Portable battery pack for jump starting 911?

M491Cabriolet · 04:04 PM

It's happened more than once that I've left my Porsche 911 in the office parking lot with the parking lights on, stupidly killing the battery by the end of the day. Since asking colleagues to jumpstart your car isn't a good way to make friends and influence people, I was wondering if carrying one of those rechargeable battery packs in the trunk might be a good idea. That way, I could just hook it up to the battery in case of a dead battery and jumpstart my own car. I don't mind the added weight on those days I use the car to get through traffic to work. I saw some of these at Sears, but I don't know which amperage i would need and should get. My bigger engine is a 3.2. Sears sells a 750 amp, a 950 amp, and an 1150 amp model. These amps are not cold cranking amps (they tell me). Does anyone know which model would work for the purpose above? Thanks for any information.

304065 · 01-05-2008, 04:04 PM

Mauro, I don't recommend that in anything but the most dire emergency.

Here's why: an alternator is designed to bring the battery back to 100% charge from about 95% charge, such as it might experience from a long cranking period on a very cold day.

When you ask it to bring the battery back from 50% charge, it is likely to overheat and blow the diodes.

HEAT is the enemy of electrical components.

Rather than investing in the backup battery you would be better off to buy one of those boxes that interrupts the current when the battery voltage drops below a certain amount. That way, the load on the alternator is less when you start.

Again, in an emergency fine, but it has to be the kind of emergency where you are prepared to write a check for $1500 to replace the alternator, battery and all the labor when the thing cooks itself trying to do a job it wasn't designed for.

M491Cabriolet · 04:04 PM

John,

Thank you VERY much! That box that interrupts the current when the battery voltage drops below a certain amount sounds super cool. Do you know what they are called and/or where i could buy one (or two)? Again, thanks for helping me avoid making a mistake. I owe you one!

rnln · 01-05-2008, 05:44 PM

Quote:

Originally Posted by john_cramer

... you would be better off to buy one of those boxes that interrupts the current when the battery voltage drops below a certain amount....

Can someone elaborate this a little more detail? Although I never heard of this but what I understand from the above is some device to connect between the batery and car components. The device is smart enough to cut off its' switch when the batery is below a certain voltage, but the voltage is still high enough to start the car, to save the batery's voltage. And when you come back to the car, you still can start the car up.
Did I get the idea right?

richde · 01-06-2008, 05:22 AM

Wire the lights so they only work when the ignition is "on?"

ljowdy · 05:54 AM

this is one of several "low voltage cut off switches" on the market.

"They work!!!"

http://www.batterymart.com/c-prioritystart.html?utm_source=google&utm_medium=ppc&utm_term=prioritystartcampaign

Gator · 01-06-2008, 05:38 AM

I have been using the 750 amp Sears model for about 3 years. I honestly have never used it on my Porsche but I have used it on many other cars and trucks. I always have it in my 72 pick up in case I get a flat, need light or have a run down battery out in the woods. It also comes in handy down here to re charge your cell phone or run a small tv during the hurricane power outages. Oh yeah, I also carry a tire plug kit when I go to the deer woods.

dagriff · 01-06-2008, 10:12 AM

Quote:

Originally Posted by Gator

I have been using the 750 amp Sears model for about 3 years. I honestly have never used it on my Porsche but I have used it on many other cars and trucks. I always have it in my 72 pick up in case I get a flat, need light or have a run down battery out in the woods. It also comes in handy down here to re charge your cell phone or run a small tv during the hurricane power outages. Oh yeah, I also carry a tire plug kit when I go to the deer woods.

Totally agree.
Get one anyway, you won't know how useful they are until you have had one and it dies!
I just replaced my 10 year old one (My fault, I fried it..) with a 700amp that has a built in light & two "Cigar Lighter" outlets. The night I got it we had a power out.