I hate it when my battery dies! I always have to get a jump from a stranger, so I bought one of those cigarette-lighter chargers (a great idea, but this is a common enough problem that it shouldn't have to be bought separately).
Cars should have a built-in "emergency battery" that is connected when a switch in the dash is flipped. It just needs to be enough to start the car so the primary battery can be charged. When the car has started, the switch flips back to the primary battery. Dead battery is no longer dead!-- jivetalkinrobot, Jul 29 2003 (?) Battery Switch http://www.bep.co.n...t.cfm?productid=572Just put this and another battery in your car. [Cedar Park, Oct 05 2004, last modified Oct 17 2004] BatteryMinder http://www.vdcelectronics.com/A type of built-in battery charger [jeffman, Oct 05 2004, last modified Oct 17 2004] You need the emergency wind-up battery recharger, which we discussed from time to time.-- DrCurry, Jul 29 2003 Switches for secondary batteries are very baked in the boating world. I don't see the invention here.-- Cedar Park, Jul 29 2003 "deep cycle marine battery?" Is that just a ni-cad?-- bristolz, Jul 29 2003 I seem to remember a car battery that was advertised with its own built-in switch in case you left your lights on. In this case, you open the hood, switch the switch on the battery, and drive off. Don't know what type that was, though I would suspect that the reduction in power required to have a 'little battery' wasn't acceptable.-- Zipwow, Jul 29 2003 lead acid batteries have a self discharge rate. if the primary is dead, then so will the secondary. unless, of course, the primary is dead because you left the lights on or something. in which case, you need a feature that saves the batter rather than a spare.-- barkinghugh, Jul 29 2003 Completely agree with [barkinghugh]. I have an intercom unit with a back-up battery, but that uses alkalines.-- FloridaManatee, Jul 30 2003 How good does the batteryminder work if anyone has one the website says you could leave the battery plugged in to the batteryminder for years and the battery would remain as good as new. (see Link)-- jeffman, Jul 30 2003 My car has a lights left on feature built in already. Turns them off after about 5 mins or something without key in ignition. Plus its got a little beeping thingy too. So as for saving part of the existing battery its baked well an truely.
However I presume this is more of a solution for cars with knackered alternators/poor batteries. In this case (and this is if you really just dont want to replace the respective part) Maybe a giant capacitor that charges up at the same time as your normal battery (via diodes or whatever to ensure its still charged when u stop) but does not power any functions in the car (those diodes again) and you have your little switch there and one quick flip and you've got enough power for one shot at starting.
You would have to be careful with the whole capacitor charging the sick battery but that could be delt with i think.-- fusion_cat, Sep 23 2005 random, halfbakery