Peter 642

I've been told not to charge a dead battery with a running car despite the fact that "everyone does it." Used to do it on my 944 until a neighbor pointed out that at any point I could kill my electronics with a few stray volts. Today, passing a beautiful 993 stuck on the side of the road, recharging via another running car; I imparted said wisdom, suggesting kindly that it may not be ideal to have running cars used to charge dead cars. Owner of said car seemed unimpressed, thanked me, and went about his business, which in this case was having the "charg-er" vehicle step on the gas to get the motor running faster, to (I guess) re-charge the alternator so that more energy could be transferred to the "charg-ee," dead car's battery?

Am I wrong that this procedure is a strict and specifically in our cars, worse perhaps in a 993, that Porsche electronics would not be suited for this type of re-charge? Am I over-reacting?

Dennis C

I've never really heard of anyone doing this. My experience has been that once the dead vehicle starts, the cables are removed and the alternator charges the battery.

SSST

Was he charging the battery or trying to jump it? Modern electrical systems have a lot of protection built-in. If he was trying to jump start it then probably no harm assuming he was following proper procedures. It is an innefficient way to charge a battery though.

If he was stuck on the side of the road, I would guess he has a different problem than a bad battery.

logray

An alternator won't often provide peak charging capacity until at proper RPM. I've used this method many times in the past - it works. Of course, bump starting works too... but does the manual say you shouldn't bump start? LOL.

I remember one time working on a VW, with the steering wheel in my lap. I accidentally shorted a wire and it literally caught on fire and burned all the way into the instrument cluster. Lesson learned.

That being said I agree with SSST, live with the rest of us and do with the original poster saw a fellow owner doing (everything will be OK).

If you are **** and don't like to break things - unplug your battery and put it on a charger.

Peter 642

I suspect, given the absolute perfect condition of the car, that the good weather finally got it out of the garage and that it was making its maiden voyage for the season, and probably just needed power. I would think that running it at highway RPMs/speed for an hour or two is a safer way of charging, and that for the hundred bucks a Sears charger or other is just a bit safer, but it's good to hear that I'm probably over-reacting.

Logray, how on earth did you get your car to fly?

logray

Didn't you know that our cars can fly and drive on walls? Try it some time. :-)

As for the charging, the generator will not charge the battery like a "smart" external charger will. I've brought many batteries back to life on a bench with a good "smart charger".

But that being said, as you pointed out Peter # 642 - sometimes a good drive (and then continue to drive it every day) makes a battery last a lot longer than it otherwise would.

Quix

And how tall were those trees reflected in the windshield?

Eharrison

Well, on the side of the road I wouldn't think a second not to get a jump. How many times have I heard of someone regretting "that jump".

Oh and I've never heard of a few stray volts.... Reminds me of this....

Peter 642

Heh...heh...you said pendulum ball...

Yep; I'm over-reacting.

nick49

I think your neighbor had good intentions but no sound knowledge of automotive electrics. If you have a battery that is so low it won't start the car it most likely has less than 12 volts. A battery fully charged on a vehicle running at high idle or more could be maxing out at around just under 15 volts or there abouts. This voltage is the maximum regulated DC that any 12V system should ever see and all 12V accessories, motors, lights, etc are safely within their parameters of design consuming current at this voltage.

So jumping or attempting a charge on a low battery with a running car even at high rpm will not hurt anythin providing the cables are hooked up + to + and - to -. There are no stray volts unless the car doing the charging has a defective rectifier or regulator. If that's the case it would damage the car doing the charging as well. The battery can act as a buffer and absorb some voltage spikes if they were ever to occur which is unlikely except in the case of a diode failure.

sjfehr

Charging a battery from another car's alternator is no more dangerous than charging from the first car's alternator. You want to have the jump car running to avoid killing its battery, too.

One thing you should not do, though, is connect the black cable to the negative terminal on the car being jumped; if the battery has vented, the spark of the connection could ignite it. The entire chassis is grounded, so it's safer and just as effective to connect anywhere else on the car. Connect + first, and then - to metal elsewhere on the car.

nick49

That's right! Exploding batteries sending corrosive acid in every direction gets pretty messy. That's why Porsche on the 996 recommends jump starting well away from the battery in the engine compartment.

wwest

Either way, continuing charge after "target" start, or not, no harm can be done unless the donor vehicle has a defective charging system. Provided alternative methods are available charging a battery by running an engine is not a wise choice.

thirteeneast

10,000 Ohm's so what, If it said amps then I would not touch it.

thirteeneast

battery’s with dead cells still often read at 12v, But its the load that’s affected.
They will not have the cranking amps required to start the car.

Its all about the AMPS.