Priority Start

[kleeber]

Haven't tested this in the wilderness, just my driveway, but it has kept me from having a dead battery a few times now:

http://www.batterystuff.com/battery-products/accessories/PS12Vauto.html

Basically, it's just a solenoid that disconnects your battery when the voltage drops below a certain level, leaving you with plenty of juice to start your engine.

I got it because I have a late model car that will fail to "go to sleep" and will drain the battery in a few days unless you lock it with the key fob. By "drain" I mean so little voltage the dash lights won't even come on. (Dealer says this isn't a flaw...don't get me started on that one...)

The priority start is supposed to re-engage when you turn the key again, but my car doesn't do this--too many computer circuits, I guess. I just pop the hood, pull off and re-attach the little black wire, and in half a minute after I discovered my "dead" battery I'm on the road. Besides the convenience, it is keeping my battery from getting hammered.

[mil-dot]

we got these in an entire fleet of Police patrol vehicles.  They SUCK!!!!!   the batteries are usually dead in cold weather and still won't start.  Buy an OPTIMA battery and never look back

[kleeber]

The Priority Start disconnects your battery when the voltage falls to a low level (about 11.7 volts). If an unwanted current draw--such as lights left on, door ajar, etc.--is what is pulling your charge down, this will help keep your voltage from falling below that level.  If your voltage is falling because your battery won't hold charge or the weather has turned cold, it can't prevent it from continuing to fall. In that case, as you say, your best option is to have a good battery, a spare, or some other kind of backup plan.