Jump starting a BBi

Hi Henry,

Boxers are safe a

Hi Henry,

Boxers are safe as long as you don't hook the cables up backwards. Don't laugh folks, it happens by DIY'rs far more than one might think!

The Alt. & Digiplex will be fine. The jump starting issue mainly came about with the advent of actual computers, because they operate at 9 volts and not 12.

Once the engine is running the jumper must be quickly disconnected or shut off to prevent the voltage from the jumper/charger from over powering the Alternators charging output.

HTH's

Regards, JRV

Drew,

you may be right. Vol

Drew,

you may be right. Voltage spikes are dangerous to certian auto components.

This is a good subject to get to the bottom of.

Henry,

TR's are some on the cars with computers that shouldn't be jumped.

This exerpt is from the old FA

This exerpt is from the old FAF "Ferrari Owners Survivors Manual" circa mid 80's that was their parts catalogue and had assorted technical tips.

"Why Do Dinoplex units Fail?"

"The Marelli dinoplex unit is an extremely high voltage output device that performs excellently most of the time. Its only fault is it seems to be very sensitive to high input voltage. Most problems seem to occur soon after a dead battery has been jump started. Problems can also occur if the car has been sitting long enough for the battery to drain down, when the car does start, the alternator puts out high current and voltage and sometimes the regulator apparently doesn't catch the over-voltage situation soon enough to prevent damage to the Dinoplex internal circuitry. Because of this apparent sensitivity to high voltage surges from either a jump start battery and alternator or from the cars own alternator, we strongly recommend that you keep the battery fully charged. Thus, if your car sits for long periods use a charger, after disconnecting the battery from the car, before starting."

I have always gone by the above rule. I would think that somewhere along the line with going from maybe the Dinoplex unit to Digiplex to whatever is being used currently the problem got solved.

Anybody?

Drew,

Honestly it makes se

Drew,

Honestly it makes sense to heed that advice. I fully concur that dead batteries are detremental to several systems and components. The overcharge they refer to is know as Full Fielding, this is a state where the Alt. is charging to it's maximun ability which can be as much as 14v in some cases depending on Alt..

Thanks for digging up that info.

Regards, JRV

I believe that there is much f

I believe that there is much folklore about the dangers of jumping from a second battery. JRV observation noted (wrong polarity), the main power bus of any car cannot tell the difference between the 12(+) volts of the dead battery or the battery used to jump the car. In fact, when the two batteries are placed in parallel, there will be a current flow from the higher voltage battery to the lower voltage ("dead") battery, effectively charging it. Nonetheless, the system will not see any "spikes" or "transients" that are not already inherent in the car. Leaving both batteries connected for a period of time will do no harm, other than use some of the alternators current output to charge the jumper battery, which is not desired. One can make a case, however, for leaving the second battery in parallel for a few minutes to avoid the regulator sensing low voltage and inducing the field current to result in 40+ amperes output of the alternator, testing its upper limits of diodes and armature windings.

Long story short, no harm to be done by using a battery to jump start most cars. I qualify "most", and would have stated "any" car, but JRV suggests that the computers in modern cars use 9 volts. I have not heard of this, and am perplexed as to why the engineers would design the black box to use 9 volts. But I get ahead of myself. Most computer logic (and I assume they use off-the-shelf memory and logic chips) prefer 5 volts (TTL standard). CMOS chips have a wider range of input voltage and power voltage and are designed for voltages from 5 - to 12 (approximately based on particular chip). I mention black box because the logic in the box requires a rather constant voltage, be it 9 or 12 volts. Thus, they most certainly have a voltage regulator (different than the alternator voltage regulator) that filters the varying voltage on the bus (11 to 14, lets say) and maintains a constant 9. They do this with capacitors, zener diodes, filters, etc. The engineers who design automotive computers know that batteries go dead, or idle speed drops momentarily when a clutch is let out at a stop light, in the rain, with the wipers on, at night, with the brights on, and the air conditioning fan blowing full blast on the windshield, with the blinker on, and the radio, with the cigarette lighter in. (You get the picture). If the computer could not tolerate this transient drop of bus voltage to 11 volts, than the car would stall. Not a good thing in the rain.

Long winded. Sorry.

Jim S.

Jim,

Agree about using &#34

Jim,

Agree about using "a battery" to jump another car.

The problems "can" (as in there is a possibility however remote) occur when another car or a Battery Chargers "jump start" function is used.

Jump Starting was safe & common with cars that had generators & no electronic components. With the advent of Alternators & Multi-Electronic Componitized cars jump starting took on an element of risk. The problems occur not when charging a battery or even hooking up the cables backwards, but when the key is turned on, on the dead car. It's systems are then at the mercy of accidental reversed polarity or a voltage/amperage that it may not be compatible with, such as an Alt. with much higher output (70-80-90amps) than it's stock version/s.

The other issue of dead batteries is the excessive heat components produce when attempting to operate at LOW Voltage/Amperage Levels. That heat is sometimes 3-5 times as high as the same duration of operation at normal electrical levels.

The long and short of it is that there are several different concerns and reasons why jumping is risky on electroisized cars.

Am I full of it?

Regards, JRV

JRV - I hope that I can help h

JRV - I hope that I can help here, from an electrical engineering perspective. 1) Your concern regarding reverse polarity in a dead battery versus reverse polarity in a well charged battery is interesting, but I am unable to assess the consequences without a great deal more information. Not trying to evade the issue, but the voltage seen on the power bus (I use this term to describe the vehicles +voltage side for all equipment) will be the algebraic sum of the two battery voltages (when placed in series and reverse polarity). Thus, if the bad battery is at 8 volts, and the good battery (12.5 volts) is connected in a reverse polarity, the bus will momentarily see –4.5 volts. Momentarily because one of several things will happen; a) the offending technician (I try to be benevolent) will evacuate one of several body orifices and realize the mistake, or 2) the lead post or plates will get so hot as to melt and/or expand rapidly (explode). This technician has created an infinite current situation that will last for only so long.

Your concern about the alternator of the helping-car putting out too much current (70-80-90 amps) is quite valuable from a pedagogical perspective. This is a common misconception that current drives issues, or voltage drives issues, but it is important to understand when a source of electricity is a constant current source or a constant voltage source. A load by definition will only draw as much current as its apparent resistance will allow. This is very important, and if you understand this, then you know more about Electrical Engineering than I learned in 5 years of higher education. A load (a computer, a spark plug, a radio, head lights, etc.) has an apparent resistance. Each device has two wires. Disconnect these wires, look into the device sighting down the wires, and what resistance do you see? I am trying to make a mechanical metaphor, but simply stated, each load looks to the circuit as a resistance (in direct current) or impedance (in alternating current). How much current the device will ALLOW (draw) to pass through it is defined by the simple equation known as Ohm's Law; current through the device will equal the voltage applied divided by the device resistance (load). Now here is the important part. Nowhere in this equation does it suggest that a source (of current) that puts out 70 amperes will PUSH less current through the device than a source that can put out 90 amperes! The only caveat is if the source is a CONSTANT CURRENT SOURCE, and this device in question is THE ONLY LOAD IN THE CIRCUIT. In that scenario, yes, whatever current the source puts out as a constant current source will be rammed through the load (with certain qualifications).

Alternators are current sources (NOT VOLTAGE SOURCES). So it would seem, that all of the above discussion should be for a constant CURRENT source. Here is the trick. All of the current is directed to the battery, where the current moves electrons of the lead acid plates, causing the battery voltage to rise. IT IS THE BATTERY THAT SUPPLIES VOLTAGE TO THE BUS, not the alternator. The battery wants to look like a constant voltage source, and Ohm's law applies (Voltage=Current x Resistance; rearranging, current=voltage/resistance).

Now to your thoughts about low voltage and hot components. Unfortunately, it violates the Second Law of Thermodynamics (I never liked that law anyway). The statement suggests that by applying less voltage you can generate more heat (no no no, can't do that). What I think you are trying to say is that for devices requiring so many Watts of power (the product of current and voltage), that when less voltage is applied, you must deliver more current. Heat loss in a device is the product of the current squared multiplied by the resistance. More current, more heat. But, if the load looks like a resistance, and you lower the voltage, then by Ohm's Law you will have less current flow and less heat. Yes, there is a voltage below which computer components won't work, but it is not due to heat.

Now on to Jeff Ryerson's input. The current that is used by the various loads in the car is supplied by the battery. The alternator simply raises the battery's potential (read voltage) to supply that current. Your concern is well founded, but for a slightly different reason. I do not like to jump-start a dead battery for only one reason: the low bus voltage will cause the voltage regulator to send high field current to the alternator, which will flog itself in an effort to put out current to charge the battery. Now, mind you, the engineers that design this stuff are pretty smart, and I trust that they designed the alternator to put out as much current as it can for as long as it may need to, but diodes and shellac insulation (windings) are not great fans of heat (high current). Thus, when I jump-start my car, I am careful to watch the ampere meter to keep the charging current below 20 amps if possible. It will only take a few minutes for the battery plates to be sufficiently charged that the charging current will fall, but I prefer that the regulator does not ask the alternator to pump out 80 amps for a long period of time. It's okay at a stop light for a few seconds after idling, but not for 5 minutes.

Whew, time flies when your having fun. Hope I haven't over stepped my invitation.

Jim S.

One of my favorite bumper stic

One of my favorite bumper stickers of all time is;

"Repeal Ohm's Law"

As in many areas of life, the laws of physics must be tempered by the creativity of the user. Stated in the alternative,

"Everytime they make something idiot-proof, we find smarter idiots."

I have yet to see jumping a car hurt anything, unless the polarity is reversed. I have seen hundreds in my short lifetime (having been born in the first half of the previous century).

Jim S.