What is the Voltage of a 12V battery after a Week Without Starting the Engine?

All 12-volt car batteries have a self-discharge rate of some kind. That means if a fully-charged car battery is sitting on your work bench at 12.6 volts in December, if you check back a few months later, it will probably measure a voltage level lower than that. With that being the case, one must acknowledge that if a 12V battery is connected to a car that has electrical accessories using energy, that the battery will discharge even faster.

The 12-volt battery in this 2001 BMW Z3 is fully-charged at about 12.6-12.8 volts. While this car is 20 years old, it does have some electrical accessories that stay on, even when the car is parked, like the clock in the radio and the seat memory. Over time, those accessories will discharge the battery and we let it sit for a week, to see what the voltage would measure after seven days of not starting the engine. As you can see from the photo, the battery measure 12.46 volts. 

That's not a huge voltage drop and certainly plenty left to start the engine, but if we let it sit for a few more weeks, the voltage would be sure to drop further. People often don't think of batteries as consumable items with finite lifespans, but they are. Car batteries move closer to their end of use with each day that passes after they are built, whether they get used or not.

Newer cars and trucks that are even more laden with electrical accessories can discharge car batteries even faster. Accessories like car alarms, apps and OnStar that stay on, even when the vehicle is turned off can quickly discharge car batteries in newer vehicles. When was the last time you measured the voltage of your car battery? If it doesn't measure at least 12.6 volts, it could probably use a maintenance charge.

When any lead-acid battery is discharged below 12.4 volts, sulfation can begin forming in the plates of the battery, which diminishes battery capacity and shortens battery lifespan. That means keeping your battery fully-charged whenever possible will maximize battery performance and lifespan and a great way to do that is with OPTIMA chargers & maintainers. These chargers are microprocessor-controlled and have one-touch functionality. You don't need to be a mechanic or a gearhead to know how to use one, just look at the pictures on the charger and press the option that matches your battery.

The OPTIMA charger will fully-charge your battery and then keep it properly-maintained without overcharging it, until you are ready to use it again. Connecting the charger is as easy as attaching the black clamp to a ground (like a metal bolt inside the engine compartment and attaching the red clamp to the positive terminal on the battery.