Can you leave a power bank in a hot car?

Not in a parked car in the sun once it is warmer than about 20 degrees C (68 degrees F) outside. A sunny cabin runs 22 to 25 degrees C above the outdoor air within an hour, which passes the 40 to 45 degrees C storage limits printed for power banks, phones and power stations. A healthy bank is very unlikely to catch fire in a car; the hottest dashboard measured here, 85 degrees C, is below the temperatures cells are tested to survive. Heat costs capacity, and it costs most when the bank is full.

How hot does a parked car get?#

How far above outside, and how fast. McLaren, Null and Quinn logged a dark sedan for 60 minutes on 16 clear days with outdoor air at 72 to 96 degrees F (22 to 36 degrees C). The cabin rose on average about 40 degrees F (22 degrees C), with 80 percent of the rise in the first 30 minutes. On the coolest day, 72 degrees F, it still reached 117 degrees F (47 degrees C), a rise of 25 degrees C. Windows opened 1.5 inches made no significant difference.

Where in the car. Vanos and colleagues measured six vehicles in Tempe, Arizona, in sun and in full shade, on three days between 25 June and 11 July 2014, with outdoor air of 32.8 to 41.5 degrees C:

A power bank in a bag on the floor, out of the sun, sits near cabin air. A phone on a dashboard mount or a bank on the seat in a sunbeam is heated directly by the sun, on average 4 to 22 degrees C above the cabin air. The same measurements set the limits for fuel in our page on propane and gas canisters in a hot car.

What temperature is too hot for a power bank?#

Each maker publishes a storage range, and for a parked car, storage is the right column: the bank is not being used.

The finished products are stricter than the bare cells: the cells are rated to 50 or 60 degrees C, the banks, phones and stations to 40 or 45. Against the car figures, average sunny cabin air (46.7 degrees C) is past the product limits and just under the cells' 50. The dashboard average (68.9) is past every storage figure in the table, including LG's one-month 60.

Will a power bank explode in a hot car?#

Two answers circulate: "lithium plus heat equals fire" and "it is fine, it is just warm". The numbers support neither.

Why a healthy bank should not ignite at car temperatures. LG's MJ1 specification includes the UL 1642 heating test: charged cells are heated at 5 degrees C per minute to 130 degrees C and held for 10 minutes, and the requirement is "No explode, No fire". Anker's safety sheet sets its warning line at 100 degrees C: "Exposure to fire or temperature above 100 degrees C may cause explosion." The hottest reading in either car study, an 85.0 degree C dashboard in Arizona, is 15 degrees C under Anker's line and 45 under the oven test.

Why that is not a promise. The test is passed by new, undamaged cells, and Anker warns that "damaged or modified batteries may exhibit unpredictable behavior resulting in fire, explosion or risk of injury".

Why "it is fine" is also wrong. The same LG document tells pack makers: "Do not place the battery in direct sunlight or heat." Apple: "Don't leave your device in a car, because temperatures in parked cars can exceed this range." The damage they guard against is slow and invisible: lost capacity.

What heat actually does: capacity lost in storage#

Lithium-ion cells lose capacity while they sit, which is called calendar aging. Keil and colleagues at the Technical University of Munich stored three types of 18650 cell (NCA, NMC and lithium iron phosphate) at 16 states of charge and at 25, 40 and 50 degrees C for 9 to 10 months. What matters in a car:

• Heat accelerates it in every chemistry. All three cell types "exhibit an increased calendar aging with higher storage temperature". For the LFP cells the rate after nine months was about 0.2 percentage points a month at 25 degrees C and about 0.5 at 50.
• Charge level matters as much. Capacity fade steps up once the graphite anode is more than about half filled, which for these cells began at about 60 percent state of charge (NCA and NMC) and above 70 percent (LFP). The authors' advice: "cells should not be stored at high SoC".
• The first weeks cost most. Loss followed roughly the square root of time, so new cells give up capacity fastest.

Read from the paper's Figure 2, to about a percentage point, the NCA cells after 10 months:

Kept full at 50 degrees C, the cells lost about 12 to 13 percent; kept half full at 25, about 3 percent. The NMC cells were worse at the hot, full corner, about 77 percent left after 10 months at 50 degrees C and 100 percent charge.

The figure most pages repeat is an estimate. Battery University's widely copied table gives 65 percent remaining after a year at 40 degrees C and full charge, and 60 percent after only 3 months at 60. Its own caption calls these "Estimated recoverable capacity". The measured cells held up better at 40 and 50 degrees C, though both agree that hot and full is the worst combination.

An afternoon is not a summer#

What matters is how long the bank lives in the car. LG grades its storage limits by time: 60 degrees C for a month, 45 for three months, 20 for a year. Its 60 degree C test stores charged cells for a full week and requires at least 80 percent capacity recovery. An afternoon in a hot car is hours at temperatures the cell maker allows for weeks: it costs some capacity, it does not ruin the bank.

The bank that suffers is the one kept in the door pocket all summer "for emergencies", topped up to full and never used: high charge, high temperature and months of time, every factor Keil measured.

Can you charge a power bank or phone in a hot car?#

Not while it is parked in the sun. The Panasonic and LG cells and the EcoFlow station all stop their charging range at 45 degrees C, below average sunny cabin air. Apple says a hot iPhone's charging "slows or stops", and it may show "Temperature: iPhone needs to cool down".

Two setups break this rule: a bank left charging from the 12 volt socket in a parked car (the vehicle side is in 12 volt power and camping fridges), and a dashboard panel feeding a bank on a sunny seat. Set a panel up outside with the bank in shade, as in solar for camping. While you drive with the air conditioning on, charging is normal.

A decision rule by outdoor temperature#

Adding McLaren's measured rise to the printed limits gives a rule you can apply at a trailhead car park. Sunny cabin air after an hour is roughly outdoor air plus 22 degrees C on average, and plus 25 on their coolest day.

Where the rule stops applying. It describes a closed car in clear sun. Full shade cut Vanos's cabins by 8.4 degrees C on average. Vehicle size changes the speed more than the end point: Vanos's cars heated at 0.30 to 0.42 degrees C per minute and reached similar temperatures. Neither study measured the glove box, console or boot (trunk), so this page does not claim any of them is cooler than cabin air.

What to do with a bank on a summer road trip#

• Take it with you when you leave the car for more than a few minutes on a warm day. Even Nitecore's 77 Wh bank weighs 315 g; it rides in a pocket or daypack. Put it on your car camping checklist as something that leaves the car with you.
• If it must stay, put it low and out of the sun: in a bag on the floor, not on the seat, dash or parcel shelf. The glove box is shaded, but nobody has published its temperature; test it before trusting it.
• Do not leave it full if it lives in the car. Store it at about half charge; on Nitecore's four-light gauge, two steady lights is about 50 percent. Keil's cells aged distinctly faster above about 60 percent.
• Retire a bank that is swollen, dented or has been crushed. None of the safety margins above apply to it.

Between trips, a bank belongs indoors at room temperature and about half charge, as in storing gear between trips. A smaller bank sized with the method in power banks and battery sizing is also easier to keep in a pocket.

Check your own car and your own bank#

The car. A cheap minimum and maximum thermometer answers the question for your vehicle.

1. Put the thermometer where the bank usually rides: glove box, console, door pocket or floor.
2. Park in the sun on a clear day, windows closed, from late morning.
3. Read the maximum after 60 and 120 minutes, and note the outdoor temperature.
4. If the reading passes 40 to 45 degrees C, that spot is outside the bank's storage rating.

The bank. With an inline USB power meter, record the watt-hours a full bank delivers in spring and again at the end of summer. For reference, Keil's cells stored half full at 25 degrees C lost about 3 percent in 10 months; a much larger loss means heat, full charge or heavy use has taken its share.

Common mistakes#

Cracking a window. McLaren found windows opened 1.5 inches made no significant difference to how fast or how far the cabin heated. Shade does help.

Using a dashboard mount as a parking spot. A phone on a windscreen or dash mount sits on the hottest surface measured, 68.9 degrees C on average in Arizona. Take it down when you park.

Keeping the emergency bank topped up to 100 percent. Full charge is the condition that makes heat expensive. Half of a 10,000 mAh bank, about 15.7 Wh delivered, is still roughly one full charge of a 14.8 Wh phone.

Frequently asked questions#

Can a power bank explode in a hot car?#

Very unlikely, if it is undamaged. Cells such as LG's MJ1 must survive 130 degrees C for 10 minutes without fire under UL 1642, and Anker's warning starts above 100 degrees C. The hottest car surface in the studies here was an 85 degree C dashboard. A swollen, dented or crushed bank is different: damaged cells can behave unpredictably.

What temperature is too hot for a power bank?#

For storage, 40 to 45 degrees C (104 to 113 degrees F) for finished banks, phones and power stations: Nitecore says 40, Apple and EcoFlow 45. Bare cells are rated higher, 50 degrees C for Panasonic's NCR18650B and 60 for a month for LG's MJ1. For charging, 45 degrees C is the upper limit in every cell datasheet checked here.

Can I leave my phone in a hot car?#

Apple says not to: iPhones should be stored between minus 20 and 45 degrees C, and "temperatures in parked cars can exceed this range". A sunny cabin passes 45 degrees C within an hour when it is above about 20 degrees C outside. A hot iPhone may slow or stop charging and show a temperature warning until it cools.

Is the glove box a safe place for a power bank?#

It is out of direct sun, which helps, but no published study has measured glove box temperatures, so it cannot be assumed to stay under the 40 to 45 degree C storage limits. Leave a minimum and maximum thermometer in it for a sunny afternoon before trusting it.

Should I store a power bank fully charged in the car?#

No. Storage at a high state of charge speeds capacity loss, and heat multiplies it. In a 10-month laboratory study, cells kept full at 50 degrees C lost about 12 to 13 percent of their capacity against about 3 percent for cells kept half full at 25. Keep a car bank at about half charge.

Can you leave a portable power station in a hot car?#

Not in a sunny parked car on a warm day. EcoFlow's RIVER 2 is rated for storage from minus 10 to 45 degrees C and calls 20 to 30 best, and its lithium iron phosphate chemistry still ages faster in heat. A sunny cabin passes 45 degrees C within an hour once outdoor air reaches about 20 to 23 degrees C.