What You Need to Store a Car Over Winter

A battery maintainer first - a flat battery is the thing that actually goes wrong. Then a fuel stabilizer if the tank will sit more than a couple of months, a desiccant if the car is shut up and unventilated, and a cover matched to indoors or outdoors. Leave the tires at the door-jamb placard pressure: Michelin says over-inflating "will not help flat spotting" and "in some cases it could make it worse".

Storing a car is a short list of cheap things done once, and a long list of things the internet will sell you that do not matter. This page is ordered by what actually fails.

How we judged this list

One test: does the failure it prevents actually happen in three to six months of sitting? That ruled out most of what gets recommended for storage - fogging oil, raising the car on stands, draining fluids - which are measures for multi-year layups, not a winter. What survived is four items, and one of them is under twenty dollars.

Within each job we preferred products that publish a figure: a treat volume and a dose, a gram capacity, a published output current, a named inner layer.

1. The battery, which is the one that always goes

A parked car is not an idle car electrically. Alarms, keyless-entry receivers, body control modules and clocks all draw continuously, and a lead-acid battery also self-discharges, faster when it is partly discharged to begin with. Three months of that is enough to leave a healthy battery flat and a four-year-old battery dead for good.

The correct tool is a maintainer, not a charger, and the distinction is real: a maintainer puts out under two amps and holds a full battery full indefinitely, while a charger pushes six to fifteen amps to refill a flat one. Battery maintainer vs battery charger has the arithmetic, including why a dumb trickle charger left on all winter cooks the battery it was meant to save.

If there is no outlet where the car is parked, you have two options and one non-option. The options are a solar maintainer on the dash, or disconnecting the negative terminal - which stops the draw but loses radio presets and, on some cars, means a relearn procedure. The non-option is starting it every couple of weeks and letting it idle: how often to start your car in winter explains why a short cold idle can leave the battery lower than before you turned the key.

2. The fuel, if it will sit more than a couple of months

Gasoline degrades. The light fractions evaporate, the rest oxidizes, and the gum that forms settles in the places you least want it. Ethanol adds a second mechanism, because it attracts water.

STA-BIL publishes the figures that make this easy to get right: 1 oz per 2.5 gallons, a 32 oz bottle treating 80 gallons, fuel kept fresh "for up to 24 months" in storage, and protection against the effects of both ethanol and non-ethanol fuels.

Two details from the same listing that people miss, and they matter:

• Store it with a full tank. Air space is where condensation forms. A full tank has almost none.
• Run the engine for five minutes after dosing. STA-BIL's instruction is explicit - that is what carries treated fuel out of the tank and into the lines, rail and injectors. Pouring it in and parking the car treats the tank only.

A car still being driven every week or two does not need this; fresh fuel is already going through it. And this is a different product from a water remover - if the question is freeze-up in a line rather than fuel going stale, HEET vs Iso-HEET is that page.

3. The moisture, if the car is shut up

A sealed car with damp carpet is a terrarium. Moisture evaporates during the warm part of the day and condenses on the coldest surface at night, which is the glass. Weeks of that produces a musty interior and a film on the inside of every window.

A rechargeable silica gel desiccant is the cheap fix, and the one we picked publishes a number to size it by: 150 g of moisture adsorbed per 500 g bag, up to 180 g, reactivated in a microwave in 10 to 16 minutes, for more than ten cycles. Its own listing names the symptom - reducing condensation on car windows on cold winter mornings.

What it will not do is beat a source. A wet floor mat, a leaking door seal or a blocked sunroof drain will keep feeding moisture faster than any desiccant removes it. Dry the interior properly before you shut the car up - and if the carpet is wet with brine rather than water, deal with the salt first.

4. The tires - and the one piece of advice makers disagree on

A tire sitting under load for a long time takes a set in the flattened contact patch. Michelin describes the nylon cap staying "in this flat state for a long time" until the tire takes a set to that shape. Continental puts a timeframe on the serious version: semi-permanent flat spots can form when tires stand under vehicle load for a month or longer, made worse by high temperatures and low pressure.

Here is where the makers part company, and it is worth knowing rather than guessing:

Our reading, and the reason the recommendation at the top of this page is what it is: placard pressure is the position of the maker that has published a direct answer to the over-inflation question, and it is also the pressure you want the car to be at when you drive it again. A temporary flat spot usually works itself out within the first few miles of driving. Chasing a semi-permanent one with extra PSI is, on Michelin's own account, as likely to hurt as help.

Set the pressures when the tires are cold, and recheck them before the car moves in the spring - the stored car will have lost pressure to the cold exactly as a driven one does. The cold-pressure arithmetic is here.

The cover, and the mistake people make with it

Indoors and outdoors are different products. A breathable single-layer cover keeps dust off a garaged car without trapping moisture against it. A multi-layer outdoor cover resists rain, snow and UV - and needs securing, because a cover that moves is the thing that damages paint.

The rule that matters more than the cover you choose: cover a clean, dry car. Do car covers scratch paint has the mechanism - grit plus movement - and the full cover roundup compares the outdoor options on layers, lining and securing.

The short version

Fill the tank and dose it with stabilizer, run the engine five minutes, put a maintainer on the battery, dry the interior and add a desiccant if it will be shut up, set the tires to placard pressure, and put a clean cover on a clean car. Four purchases, one afternoon, and the car starts in March.