Quote:
Putting two alternators in parallel should be a good thing for voltage supply, but since both are limited to supplying 14v you only get 14v. Spikes occur all the time while the alternator is running because you rectify an AC to DC (if higher order harmonics is what you'd call a spike).
Uffe, you seem to be a reasonable fellow judging from your posts. Since I enjoy an honest technical discussion a well as anyone, I'll share my impressions on your statement quoted above. I'm open to discussion on it from you, in fact I'd welcome it. For others who are bored by this kind of post, be warned! It's long!
It sounds as if your talking about steady state dc voltage, at equilibrium, but not accounting for a period of time when the current surges between the two systems, before they reach voltage and current equilibrium. (I've read documents that state that most electronic stress and eventual component failure occurs during initial inrush current stages of power-up). Certainly the alternators are not connected in series, adding their voltages to 28 - 29 volts, but they are feeding two different different batteries, but a common load, with different voltages for short periods of time, connected by cables with resistance, inductance and capacitance. That common load of parallel connected batteries will experience an initial huge inrush of current when the dead vehicle starts and then begins charging, adding its output current in parallel to the other charging system, when measured over microseconds and even milliseconds. The voltage rise that occurs when that increasing current rush encounters inductive components will produce inductive spikes that can reach devices on the system bus without being fully neutralized by the capacitance of the battery, as well as minute periods of time of overvoltage. The spikes will ride on the dc voltage component of the power bus. There will be minute amounts of time, measured in milliseconds or longer, before the alternators can reach their design regulated voltage target, when the voltage can and does surge much higher than it should because two charging sources end up producing excessive current into the same parallel battery load. That is what I'm referring to, not the voltage as measured over a 1, 5 or 10 minute sample period. If the short duration spikes encounter series or parallel properties known as resonance, the spikes can be thousands of times greater than the input voltage, which fortunately will be damped quickly by the battery, but it will exist for minute amounts of time. It works everyday in RF amplifiers, with much smaller currents, by design. If it happens in your charging system to the point of early death of your ECM its bad luck, but still using the same principles.
If this period of non equilibrium were not possible or even common, there would be no phenomena known as hysteresis effects in amplifiers, voltage regulators and other systems where feedback loops take a finite amount of time to stabilize and not oscillate to some degree. In engineering terms I've seen written many times, behavior during this transition period is designated as "undefined." (This effect exists in many systems - in RF systems, one term that describes a variant of this property is AFC hunting. It's effect is similar to your vacuum cruise control, once set, taking some time to reach the desired target, as it overshoots, then undershoots, then finally reaches target speed). The effects of this are short lived(hopefully) and are why you don't suffer electronic voltage damage every single time you jump start a vehicle. There is likely some voltage protection within the ECM, TCM and other sensitive components, but obviously not enough to withstand reverse polarity or excessive voltage. You can prove this without any specific knowledge, but it will cost several hundred dollars. Take your ECM out of the vehicle, connect it to a bench dc power supply and crank it up to 100 volts. Leave it that way long enough and it will permanently damage some internal components, because it's being operated outside of its design parameters for input voltage, similar to whats occurring when jump starting incorrectly. The 100 V pure dc current bench setup will just produce failure faster and more reliably. Since Chrysler doesn't specify what the voltage protection is, in terms of voltage, power and electrical heat dissipation, we simply don't know how well it's protected. They do specify how to jump start a vehicle, right in the owners manual, which should be more helpful to most. You'd think they would spec a self-healing crowbar circuit with a time delay on the power leads inside of the ECM. It would certainly help.
There is a black box way to compare what happens during jump starting, using parallel connected D.C. power supplies to a common load, which is what the two alternators represent in the vehicles connected by jumper cables. Look at the manufactures requirements for connecting parallel dc power supplies, if they don't have a regulating module built-in, designed for that purpose. They'll void your warranty if you do that on their new equipment. Why do the manufacturers state to not connect them in parallel to the same load, unless a load sharing module is used, if they have one available? They will describe the oscillations that can occur between the two DC power supplies via the common load path, as uncoordinated voltage regulation occurs, that can damage both power supplies and void their warranty.
Regards,
Ranger1
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