It is not healthy for your car battery to drop below 75%.
Cold temperatures greatly effects how well your battery can perform.
Engine oil is thicker at low temperatures, which increases the load or effort on your starter. This increases the amperage or power needed to turn your engine over. The amount of amperage needed can double at zero degrees F. A cold battery has a decreased ability to supply amperage. At zero degrees your battery can only work at 65% efficiency.
In short a weak battery on a cold day can lead to a no start condition. This can lead to a stressful situation! You can prevent this with some battery winterization steps.
You should make it a habit of checking your battery voltage once a week or at least once a month.
Is your battery ready for the cold weather? Use this chart:
You will need a volt meter to check your voltage level. The vehicle should be off with the key out. Check across the positive + Red Terminal and negative - Black Terminal.
Battery Voltage Chart: State of charger at 80 F
12.66 = 100%
12.45 = 75%
12.24 = 50%
12.06 = 25%
11.89 = 0%
For cold weather voltage will drop .01 volts for every 10 F
A fully charged battery should be:
At 32 F you should see around 12.587 Volts
At 0 F you should see 12.516 Volts
If the battery is at 75% or lower it should be charged. The best method to charge your battery is to use a portable charger at the lowest amperage setting. You can drive your vehicle to charge the battery but...
You should have all of the electrical loads turned off. Your vehicle is consuming power when running and the alternator output can only work so well to put energy back into the battery. If your vehicle is using a lot of energy such as the radio, heated seats, lights (keep them on at night), blower motor et... it will take longer to charge your battery. It would be recommended to drive at least 30 minutes above 40 mph with minimal electronics turned on to help charge your battery. That is why using a battery charger is the best method.
If your battery is not maintained at a minimum of 75% it can be permanently damaged.
Never Charge a Frozen Battery -- It Can Explode!
Never Charge a Leaking Battery -- That is Sulfuric Acid!
Never Charge a Battery In An Enclosed Space -- On a BMW the batter is often in the trunk or hatch area. The battery has a vent installed that out gases outside the vehicle. If the vent is off the gases can build up in the trunk can this gas is explosive!
If you can, it is best practice to remove the battery and let it warm up inside before charging. However, you can charge a cold battery in your garage if this is not feasible. If you do remove the battery make sure to disconnect the negative first and then the positive. Installation is reverse, install the positive first and then the negative.
An average life span of a battery is 4 to 5 years. Some batteries have a date stamp on the terminal or a sticker that will tell you the battery date. Battery replacement as a maintenance item is an often overlooked item until your vehicle does not start.
A weak or dead battery does not necessarily mean it needs to be replaced. A good battery can be depleted from short distance driving, leaving the lights on accidentally, or a parasitic drain (See my video on this), or vehicle storage time. There is always a small drain on a battery to maintain radio settings and other items of around 50 milliamps or .050 amps. That is why you should always check and maintain your car battery.