most jumper cables are close-coupled significantly reduces the inductance of the circuit compared to two widely-separated cables.
But let's assume that it is 32 uH and do some back-of-the napkin math. How fast is the circuit actually broken by disconnecting a cable? What is the relative voltage difference at the time of disconnection?
I would argue that the circuit actually "breaks" relatively slowly when you un-clamp a cable, because the teeth retract from where they've sunk into the terminal and the resistance ramps up over maybe a millisecond as they do so. Its not like snapping a relay open, which itself is on the order of 10s to 100s of microseconds. I'll be conservative and call it 500 uS. Now the next thing to consider is the current flow. The worst case would be if the recipient car's alternator is actually not working AT ALL, even after that car starts, so that the donor car is both running the recipient car's electronics (maybe 10A) and chargning its battery (full alternator output initially, but even a very, very dead battery will quickly build enough surface charge and internal resistance to drop that down to about 20a. So let's be generous again and say that 50 amps are flowing at the time of disconnection:
So from that, we can guesstimate that dI/dT is on the order of 50A/500uS.
So the voltage "spike" due to the cable inductance will be (50/500e-6)*32e-6. By my trusty HP15c, that gives me a voltage spike due to the cable inductance of 3.2 volts.
That's not going to damage ANYTHING. Even if some of our guesstimates are off by a factor of 2, its still not an issue.
So I'm back to my first claim: the biggest risks in jump-starting are a) operator error, and b) the recipient car having a full-field charging system fault that makes it's system voltage surge high when it first starts.