High Amp Booster Jump - Nature of PCM Damage?

[DrumrBoy]

Was just reading another post and came across the notion that the portable booster jumper units can fry the PCM. I don't have one (so I don't use 'em)....i always just slow charge a weak battery on a Battery Tender....

BUT recently, my car has been jumped a few times by one of those handheld units.

When a PCM board is damaged by such, is it usually non-functional or is it generally hard to detect (i.e. some flakey problems vs catastrophic failure)? I am chasing down some flakey idle stuff and am trying to eliminate this as a variable.

I know there's not always-correct answer, just looking for the general reaction....

 

[dave6666]

Just paint your hood vents white.






















Really though... Would be curious why the frequent need for assisted starts.

On the PCM... A high amp booster will provide current as needed. If there is a path it will go there, but what is different about the current path when using a booster than using your normal battery? Unless there is additional voltage, or it is not a true DC source like a battery is. If the thing is still connected to a wall socket or 220VAC 3 phase outlet then...

I disconnect my PCM when welding on my car but that is 30 to 40 volts. A good quality battery power only booster IMO would be fine.

 

[Steve-Indy]

DrumrBoy, I hope that MY post was not misleading.

If indeed it was my post, I was referring to the large A/C powered chargers on roll-about wheels (like those seen in commercial garages)...that seem to kill PCM's on Gen II's and an occasional Dodge Dakota when they activate the "boost to start" switch. I can name 6-7 such Viper cases alone.
NOTE that this is NOT a portable starter pack...which are just batteries. I have a couple of these and have had no ill effects jumping countless cars and many Vipers. My comments are personal observations, not proven...though my EE consultant concurs.

 

[dave6666]

Seems like we may have a consensus growing...

Batteries OK...

 

[Steve-Indy]

Dave, some of my worries specific to Gen II's has to do with the location of the jumper terminals and the fact that the PCM is right there!! On a Gen I, the PCM is on the other side of the car---and, so far, I have not come across this problem.

Also, many of the "pro units" pack a wallop...250-400 amps for some of the Snap-On chargers equipped with the "cranking assist for boost starting".

Scares me!!!

 

[dave6666]

Gotcha Steve. And agree. But a few points to continue the discussion...

If the booster is true battery powered (it's charging circuit aside), then you have true DC power at the helm. If it is plugged into the wall then it is some hybrid form of DC. The el-cheapo battery chargers are nothing more than half wave rectifiers. Barely DC at that point.

Secondly, current has to have a path. It will not flow if it has no where to go. Electrical energy can make it's own path but it has to break down an insulator typically to do so. 12 volts does not really fit that bill. It will not break down insulation. Lightning which of course can break down insulation and almost every other non conductor, runs 1,000,000 volts or so.

Likely the highest risk of damage comes at the moment the cables are connected. In a perfect world you might disconnect the PCM, then connect the jump booster, then reconnect the PCM. Point is, there could be an instantaneous surge right when you connect. That surge aimed right at the drained battery and other milliamp devices like the PCM. But your battery is just as big (if not even more CCA) and it causes no issues when you connect it right?

Pure DC - unplug the jumper device from the wall socket - might be the kicker here.

 

[dave6666]

More thinking...

Maybe the reason that you could get a surge when connecting a booster versus installing a battery is that you have in the case of the booster, an electrical vacuum - the dead battery. Current wants to rush in to the dead battery and of course elsewhere. When connecting just a battery this condition does not exist.

 

[Purdue_Boiler_Viper]

Gotcha Steve. And agree. But a few points to continue the discussion...

If the booster is true battery powered (it's charging circuit aside), then you have true DC power at the helm. If it is plugged into the wall then it is some hybrid form of DC. The el-cheapo battery chargers are nothing more than half wave rectifiers. Barely DC at that point.

Secondly, current has to have a path. It will not flow if it has no where to go. Electrical energy can make it's own path but it has to break down an insulator typically to do so. 12 volts does not really fit that bill. It will not break down insulation. Lightning which of course can break down insulation and almost every other non conductor, runs 1,000,000 volts or so.

Likely the highest risk of damage comes at the moment the cables are connected. In a perfect world you might disconnect the PCM, then connect the jump booster, then reconnect the PCM. Point is, there could be an instantaneous surge right when you connect. That surge aimed right at the drained battery and other milliamp devices like the PCM. But your battery is just as big (if not even more CCA) and it causes no issues when you connect it right?

Pure DC - unplug the jumper device from the wall socket - might be the kicker here.

I still don't follow. V=IR. Ohms Law. So, I = V/R. You can have a bazillon amp capable supply, but it will only supply V/R amps regardless of its capacity. Am I missing something?

 

[dave6666]

Ohms law has nothing to do with it. It is simply the current path and the existence thereof or there not. The biggest battery in the world that is not connected to anything has no current draw. Potential I = humungo. But you can still connect that humungo battery to a flashlight bulb and it will happily supply the milliamp current the bulb wants without frying the bulb.

 

[dave6666]

Here's another example following the humungo battery - flashlight bulb theme.

My house has a 200 amp electrical supply. I switch on a 60 watt bulb that draws 2 amps and it does not blow.

OK. Now I know the wire to that bulb is smaller than the 200 amp wire and there's a breaker in there too, but current has to first have a path for ohms law to apply. Otherwise R = infinite.

 

[dave6666]

Also, while I'm on a roll, V with a wet cell lead acid battery is fixed at 2.2 volts per cell. A six cell battery produces 13.2 volts.

Therefore, given an actual current flow situation and without reducing or limiting the voltage from the battery, in the Ohms law formula, V is then fixed at 13.2 unless the battery goes dead or you over draw it.

Sorry man. I have many years of electrical training/education/experience so I lay it thick maybe.

 

[Purdue_Boiler_Viper]

Also, while I'm on a roll, V with a wet cell lead acid battery is fixed at 2.2 volts per cell. A six cell battery produces 13.2 volts.

Therefore, given an actual current flow situation and without reducing or limiting the voltage from the battery, in the Ohms law formula, V is then fixed at 13.2 unless the battery goes dead or you over draw it.

Sorry man. I have many years of electrical training/education/experience so I lay it thick maybe.

Let us go back to the origin of this thread. High Amp booster Jump - Nature of PCM damage? What would cause PCM damage? I have a few milliseconds of experience with electronic circuit damage. All of it is caused by heat, which is in turn caused by too much current driven through a given resistance. Think of your toaster. Voltage is constant (115V RMS AC), and current is dictated by the constant resistance of the toaster elements. You can hook up Three Mile Island to the toaster, but if the voltage is still 115V RMS AC, and the toaster resistance does not change, you will not blow the toaster. You can have 100 Amp service, or 200 Amp, or 50000000 Amp and it makes no difference. You still only have the constant 115V, and the resistance of the device in question.

I am assuming that PCM damage would also be caused by too much current. But such current is always governed by Ohm's Law. I =V/R. Without a change in V, or a change in R, a huge current source is irrelevant. I can hook up one billion Energizer batteries in parallel, and still, the current through the PCM, or flashlight bulb, or your finger, is dependent upon Ohm's law. If V and R are constant, if would not matter if you hooked up a 13.2V Booster, cat battery, or Nuclear reactor. The current flow would be the same.

Please explain further.

 

[dave6666]

Got it. Here we go...

Ohms law is typically thought of and used as a static law. Not that as things change Ohms law ceases to apply, but a resistor with XX amps going through it will have a voltage drop of XX is common Ohms law stuff. If you have a thermistor and you change the temperature then Ohms laws still is the way you look at the impact of the resistance changing as the temperature changes. And you know, thermistors change kinda gradually as the temperature changes. Certainly not a milli or micro second response device.

Let's move forward to a current surge. We are talking physics here which quite honestly extend beyond my training to be able to explain the movement of electrons, but we are talking huge orders of magnitude and instantaneous change. Ohms law ain't typically used when explaining the physics of that level of dynamic change.

Back to the PCM, Steve's concern (PCM proximity to jump post), and my analogy of the dead battery being key.

You hook up a jumping device - a huge current source - and the dead battery at the back of a Gen 2 car causes a huge inrush of current flow as it is dead, and hungry. The PCM - close by the origin of this inrush current - gets a "bump" from this current. Current has been "forced" where it would otherwise not be drawn. Pushed if you will.

Now you previously noted the heat factor. An elementary part of the power formula, P (watts) = V x I. If you force the amps, and the volts are fixed, the power to this device goes up. And in the case of the PCM the device is tiny little microchip sized stuff. Not much room for additional forced power to be absorbed, which of course must be dissipated by heat. BBQ...

That is why Ohms law does not make a good troubleshooting theory when talking about an instantaneous power surge. A power surge happens wicked fast, then forces current and imposes an increase in power to the device which has to be displaced as heat or if the device cannot handle it, melted device.

In the case of a memory bit or a program bit like the PCM probably contains, you could BBQ a data bit or code bit stored in a location in the chip. The current surge will find a path and if there are a bunch of these paths then the selection process may be random as to whom gets the BBQ sauce.

If someone has some physics training then please chime in as to the phenomena of current surge.

 

[Steve-Indy]

From my post in the paralled thread:

"Hi, Pat !! Thanks for jumping in here and in parallel thread. The "'boost to start" option exists on the commercial size chargers that normally charge a battery at anywhere from 35-70 amps (DC)...then, at the time one elects to start the vehicle, one flips the "crank assist output" switch to go 50-400 amps (though the voltage is reduced by my memory).

Dave, Pat, and All...I am NOT at all sure about this phenomenon, just sharing my concern about the possibility of damage under these avoidable circumstances. My "series" of adverse outcomes could be true, true but unrelated...but in each case, the Tech confirmed "dead PCM"...and the points along the path were ...car ran fine, battery dead, charged battery and flipped the boost-to-start switch, hit starter & cranked engine, no start...nothing...no spark & no fuel...all in Gen II's plus a Dakota or two. These vehicles do use the JTEC unit.

Most of you know, I really don't know my asp about electronics...just trying to pass along "clinical observations"...for YOU electrical engineers to sort.

 

[dave6666]

Sometimes Steve the empirical study trumps the theoretical study. I was just trying to rationalize how it might happen, and short of giving myself a class on inrush current phenomenas, have called it good noting that there is a need to be careful.

 

[DrumrBoy]

Dang, I knew Ohms law would enter the discussion....that's why I became an ME

Steve - yes, your post made me think about it but didn't realize until y'all clarified that it was the AC boosters that were the subject of the conversation. Mine needed repeated jumps from one of those handheld units (probably just a battery as Dave stated)....I had no idea how they worked but didn't assume they were just a battery since they're so small. Being a knuckle-dragging ME of sorts, I subconsciously thought they might be some giant capacitor or something.

The reason for so many jumps is the shop where it sat for 2 months (while the battery died) moves cars around every few days and used this handheld to jump it frequently.

I just changed the battery because it seemed to not take a charge from the Tender very well and all is good.

One more variable eliminated.

Anyone for Maxwell's equations?

 

[mjorgensen Woodhouse]

We have also seen this on the Gen1/2 cars and always thought it was the spark and surge when connecting the jumpers and or getting the cables back wards for the inexperienced "jumpee" lol.

 

[Jack B]

Lets try a different twist,

1. The battery has at least three branch circuits leaving the rear of the car.

2. One goes to the post at the pcm.

3. One goes directly to the starter.

4. When you attach a boost type charger to te pcm post the current path is back to the battery and then forward to the starter.

a. Since the conductivity (due to greater cross section) of the frame is greater than the actual battery conductors you are going to get significant current returning to the battery in parallel paths (uncharted) thru other than the actual conductors. If you have elctronics in that path you can have trouble.

My guess is that when a new car is developed, there are probably some that have had substantial issues with stray current, but, it is designed out of the car in te R&D stage.

5. That is not the only possible failure mode, in general, more electronics fail from a high rate-of-rise of current (di/dt). A voltage is developed (Ldi/dt) that will cause the failure of the semiconductor circuit. This is called a surge, transient or impulse and is measured in micro-seconds. You can detect this form of anomaly with a capture scope.

Probably most PCM's have a regulating type of converter(e/e) in the front end, this would be susceptible to failure from impulse. I personally would not connect a boost/start charger to any car with a PCM, that has the physical proximity of the vipers engine compartment's positive post to the pcm.