The amount of time, usually referred to as "airport", "garage", or "storage" time, that you can leave your vehicle parked and still start your engine is dependent on such things as the battery's initial State-of-Charge (SoC), the Reserve Capacity (or amp hour capacity), the amount of natural self-discharge and parasitic (ignition key off) load, temperature and battery type (plate chemistry). Car manufacturers normally design for at least 14 days or more "airport" time; based on a fully charged battery in good condition, moderate weather, and no additions to the original car's parasitic load (for example, an after market alarm system). The number of days will vary based on the temperature. When a battery drops below 100% SoC, sulfation starts slowly occurring, and this will reduce the capacity of the battery and if left unchecked, will kill the battery.
If you leave your vehicle parked for more than two weeks, then you have several options:
15.1. The best long term (over one month) option is to continuously float charge your car battery by connecting a "smart" battery charger or a voltage regulated float charger because it will keep the battery fully charged, thus eliminating sulfation. If there is no AC power available, use a five watt (or greater) solar float charger. These options will allow you to park you vehicle indefinitely, but the battery should be checked periodically. You will need a "float" charging voltage between 13.2 and 13.8 VDC at 77° F (25° C) and at least .5 amps (500 milliamps) to overcome the vehicle's parasitic load and the natural self-discharge of the battery. Do not use a cheap "trickle" charger, because it will overcharge your battery and dry out the electrolyte.
15.2. Disconnect the grounded battery cable (which is normally the NEGATIVE (-) cable) to remove the parasitic load, but be sure that you have saved any security codes or radio stations presets that will have to be reprogrammed, but the battery's natural self-discharge will continue. This option will work from one month to six months depending battery type and temperature.
15.3. Replace the battery with the largest AGM (Ca/Ca) or Spiral Wound AGM VRLA battery that will fit with very low self-discharge rates. For periods greater than two months, also disconnect the grounded battery cable to remove the parasitic load. This option will work for six months to twelve months depending battery type and temperature.
15.4. Install a battery with a larger reserve capacity or connect an identical battery in parallel, but the battery's natural self-discharge will continue. For periods greater than two months, also disconnect the grounded battery cable to remove the parasitic load. This option will work for two months to twelve months depending battery type and temperature.
15.5. Replace the battery when you are ready to drive the vehicle again, especially if the battery is over three years old and in a hot climate.
15.6. Have someone drive your car during the day at highway speeds every two weeks for at least 15 minutes to keep the battery charged.
15.7. Park you car where jumper cables are reachable or obtain a jump starter (recommended especially when head-end parking is required). Jump start the battery and hope that there is no latent damage.
15.8. Install a low voltage disconnect. This is especially helpful if the driver forgets to turn the headlights off.
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