Good point about the gas in the sealed trunk. That isolator, BTW, is a couple of really expensive diodes, but it has a heatsink and some nice terminals and I'm sure it comes with instructions for those that aren't electrically inclined.

Just curious... Do you live in an area that's fairly warm, like in the South or Southwest? I thought I remember reading somewhere that sealed lead-acid batteries don't care for elevated temperatures (like in a closed trunk), it greatly shortens their life.

I couldn't fine a pair of 120A diodes and a big heatsink for less than what they sell them for off the shelf. And then add in the voltage sense circuit.

"Voltage Sense Circuit" Okay, well I'm lost. I visited the Surepower.com website and took a quick look at their offerings and saw no mention of a "voltage sense circuit". I'm really curious as to what this circuit supposedly does...

I bought one a few years back from Sportsmans Guide, I believe. It has the air compressor on it as well. Didn't cost more than $50-60 shipped (they're fairly heavy). The air compressor's fairly slow and noisy but works and the jumper's great. Supposed to be able to get several jumps from a single charge. Mine has held it's charge, and I've been surprised that they weren't more popular. They cost about the same as a year's AAA membership and you won't have to wait 3-4 hours for help to arrive. The prices do vary alot, but I'm not sure you get much more for your money by going with the pricier ones.

The E terminal on the 4 terminal isolators. GM and ai think some other brands alternators need that 4th terminal to be connected to an ignition switched source and a wire running back to the sense pin in the alternator. It works out well though as the alternator now measures the voltage past the diodes so it conpensates for the voltage drop in the diode.

Being a ham radio guy into portable operations, I keep a motorcycle battery sized sealed battery around and well charged. It's not always in the car, but if I'm headed to the sticks, I sure want it with me. It was about $80 and most of the time I just use a $25 2amp charger for it.

The sealed lead-acid batteries in almost all of those things need pretty tender loving care. Charge them with more than 14.4 volts and you cook them. Let them drain too low (~10.5v under load / 12v idle) and they're shot. I'm pretty careful with my batteries but I still fry about one a year now. Those cheap wall-wart chargers on most of those jumper packs are asking for trouble. You're better off with one of the packs that you can plug into the cig lighter to charge, since that's a nice regulated voltage source. (Any spikes when starting aren't too horrible for batteries, but a long-term over-voltage is fatal).