QUOTE (pen411 @ Jun 21 2010, 04:30 PM) index.php?act=findpost&pid=335047
You would have to leave a cell charger or other low power device plugged in for probably a few months to discharge the battery to the point you couldn't start the vehicle. At that time frame the battery will charge just from sitting plus all the stuff that gets power already such as the ECM PCM radio, security. Etc
Look at it this way. Your phone can run a week or longer on standby using the tiny little battery it has.
I know -- big honking battery ought to manage this sort of thing forever. Sure looks like it should. It won't.
Don't join the folks who have made the mistake. Some of these devices consume something on the order of 100mA, some far more. Now that doesn't sound like a whole lot, but leave your car at an airport for a week with that kind of consumption and you'll be looking for the guy to give you a jump start when you get back. Another one that bites people are those little RF modulators into which you might plug an MP3 player. Your battery won't last a week with that kind of load, either. Even at 110mA, you're eating up about 32Wh per day.
Remember from your own experience -- a single 1A (12W) dome light left on overnight can leave you stranded in the morning. In five days, a 100mA load can produce the same result. 1A x 12 hrs = 0.1A x 5 days.
I designed the firmware for a product that requires about 110mA of current even when "off" (actually, it's in a "sleep" mode) when connected to external power. The CPU has to manage the charging circuit, and remains on full time. I can't tell you how many times we've warned LEO and other government agencies that when they hardwire the power for these things, they be sure to do so on the SWITCHED side of the ignition. We still get calls about the product killing batteries over the period of a 5 ~ 7 day period because they ignored the warning.
Any time I hear about someone with what appears to be a decent battery and a functional charging system complain about the battery going dead if they don't start the vehicle at least weekly, we go on the hunt for parasitic losses that exceed those of the normal on-board stuff you mentioned above (not all of which is powered with the ignition off, BTW). 9 times out of 10, it's an accessory wired on the wrong side.
A vehicle battery doesn't have to have every last ounce of power drained from it before the vehicle won't start. Load it up until it drops to 10VDC and many vehicles won't even try. Since the voltage tends to drop immediately when the starter load is applied, you have to start out quite a bit above that to have success. It's also not recommended to keep discharging a normal lead-acid battery below about 50% of capacity (a s.g. of around 1.190~1.195) which occurs at a rest voltage of a surprisingly high 12.25V or so. I say "normal" since most people don't run deep cycle batteries under the hood. The kinds of small parasitic losses I mentioned can cause this to occur on a regular basis, significantly reducing the life of the battery.