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How long can my car sit before the battery dies?
A parked car is never fully off: alarms, keyless entry, clocks and telematics keep drawing a small current. Enter your battery's size and the car's parked draw to see how long that drain takes to bring a full battery down to half charge. It's the same arithmetic as our article How long can a car sit before the battery dies?
An estimate, not a battery test. This is arithmetic on the numbers you enter, not a measurement of your car. Real life is usually worse: see the assumptions below.
Work it out
The arithmetic
Days to half charge = (Ah × 0.5 × 1,000) ÷ mA ÷ 24
Half the battery's amp-hours, turned into milliamp-hours, divided by the draw in milliamps gives hours; divide by 24 for days. It's our own arithmetic (draw × 24 hours a day), not a manufacturer figure.
Here is what the draws in the article mean for an example 60 Ah battery that starts fully charged:
| Parked draw | Charge used per day | Days from full to half (30 Ah) |
|---|---|---|
| 30 mA (typical Subaru) | 0.72 Ah | about 42 days (6 weeks) |
| 50 mA | 1.2 Ah | about 25 days (3½ weeks) |
| 70 mA (Subaru's upper limit) | 1.68 Ah | about 18 days (2½ weeks) |
| 140 mA (a telematics module stuck awake) | 3.36 Ah | about 9 days |
| 300 mA (a fault) | 7.2 Ah | about 4 days |
Subaru's dealer bulletin 15-308-23R sets an acceptable parked draw of 70 mA or less and says a typical Subaru draws 30–35 mA once asleep. Other makers publish their own limits in their service manuals.
The assumptions (real life is usually worse)
- The battery starts fully charged. It rarely is: Varta says that with infrequent use or short distances "the alternator cannot fully charge the battery."
- The battery is healthy. A worn battery reaches half charge sooner. Subaru's bulletin notes that "weak batteries mimic parasitic draw." No source gives a figure for how much sooner, so the calculator doesn't adjust for it.
- The draw is constant, and self-discharge and temperature are left out. Varta says high temperatures speed up self-discharge. Cold cuts the starting power left in the battery, so a half-charged battery that would start the car in September may not in January.
- "Half charge" is the line because by then, around 12.2 V on a flooded battery (East Penn's chart), battery makers already say to recharge. It isn't the point where the car won't start; it's the point to act. Check yours with the battery voltage checker.
What makes a battery die sooner than the arithmetic says
- A module that won't sleep. Subaru notes that where cell signal is weak, such as an underground garage, the telematics module can stay awake "drawing up to 140 mA for 14 days" before it hibernates.
- The key fob too close. A fob near the car can keep keyless-entry modules polling. Subaru's test procedure keeps it at least 4.6 m (15 ft) away.
- Aftermarket devices such as alarms, remote starters, dash cams and GPS trackers draw power continuously. A plug-in OBD dongle left in the port is the same kind of load.
- A real fault. A few hundred milliamps from a stuck relay or a faulty module can flatten a battery in days. If yours dies after only a few days, read how to find a parasitic battery drain.
What carmakers do
MINI checks the battery voltage of cars on its dealer lots every four weeks and recharges any that read below 12.5 V; with the battery disconnected it stretches that to 12 weeks. Lexus tells dealers to disconnect the 12 V battery's negative terminal on its 2015–2016 hybrids for storage of 30 days or more. Varta says to charge a battery to 12.7–12.8 V before storage and check it every two months. Check your owner's manual before disconnecting anything: some cars lose stored settings, and Toyota Canada advises against disconnecting the battery at all on models with a battery sensor on the negative terminal.
Method
Formula. Days to half charge = (Ah × 0.5 × 1,000) ÷ mA ÷ 24. Charge used per day = mA × 24 ÷ 1,000 Ah. The answer is rounded to the nearest day ("less than a day" below one day); from 14 days up, it is also given in weeks, rounded to the nearest half week. Capacity must be 1–500 Ah and the draw 1–20,000 mA. Choosing "aging" adds a note but never changes the number.
Assumptions. The battery starts fully charged and healthy, the draw is constant, and self-discharge and temperature are left out. Half charge is the end point because by then, around 12.2 V on a flooded battery, battery makers already say to recharge; it is not the point where the car won't start.
Sources. The arithmetic is ours. The reference draws (30, 70 and 140 mA) are from Subaru's bulletin 15-308-23R, the 12.2 V half-charge figure from East Penn's Form 0004, and the notes from MINI and Varta, all listed below and cited in our article.
Method version 1, .
Sources
- Subaru of America (via NHTSA), 15-308-23R: Testing Telematics Systems for Dark/Standby Current: the 70 mA limit, the typical 30–35 mA, the 140 mA telematics case, the fob distance and "weak batteries mimic parasitic draw".
- Subaru of America (via NHTSA), 07-236-25: Dark Current Parasitic Draw Diagnostic & Testing Procedures: aftermarket devices that draw power continuously.
- MINI USA (via NHTSA), SI M61 01 02: Battery Maintenance Requirements: the four-week and 12-week checks.
- Lexus (via NHTSA), L-SB-0011-14: Maintenance for HV & Auxiliary Batteries: disconnect for storage of 30 days or more.
- VARTA Automotive (Clarios), How to Store Vehicle Batteries: charge before storage, check every two months.
- VARTA Automotive (Clarios), Car batteries: After the summer heat comes the deadly freeze: short trips and self-discharge.
- Toyota Canada, How to Prepare Your 12V Battery for Cold Weather Driving: don't disconnect the battery on models with a battery sensor on the negative terminal.
- East Penn Manufacturing, Battery Care & Maintenance (Form 0004): 12.2 V is about half charge on a flooded battery.
The full explanation, with storage advice and what to do when you come back to a flat battery: How long can a car sit before the battery dies?