Will a Car Battery Die Sitting All Winter?
2 lessons in this series, 5 min in total
- Will a Car Battery Die Sitting All Winter? 2 min read
- How to Keep a Car Battery Alive in Winter Storage 3 min read
The figures here come from Yuasa’s storage guide, Battery University, Universal Technical Institute and the Battery Tender Junior manual, each credited where it appears. See how we research.
Two slow leaks: self-discharge and parasitic drain
Self-discharge is the charge every battery loses on its own, even with nothing connected. Battery University puts lead-acid self-discharge at roughly 3% a month at 68°F (20°C) and says it typically doubles with every 18°F (10°C) rise. Yuasa’s motorcycle storage guide says a battery connected to nothing can lose up to 1% of its charge a day at 70°F (21°C).
Parasitic drain is the power the car keeps using after you switch it off. According to Universal Technical Institute, the engine computer, alarm and clock keep drawing current. It puts normal draw at 50 to 85 milliamps (mA) in newer cars and less than 50 mA in older ones.
Here is what that means over a month. A 50 mA draw for 24 hours is 1.2 amp-hours (Ah) a day, or about 36Ah over 30 days. That’s more than half of a 60Ah battery, before self-discharge is counted.
If a dash cam records in parking mode or an OBD2 adapter stays plugged in, also read does dash cam parking mode drain your battery and can you leave an OBD2 scanner plugged in.
Why a flat battery freezes
A flat battery freezes far sooner than a full one. Yuasa explains that as a battery discharges, its electrolyte turns mostly to water, so it freezes at a higher temperature. Freezing can crack the case and buckle the plates.
| Battery state | Approx. freezing point | Source |
|---|---|---|
| Fully charged | -75°F (-59°C) | Yuasa |
| Fully charged starter battery (specific gravity 1.265) | -67°F (-55°C) | Battery University |
| Low charge (specific gravity 1.15) | 5°F (-15°C) | BCI, via Battery University |
| Discharged (dead) | 27°F (-3°C) | Yuasa |
The exact figures differ by source, but the pattern is the same. A full battery handles deep cold, while a dead one can freeze at 27°F (-3°C), just below the point where water freezes.
The voltage that means recharge
Resting voltage is the reading across a battery’s posts when it is neither charging nor powering anything. It is the number to watch on a stored battery. Yuasa says to charge when a voltmeter reads less than 12.25 volts, while Battery University suggests recharging a little earlier, at about 12.42 volts, or 70% charged.
For a full battery, Battery University lists 12.65 volts at rest for a fully charged starter battery, and the Junior manual puts a full 12V lead-acid battery near 12.9 volts. The figures differ by source. The Junior manual puts a fully discharged battery near 11.4 volts, so empty and full sit only about 1.5 volts apart.
That narrow range is why the reading needs care. For the steps, see how to test a car battery with a multimeter.
If your car will sit for weeks, plan how to keep a stored battery charged before the cold sets in.
Sources
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- Yuasa: Winter battery storage
- Battery University: BU-702, How to store batteries
- Battery University: BU-802b, What does elevated self-discharge do?
- Battery University: BU-903, How to measure state-of-charge
- Battery University: BU-410, Charging at high and low temperatures
- Universal Technical Institute: Parasitic battery drain
- Battery Tender Junior manual, P/N 392-0247-RF (PDF)
