Where a jump starter should live in your car depends on your climate, not a single universal rule. In a hot climate, the trunk's summer heat is the bigger threat to the pack's battery. In a cold climate, the trunk's exposure to freezing temperatures overnight is the bigger threat, and the cabin (a glovebox or under a seat) tends to run noticeably warmer than the trunk during the coldest hours, giving the pack a real advantage. Most generic advice just says "keep it in the trunk" and stops there, which is only half the answer.
Why the trunk is not neutral: heat in summer, cold in winter
The trunk is the roomiest, most out-of-the-way spot in most vehicles, and it is genuinely fine for a jump starter for a large part of the year in a lot of climates. The problem is at the extremes. A closed trunk in direct summer sun can get very hot, and sustained heat is generally harder on lithium battery chemistry than moderate temperatures. In winter, the trunk is also the part of the car most exposed to outside cold, with no engine or cabin heat nearby to buffer it, especially overnight or during a multi-day cold snap.
None of this means the trunk is a bad place to keep a jump starter. It means the trunk is not a temperature-neutral location, and if you live somewhere with real seasonal extremes in either direction, it is worth knowing what that costs you.
The cabin option: glovebox and under-seat, and the thermal-buffer idea
The alternative is keeping the pack somewhere inside the passenger cabin: the glovebox if it fits, or the floor space under a front or back seat if it does not. The idea behind this option is that the cabin is partially insulated by the car's own structure and residual heat from recent driving, so it does not swing as far toward either extreme as the trunk does. Some sources describe this as giving the battery a "thermal buffer" during a cold snap specifically.
If you choose under-seat storage, secure the unit so it cannot roll or slide into the pedals or under a seat track, and make sure it is not blocking anyone's foot room. A jump starter loose on a cabin floor is a hazard in its own right, separate from the temperature question.
Hot-climate guidance
If you live somewhere with genuinely hot summers, favor keeping the jump starter out of a trunk that bakes in direct sun for hours, or at minimum keep it inside its protective case and out of any spot where sunlight hits it directly through a rear window or parcel shelf. A shaded corner of the trunk, or bringing it into the cabin during the hottest months, reduces the sustained heat exposure that is harder on the pack than almost anything else.
Cold-climate guidance
If you live somewhere with hard winters, the cabin is the better default, specifically the glovebox or under a seat rather than the trunk. Reported comparisons put a vehicle's cabin as much as 10 to 20°F warmer than its trunk during the early hours of a freeze, a meaningful gap when the difference is between a pack that still delivers a strong jump and one that is temporarily sluggish from cold. For the fuller picture of how cold affects jump starter performance itself, not just where to store it, see do jump starters work in cold weather.
A middle path: insulated or protective cases
If neither pure trunk nor pure cabin storage is practical for your vehicle, an insulated or well-padded protective case narrows the temperature swing wherever the pack lives. This will not turn a trunk into a climate-controlled room, but it blunts the worst of both direct summer sun and the coldest overnight hours. Combine it with the general storage routine (full charge before storing it, a periodic check-in) covered in how to store a jump starter so it works when you need it, and the recharge cadence in how often to recharge a jump starter in storage.
FAQ
Is the trunk always the wrong place for a jump starter?
No. For a large part of the year in a moderate climate, the trunk is a perfectly reasonable spot. The caution is specifically about sustained summer heat and hard winter cold, where the trunk is more exposed than the cabin.
Does keeping it under a seat actually make a real difference in winter?
Reported comparisons suggest the cabin can run meaningfully warmer than the trunk during the early hours of a freeze, which can matter for how strong a jump the pack delivers on a cold morning. If you have hard winters and the option to store it under a seat safely, it is a reasonable precaution.
What if my car doesn't have room in the cabin for it?
Use the trunk, but favor a shaded spot away from direct sun in summer, keep it in its protective case year-round, and lean on the general check-in routine so you catch a problem before an emergency rather than during one.
Will a hot trunk ruin a jump starter permanently?
Not necessarily on its own, but sustained heat exposure is generally understood to be harder on lithium battery chemistry over time than moderate storage temperatures. It is one more reason for the periodic check described in how to store a jump starter so it works when you need it, rather than a reason to assume the worst about a specific unit.
Is a glovebox big enough for most jump starters?
It depends on the size of your glovebox and your specific unit; many compact jump starters fit, but check before assuming. If it does not fit, under-seat storage, secured so it cannot roll, is the next best cabin option. For everything else in choosing and living with a jump starter, see our independent buyer's guide.