Remote battery, 5 metre cable, break 200 amps, voltage spike. - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: February 20, 2016, 10:52:46 am
content: Some cars have their battery relocated to the boot / trunk to give more under-bonnet space for all sorts of go-fast things. Now consider this - there would be about 5 metres of cable from the battery positive terminal to the starter solenoid, and this length of cable will have a certain amount of inductance. When you interrupt about 200 amps of starter motor current there is going to be a positive voltage spike at the starter solenoid end of the cable of perhaps considerable magnitude. Lots of people it seems, use the starter solenoid terminal as a feed point for all sorts of car electrics instead of running a dedicated wire back to the battery.What I want to know is, roughly how much inductance would the 5 metre long starter cable have? And seeing it is not in free air but running flat against various steel surfaces, would this increase the inductance? And seeing this steel surface is the return path for the current, would this not increase the inductance even more?What I want to do is calculate roughly the energy in this voltage transient when the 200 amps is broken.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2016, 04:00:34 pm
content: Around 60mJ.If the lead is travelling over ground plane, then it looks like an unbalanced transmission line. Like microstrip on a PCB, but without dielectric (or much, anyway). The impedance is lower than 377 ohms (the impedance of free space) because the capacitance is higher and the inductance is lower by the same ratios. A typical wire-over-plane impedance is ~100 ohms, so the ratio is around 2. Going back to inductivity, that constant is 1.26 uH/m, so half is 0.63, times 5 meters is 3.1uH.The inductance could be higher due to the steel being magnetic, but I don't think it will be by all that much. It will depend on frequency. At low frequencies, where the skin depth is most of the thickness of the body panel (< 100Hz?), the inductance might be double. At high frequencies, the steel is an effective shield and magnetic fields have to pass between the wire and panel, with the lower inductance figure. (At inbetween frequencies, either an eddy current (\$ Z \sim f \$) or diffusion ( \$Z \sim \sqrt{f} \$ ) characteristic will take up the difference. The consequence for a pulse being: about half the energy is available quickly, while the rest bleeds out along a slower exponential tail, which might be too little dI/dt to be hazardous to the system.)You'll definitely want a hearty MOV (40V?) or TVS (60V?) on the electrical system, in any case. You probably don't want too low a breakdown voltage, in case of load dump (just cooks everything), but this will take care of the starter at least, and reduce sparking on it (which is where most of that energy would go otherwise, assuming it's able to spike to some 100s V in the process).Tim

author: Dave
author_rank: Super Contributor
timestamp: February 20, 2016, 04:21:33 pm
content: The alternator will usually have a load dump clamping zener that should clamp the voltage to ~40V.

author: Circlotron
author_rank: Super Contributor
timestamp: February 20, 2016, 08:56:50 pm
content: ^^ I read that alternators use high power zeners for the main rectifiers. Some race cars don't use an alternator to save weight. That might be the cause of mystery electronics blow ups.

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2016, 09:10:19 pm
content: Where's the alternator?In the bonnet of course! Now there will be just as much inductance in series with the alternator's transient suppressor diode so it won't do much for sharp transients.The TVS needs to be as near to the battery as possible.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2016, 09:18:22 pm
content: Er, near to the source of the transient. So, near the starter solenoid, I suppose. Anywhere in the engine bay will help, though.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 20, 2016, 11:53:36 pm
content: A good rule of thumb for the inductance of a conductor in the vicinity of, but not tightly coupled to, its return conductor is 8nH/cm.An appropriate TVS diode for this application would be, e.g., SLD15U-017, 5KP15A, BZW50-15 and the like.