Modules dying when 24V vehicles are jump started

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Some pointers on electronics components on vehicles;

Have you examined the failed modules to see which components are being killed? There maybe some clues to what needs to be done to save them and a easy fix.

Generally 28V is considered the operating voltage of 24V systems. Some operational conditions can exceed 28V. See below.

SAE has recommended standards for design and test of vehicle electronics. The transients in

.pdf cover some of the same conditions as SAE. Electronic components designed and tested to SAE standards usually do not have problems when installed on vehicles. Remember 12V levels must be multiplied by 2 for 28V systems.

Jump starts with dead or low batteries can cause a series of positive and negative transients from multiple components

Protection MUST be included for reverse voltage transients and jumps.

High current electronic modules may require active protection circuits to block high supply voltages and transients.

Generally, transient suppression should protect only the electronic module, not suppress the entire vehicle electrical system. Suppressing the vehicle is the hard, expensive (it will come back to bite you), and iffy way to protect a module. Adding TVS=92s ahead of your module is a band aid for suppressing the vehicle. This subjects the TVS=92s to anything the vehicle can throw at them now and in the future. When you do not control the vehicle design, configuration, or operation you must look out for your module the best you can, regardless of what happens to the vehicle.

Look into a fast circuit to turn off power to your module with low and over voltage plus reverse voltage.

Next time design and test your modules to SAE transient and EMI standards. Also make sure all the module I/O (except power) is electrically isolated from the vehicle and other electronics.

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12V jump-start systems on Auto Club tow trucks often use 2 batteries in series in order to guarantee a good boost, especially in cold- weather areas. I don't know if similar tricks are used for 24V systems.

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Why are you quoting me here? 'Snot my problem.

Grant.

Hi Tom,

Are you familiar with "Load Dump" testing? Briefly it is a set of tests specifically designed to avoid this problem. Various Standards require that equipment designed for Automotive use must pass this test.

Briefly, a "Load Dump" is what happens when a vehicle with a flat (or faulty) battery is jump started. When the motor first fires up it charges a very high current into the loaner battery. When the leads are removed this current has nowhere to go (the main battery has a high resistance), so there is a huge voltage spike generated in the vehicle wiring due to the magnetic energy stored in the Alternator (e.g. the alternator cannot stop generating instantly).

The laboratory test for this dumps a large bank of Electrolytic capacitors (charged to 200V) across the device. It causes indicator lights to explode like flash bulbs. Lots of fun.

The design cure is either a series voltage regulator which can withstand the voltage spike, or a large MOV across the rail, sufficient to absorb the energy spike. It needs to be a big one.

Regards ................ Zim

This is all covered in MIL-STD-1275

Tom Gootee Inscribed thus:

The fact that the manufacturers fit a main power switch seems to suggest that this is a known failure condition. A bit like the warning labels on cars that tell you to disconnect the battery before performing any electric welding on the vehicle.

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Best Regards:
                     Baron.

[...]

Way to go. Although this one (as most others) is limited to 80V and for a truck with a 24V system that's borderline. Might have to roll your own here.

Not for 24V but you could ask these guys:

The market is small thouygh because most equipment is properly designed to handle those load change surges or, ahem, "unusual" jump-start methods. In aircraft it has to or it won't be certified.

--
Regards, Joerg

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[...]

That one you can buy as a COTS module, ready to wire up, at the company I pointed out in the other post. However, they use MOVs whivh have a finite number of times they can be hit. It's like an energy bank account, when the balance is used up ... *PHUT* ... a fuse in the module blows.

--
Regards, Joerg

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In addition to all the remedy suggestions, one could try to avoid to have to jumpstart in the first place. I take it there is quite a drain on the battery when the alternator isn't providing. You could remind the operator that he is living on borrowed time with an obnoxious beeper or something.

Regards, Werner

The failure mode of the device is important. Is it failing due to a low voltage feed, causing it to blow its own internal regulation circuitry, or are voltage spikes from the started windings getting injected through its power circuitry and into the device, blowing a more sensitive component?

Would not a line conditioner (DC) in the power feed to this device fix it? You know, like the car stereo guys use... a supercap bank.

The other way is to wire up a switch in the engine bay that allows it to be disconnected during a jump event.

That might turn into a good thing. If it changes the maintenance profile it may result in a user behavior change.

Depends. If the competition manages to deploy a system without that effect it would increase the "customer pissedness factor". Not a good thing.

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Regards, Joerg

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142,C1...

This paper has what looks like some really good information:

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True. OP's current problem of dead modules on a regular basis still falls far higher on that scale than dead fuses that a regular user might be expected to replace.

Regardless of the engineering fix, I would put a caution label on both the batteries and the jumper cables stating that the 'xxxx" system circuit breaker must be off before jump starting the vehicle to avoid damaging the " xxxxxx" system.

That might reduce the failures while you work on the ultimate solution.

Dan

same:

Yes, but don't use the BJT version if the load is fat. Else ... *PHUT*

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Regards, Joerg

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In the end it's all about what competitors offer. Changing a long-standing yet "sub-optimal" user behavior is next to impossible. As engineers we have to design around it. Just like engineers had to design anti-lock brakes because people notoriously tailgate during rushhour no matter how often they are told not to (on Wednesday I almost had a septic pumper truck crash into the back of my car ...).

--
Regards, Joerg

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I guess that would be a *SLOSH* instead of a *PHUT* ?:-) ...Jim Thompson

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