A very good article in the most recent Excellence Mag about dead batteries and the resulting potential problems ( too many to mention here).

I have found the same issue replacing my Audi A4 battery, where I had to "bridge" the power from the new battery with Booster Cables during replacement ( my wife had to help here).

This begs the question: would a "battery tender" or the like, a kind of trickle charger, plugged into the cigarette lighter (thanks for still having one...), be enough of a charge/maintain the Porsche ECU's and Radio codes intact?

That way changing batteries is not a problem!

Anyone on this list have any experience doing this? Any help for those of us who are forced to put away our 993 for the winter.

Air cooled Regards,

Bert
Calgary Alberta
Canada

 

Without carefully examining the open circuit voltage output of the "tender" and making sure the voltage does not go even momentarily too high, you are risking damage to the car's electronics. I have heard of people getting away doing this but I wouldn't risk it.

There are adapters available with a diode built in that use a 9V battery and plug into the cigarette lighter. This will work fine for a short "bridge" interval like changing a battery.

 

The first thing you have to do is check if the cigarette lighter is "hot" all the time and not switched by the ignition switch. In the case of my 993 and probably yours, it is "hot" all the time.

I use my spare 12V motorcycle battery to power my 993 through the cigarette lighter. You can do likewise by going to "The Source" and picking up a 10' cigarette lighter extension and connecting a pair of larger alligator clips (colour coded, of course) and wiring them to the extension. Connect them to a spare battery when the time comes. Just another item for the tool box!

 

A 'battery tender' or the Porsche Maintainer plugged into the cig lighter will provide enough current to the bus to keep the radio codes active, etc. Just make sure all the interior lights are off as well as the engine and trunk lights. They will draw too much current and will 'trip' the Porsche Maintainer. It will then take a while to reset. It has an internal thermal overload.

 

As I said, running the car electronics with the battery out and just a "tender" is risky. It all depends on how high the voltage goes without the battery load. I wouldn't do it.

 

Modern car electronics (e.g. European spec) are designed to survive 28VDC applied for 2 minutes without permanent damage (this is called the double battery jump start test, in case a truck jumper pack is connected to a car by mistake).

In addition, there is always transient overvoltage protection, due to the presence of high voltage (up to 100V) load dump pulses on the B+ line.

That having been said, I would imagine that almost all modern electronic output battery charger/maintainers are safe to connect to a vehicle without a battery.
I know many people on RL who have used the Porsche battery maintainers to keep radio codes in tact when changing batteries.

FWIW, my wife's Targa had the battery go dead this week, I didn't see any major problems with the car after removing the old battery and installing the new one.

 

Went to an Optima type AGM battery several years ago====no problems since!!

 

Chuck is incorrect. The bus doesn't care what the voltage source is. A Porsche maintainer, for instance, cannot cause damage to a vehicle's electrical system. I have used my maintainer for months with the battery out of the car to keep codes intact. It's ampacity is limited, so it will trip (and then reset when cooled) if overloaded.

 

It has nothing to do with the current capabilty. It will have current control to manage battery charging but when the battery is removed, it may not have the control to limit how high the voltage goes. Some maintainers may be OK and apparently are but without knowing the open circuit voltage specifications, you are taking a risk.

 

My reading on the Excellence article was that if your car had some problems, removing the battery would cause difficulties when you reinstalled it. If you did not have issues, then it would not be a problem to do so.

I have removed my battery several times to replace it, and other than having to reset the radio codes, there were no difficulties. Be aware that you cannot pass a (California) smog test after doing so until the readiness codes are restored in the OBDC - mine took < 100 miles to reset.

I do keep a battery tender on both cars when not driving them. The 993 seems to have a much higher battery drain when parked than the 997.

 

Read my post # 6 again.
BTW, I designed electronics for Porsche a few years ago, so I'm somewhat familiar with this area.

 

Battery charger/tender designs will vary in their quality. Even if they were designed with control to limit open circuit output voltage, a line transient could easily pass through to the car electronics if not supressed properly and exceed the alternator load dump voltage limit.

I'm just saying there is no way to know if a particular charger/tender design takes this into account. Some may be just fine but others not so good. I think we can agree on that. I prefer to see a written spec. before using one without a battery.

BTW, I also designed auto electronics (for Philips).

 

I find it hard to imagine that an OEM module would get into a car that did not have even basic TVS protection. A battery maintainer simply wouldn't have enough stored energy to blow out a front end TVS protection device on a B+ line. Just can't see it happening.

 

I'm talking about the transient protection, if any, on the line operated charger/tender. In the worst case, what kind of transient voltage and energy can pass through to the car on a lightning strike. Not all chargers are designed with this in mind. I've said my piece.