My car has a problem that there is some sort of load on the battery even when the engine is off. So the battery slowly drains with time. If left for more than a couple of weeks, the battery will be too drained to start the car. I had this happen to me, and so I bought a mobile booster pack (i.e. external battery with built in jumper cables and ability to connect to mains AC to charge) and boosted it. I drove it for a while and then had to leave the car parked for another long period of time*. Since I knew I would have to do this, I disconnected the battery until such time as I was going to start using it. After reconnecting the battery three days ago, the car started on the first attempt, and so I took it out and drove it for an errand, gassed it up, and brought it back. But today when I tried starting it, it wouldn't start at all. There wasn't even enough charge to light up the dash instrumentation. The battery had only been reconnected for 3 days and was already completely drained. This tells me that the battery is incapable of retaining a charge and must be completely dead. I took a voltmeter to it and measured 0.04 V across the terminals! I had to check it twice, because I couldn't believe it. I also looked at the resistance and got a value of some 330 ohms. I'm not sure if this is normal as an internal resistance for a lead acid battery. EDIT: never mind: these numbers were measured with the terminals connected, so the voltage does't represent the open circuit voltage, nor does the resistance represent solely the internal impedance. It seems clear to me that the battery is toast, and I have no choice but to use my booster pack to start the car and drive it non-stop to somewhere where I can get a replacement battery installed. Has anyone ever experienced something like this before? How can I avoid this happening in the future, short of driving the car regularly, which is not always possible? What could possibly be draining the battery when nothing is turned on? Why is the battery (which is not that old) already at the stage where it cannot retain a charge at all? Is it just that if you let lead acid batteries drain too low, they become irreversibly damaged? EDIT: I do see from Wikipedia that deep discharging is a really bad thing to do to a lead acid battery designed to deliver starting current, and that, in fact, even when in storage, such batteries should be charged every couple of weeks. *Please don't ask me why I have a car if I'm going to leave it parked for long periods of time: it's none of your business.