Storage is three lists, not one
The reason vehicles come out of storage worse than they went in is almost never that a step was done badly. It is that the step was on nobody's list. Splitting the job into what happens before, what happens during and what happens on the way out makes the gaps visible, and the gaps are usually in the middle list, because the middle list is the one that has no obvious trigger to make it happen.
The before list is the one everybody does. The during list is a small number of short visits whose value is entirely in catching something early. The after list is where you find out how the other two went.
The battery arithmetic, which is the one honest number here
Most of a layup plan is judgement. The battery is the part that is arithmetic. A modern vehicle draws current with everything off: the alarm, the tracker if there is one, the keyless entry receiver, the memory in the infotainment head unit. Twenty to fifty milliamps is common and some vehicles are considerably worse.
Capacity in amp hours times a thousand gives milliamp hours. Divide by the draw in milliamps to get hours, and by 24 to get days. A 60 Ah battery with a 25 mA draw runs for 2,400 hours, or 100 days, to complete flat. But complete flat is not the target, because deep discharge is what damages a lead-acid battery rather than merely inconveniencing it. Taking half the capacity as the threshold gives 50 days, which is under two months, which is why a car left connected over a winter is frequently a car that needs a jump start in March.
| Capacity | Draw | Days to 50% depth | Days to flat |
|---|---|---|---|
| 45 Ah | 25 mA | 38 | 75 |
| 60 Ah | 25 mA | 50 | 100 |
| 60 Ah | 50 mA | 25 | 50 |
| 90 Ah | 20 mA | 94 | 188 |
| 90 Ah | 60 mA | 31 | 62 |
If the draw has not been measured, treat these as an order of magnitude rather than a schedule. The useful conclusion is not the exact day count, it is that leaving a vehicle connected for a season is a decision with a predictable outcome, and that the alternatives are disconnecting it, removing it, or leaving it on a maintainer. Each of those has its own consequences, including losing stored settings on many vehicles, so the owner manual comes first.
The arguments that do not have a single right answer
Fuel level is the clearest example. A fuller tank has less air in it, and air is where the water that condenses on cold tank walls comes from. A less full tank has less fuel in it to degrade over months. Both are true, the balance depends on the fuel, the vehicle and the climate, and there is a large amount of confident advice on both sides. This calculator asks you what level you want and works out the volume; it does not pick.
The same applies to intervals. How often a stored vehicle should be moved, checked or started is a matter of opinion dressed up as fact in a great deal of what is written about it. What can be said without argument is that a tyre carrying a static load in one place for months can take a set, that moving the vehicle changes which part of the tyre is loaded, and that every move is also an engine start with everything that implies about running an engine indoors. The interval on this page is the one you entered, and it is a plan rather than a rule.
No additive, oil, battery product or charging routine is named here, and no interval is presented as correct. That is not caution for its own sake: the vehicle documentation is specific to the vehicle and a page like this is not.
The hazards, stated once
Carbon monoxide has no smell and an engine running in a building produces it faster than a cracked door clears it. If the vehicle is started, it goes outside. Petrol vapour is heavier than air and travels along the floor to the lowest ignition source, which in a garage is very often a water heater pilot, a boiler or a compressor, and an attached garage shares its air with the house it is attached to. Charging a battery gives off hydrogen and wants ventilation and no sparks nearby.
And the one that matters most: a jack is a lifting device, not a support. People are killed by vehicles falling. Anything that involves getting under a vehicle, including supporting it off its wheels for a storage period, is a job for someone equipped to do it, and there is no procedure for it on this page.
What comes next depends on the vehicle rather than the storage. Time-based service intervals do not pause because the vehicle did, and the maintenance schedule guide is where to reconcile what fell due across the gap. Tyres that sat get their own attention in the tyre care guide, and a vehicle coming out into cold weather is covered by the winter driving checklist. For fuel that has been sitting in cans rather than in the vehicle, the fuel storage calculator handles the volumes, and the running cost of a vehicle that spends part of the year off the road is the ownership cost calculator. If the space it is going into is the constraint, the garage vehicle fit calculator settles whether it fits before it is driven in.
Questions people ask
How long before a car battery goes flat sitting in storage?
Work it out rather than guessing, because the range is wide. Capacity in amp hours times a thousand gives milliamp hours; divide by the parasitic draw in milliamps and by 24 to get days to complete flat. A 60 Ah battery with a 25 mA draw is about 100 days. But complete flat is not the number to plan around, because deep discharge is what damages a lead-acid battery, so half of that is a more realistic threshold: about 50 days, or seven weeks. Vehicles with trackers, alarms or always-on telematics draw considerably more. If you have not measured the draw, the arithmetic still tells you the shape of the answer, which is that a season is longer than a connected battery usually lasts.
Should I store a car with a full tank or an empty one?
There is a genuine argument on both sides and this page does not settle it. A fuller tank leaves less air space, and air space is where the moisture that condenses on cold tank walls comes from. A less full tank leaves less fuel to degrade over months, which matters more for some fuels than others. Which consideration dominates depends on the fuel available where you are, the vehicle and the temperature swings it will see. The owner manual for the vehicle is what governs, and it is worth reading before following advice from anywhere including here. The calculator works out the volume for whatever level you decide on.
Should I start the engine periodically while it is stored?
This is one of the most confidently argued questions in the subject and the honest answer is that it depends on what you are trying to achieve. What can be said without dispute is the safety part: an engine running inside a building produces carbon monoxide faster than a partly open door clears it, carbon monoxide has no smell, and it kills. If a vehicle is going to be started, it goes outside first. Beyond that, a brief idle in a cold building does different things to an engine than a proper drive to temperature does, and whether periodic running helps or hurts is a question for the vehicle documentation rather than a general rule.
What about putting the car on stands so the tyres do not flat-spot?
Getting a vehicle off its wheels is a shop job and no procedure for it appears on this page. The point worth stating plainly is that a jack is a lifting device and not a support, and that people are killed by vehicles falling off jacks. If a vehicle is to sit off its wheels for months, it needs proper supports placed on suitable points by someone equipped to do it, on a surface that will not move, and nobody should be underneath it at any point in the storage period. The alternative most people take instead is repositioning the vehicle occasionally so the loaded part of each tyre changes, which needs no lifting at all.
What is the most commonly missed step?
Emptying the interior of anything organic, and looking for nesting on the way out. Rodents are attracted by food, paper and soft material, they get into an unmoving vehicle easily, and the damage they do to wiring looms and to insulation is expensive and time-consuming to trace. The second most commonly missed is the parking brake, which left applied for months can seize the friction material to the drum or disc; chocking the wheels instead avoids it entirely, where that is safe to do on the surface it is standing on. The third is simply writing down the date and mileage, because a service interval that fell due in the middle of the storage period is very easy to lose.