On older vehicles, when you turn the ignition off there is almost no drain on the battery, except maybe an electric clock or the dome light if you open the door. Also, the battery can be disconnected if you are storing the vehicle for a period of months to prevent it from running down.
With late model computer-equipped vehicles, it is a different story. When you turn the ignition key off (or use the "Start" button to turn the engine off), many onboard modules continue to draw a small current from the battery to keep their learned memory settings alive. A keyless entry system will also remain active as will the anti-theft system. Most modules go into a "sleep" mode after a certain length of time (10 to 30 minutes) to reduce the power drain, but even so the combined current draw from these various devices can exceed 20 to 50 milliamps, and may be as high as 300 to 400 milliamps on some vehicles.
If a late model vehicle is parked and not driven for a week, the battery can be run down as much as 50%. Let your car sit two or more weeks without being driven, and you may find the battery has fully discharged.
This is why the batteries in late model vehicles should be connected to a smart charger or trickle charger if they won't be driven for a long period of time.
Why not just disconnect the battery to prevent it from running down?
If you disconnect the battery on a late model computer-equipped vehicle, it can cause all kinds of issues. Modules that rely on the battery to maintain learned memory settings or programmed calibrations can "forget" their settings. This includes all radio channel presets, driver seat position presets, the operation of the climate control system, all-wheel drive control modules, body control modules, the PCM and transmission module, even steering angle sensor calibrations.
Some of these forgotten settings can be reset manually (like radio channels and driver seat adjustments), or will relearn automatically as the vehicle is driven (fuel trim, automatic transmission shift points, etc.). But others will not reset without doing some type of special relearn procedure, and some may may require reprogramming with a scan tool! Worse yet, some modules that have lost their power supply will never wake up, requiring a new module to fix the issue!
So if you want to store your late model vehicle for a period of time without creating battery and module memory issues, hook the battery up to a smart charger that will automatically maintain the battery charge without overcharging it, or a smart trickle charger. An ounce of prevention can save you a lot of trouble.
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My testimonial thst they work is that after 7 yrs of owning my softtail, I finally bought a new battery. Not because I had to. The battery still test good under load. Until I started using one, I was replacing batteries on my Vulcan 88 every season.