potdog

Hi all this little kitt realy works ,just had to used it left parking lights on nearly flat batttery starter just clicking ,connected to jump post and ear
[IMG]https://cimg8.ibsrv.net/gimg/rennlist.com-vbulletin/2000x1124/80-wp_20160729_07_47_42_pro_ba98ebaa771105a72dca2de125a4a4

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th on bolts on body cross brase fired up no problems.

Mrmerlin

I will caution you on using any type of jumping device for your 928,
since it has lots of cool electrical systems ,
adding outside electrons to mix with the low battery condition,
can spell the end of computers and diodes in the alternator.

Best way is to disconnect the battery and charge it for 4 hours with a 6 amp charger

NOTE a trickle charger is not a battery charger if the battery is low it needs to be charged with a battery charger not a trickle.

NOTE the alternator is not a viable way to recharge the battery though you might think that if the car starts the next time everything is good,
best to recharge the battery first then put it back into service.

NOTE running the engine with a low battery can damage the alternator.

NOTE read the WSM for battery charging advice.

NOTE dont offer to jump another car with your 928 the electron spike can kill a engine computer or damage the alternator

hacker-pschorr

Compared to new cars the 928 is a vinyl hand crank record player.

When used properly, an isolated external battery for jump starting is perfectly safe, if not all my 928's would have been fried years ago. Follow the instructions, most have an on/off switch that should not be powered on until the leads are connected.

I agree, never ever use your 928 to jump start another car and only use another vehicle to jump start your 928 if it's an emergency.

I always use the battery terminal under the hood versus connecting to the dead battery.

I have these two:

https://www.amazon.com/Anker-PowerCo...ker+jump+start

https://www.amazon.com/Booster-ES500...+start+battery

Mrmerlin

Eric I would suggest to read the WSM for further cautions from the factory

hacker-pschorr

Nothing in the WSM is going to prevent me from using an external fully charged battery to start a dead 928. In fact, you have a much higher chance of blowing something up trying to start a 928 with a slightly undercharged battery (but still enough to crank the engine) than connecting up an isolated battery pack.

In fact, the first step of working on a late model BMW is connecting an external battery while doing any diagnostic work to ensure voltage does not drop below optimal. Flash a BMW ECU without perfect voltage you will fry the computers.

I practice the same when using a Shark Tuner on a 928 that isn't running, connect up a charged external battery to make sure the ECU's are getting full voltage when flashing them.


I must admit though, I do not fully follow the WSM since I do not stand 10 meters away:


-

jcorenman

Eric, that picture is for detonating airbags
The reason there is differing advice is that there are different types of battery chargers.

"Boost" chargers (or quick-chargers) were commonplace when the WSM was written. These were simple transformer/rectifier devices with no regulation, which dumped a lot of charge quickly by applying a high voltage to the battery. If the battery was in good condition but discharged, this worked OK because the battery absorbed all of the available current and the voltage didn't get very high. But as the battery charged, or if it was toast (with high internal resistance) then the voltage went up to a level that will fry modern electronics (the 928 boxes included).

Most modern chargers are regulated for both current and voltage, and won't exceed approx 14.5 volts no matter what the circumstance. Same with a charged battery used for boosting or jump-starting, the voltage simply can't get higher than the voltage in the donor battery, typically 12.8v for a fully-charged lead-acid battery.

And same with a jump from a "donor" car, even with engine running at high RPM's the battery voltage won't exceed 14.5v or so-- assuming everything is working properly. But you never know for sure with someone else's car, so this would be my last choice.

The WSM does indeed caution against boost or quick charging, these are the old unregulated chargers. Using a modern regulated charger is perfectly safe in any size, as are any of the various isolated 12v external batteries and jump-packs.

FredR

This thing about jump starting frying LH computers and the like- is it an internet myth type of thing or a proven/well established fact that such action does indeed fry computer components?

I tend to trust opinions expressed on this list without too much question but fact is a tired battery does produce less voltage and as I have experienced all too often over here a tired battery inhibits starting and voltage during cranking can drop to 10 volts or less. By definition the generator has to provide more voltage than the battery or current will not flow into the battery that acts as an accumulator/capacitor- correct? Thus if you have two batteries cross connected at two different voltages logic suggests there is going to be quite an in-rush current until the two pieces of kit come to equilibrium. whether or not our systems can withstand this seems to be a bit of a pot luck type of thing. I have managed to avoid jump starting the 928 but that was more good luck than management.

Over here batteries last little more than two years before the heat gets to them. Tires fare a little better at 3 years but we also have some advantages [virtually no cold starting/running].

It was nice and relatively cool for this time of year this afternoon and I had a nice run in the 928 with the a/c freezing cold and the motor spinning nicely. My rear rubber is well past its sell by date so quite nice trying to get it to break - which it did not!

Rgds

Fred

hacker-pschorr

chart928s4

I use a Harbor Freight Centech 4 in 1 battery pack all the time on my car. I was nervous at first, but I measured the voltage and it was 12.3v. It works perfectly as a charger and a standalone jumpstarter - actually just a small 12 volt lead-acid battery with the 20 lbs of weight to prove it. It is 1000x easier and less nerve-wracking to simply turn the switch on and off than disconnecting/reconnecting the real battery during any electrical work. I too use the jump post under the hood. (Why else would it be called a "jump post" anyway?)

For the emergency kit, I bought a lithium Genie that is the size of a big steak. I have not used it except in tests off the car. It has circuitry designed to detect a low (down to 5v) batttery before allowing current flow, so you can charge the battery and also start the car. It might be possible to turn it into into a regular 12 volt power supply with a modified accessory cable. It is also exactly 12v and has an on/off switch, and the charge lasts for months. Specs are roughly the same as the OPs battery pack.

GregBBRD

All S4 control units will eventually die, and this always happens immediately after a no start/jumpered battery situation....always!

We follow Porsche's recommendation and never jump start an '87 or later car. Never.

I love it when people ignore Porsche's warnings and other expert 928 advise....Mr. Merlin has forgotten more about 928's than most people will ever know.

Pay attention to the experts....ignore the Internet and what you think is correct.

chart928s4

I am certainly one of the people who know less that MrMerlin has forgotten, and this is the first I've heard that these 12v devices (or jumpstarting) can fry circuits. I have read a lot of the WSM but I just have the PDFs which are hard to search - where is the section on this topic? Does it also preclude a trickle charger/maintainer?

Speedtoys

Not the first time ive heard/read a thread on it, but have never, ever, read a thread about it happening. And if it did happen once, of even 5 times, in the 20yrs of RL/etc, it's so far in the statistical noise anyhow.


Cool warranty cost-avoidance trick tho Porsche did.."Don't do the thing, cuz you'd want a new one _if_ if happened, if you did the thing..and we'd have to pay for it."

Bill Ball

If you are using a regulated source or a 12V jumper battery (like the HF item), how does the ECU know it's not your battery? I thought Jim Corenman explained it all pretty well above.

I'm not sure I understand Greg's comment that an ECU failure "always happens immediately after a no start/jumpered battery situation....always!" I've never jump started my car, but the LH failed while I was driving to work one day.

Speedtoys

There are..hundreds of millions of cars out there that share the same sensors, and darned _near_ the same ECUs from Bosch.

I see nothing unique in the 928 here, or particularly fragile in such a tremendous sample set of Bosch instrumented cars out there of the same era.

dr bob

The components in the fan controller are 16 Volt rated. I don't remember the rating for the LH or EZK controllers but might guess they are similar. From foggy memory from designing such pieces way back when, the power supply side typically has a "crowbar" for mild over-voltage spike protection. This includes a Zener diode and a few components like capacitors that deteriorate over time.

Meanwhile... The Bosch alternator in my car can make about 16.5 volts with the regulator messed up. The factory rating printed on the original regulator on my car is 14V. I remember seeing folks recommending that you find a regulator with a higher rating to try and get more charging at lower RPMs. Seems like all the critical and connected pieces are each chosen carefully so they don't fail from overvoltage, don't cook the battery but have enough voltage available to completely charge it, and still run the car.

When you jump-start the car from another while the 928 battery is discharged, the electrics in the car enjoy a roller-coaster of voltage abuse. My DD runs closer to 15 volts all the time while the engine is running. If I use actual jumper cables, that same 15 Volts can appear at the 928 battery terminals. If I immediately start the car, the 928 alternator sees the higher voltage and offers minimal excitation until the cables are removed. Then it goes from practically zero to full excitation when presented with the discharged battery and the load from the now-running car. There will be a spike, related to the capability of the alternator and how fast the regulator responds the the actual recovery in terminal voltage. It can easily be big enough to damage the relatively delicate electronic modules. Porsche recognized the risk, and for this and other reasons recommends against jump-starting the car. We lovingly refer to that connection under the hood as the "jump post" but it really shouldn't be used to try and directly start the car.

----

I just pulled a batch of AGM batteries out of the various UPS's here at our home. The one that protects the servers and another that protect the networking equipment each have a couple 18AH batteries in each box. They get replaced on a PM schedule. For grins, I connected a Schumacher 1.5 amp-capable battery maintainer to one of them, with a peak-holding DMM connected at the same time. The maintainer pushed the battery to a terminal voltage a little over 15 before it relaxed into holding mode at a little over 13. I'd cycled this maintainer out of wet-cell maintenance duty because it liked to cook the electrolyte out of the smaller 7AH capacity batteries in a few of the water toys. I'm pretty sure I don't want the 928 electronics exposed to the 15.3 peak volts, so this charger/maintainer is now relegated to the spares drawer.

----

In the ride-along tool bag for the 928 there's a set of mini booster cables made from 20 feet of 10ga speaker wire with minim battery charger clamps on either end. I added them to the bag after an early Sharktoberfest Malibu Canyon fun run, when a participant needed a boost to start at the launch point. I figure that the wire size, length, and the mini clamps will limit the amount of current and therefore voltage available while "boosting". For better or worse, those have never needed to be used in 928 service since.


Would I jump-start my car from another running car today? No. Would I connect another car with the mini booster cables for a while to help restore charge in the battery while out on the road? Sure. Just disconnect the cable clamps before cranking, so there's less chance of a spike that might damage some delicate and expensive bits. Or better, lift the ground strap and restore the battery charge level correctly before reconnecting and cranking.