Heat is what kills your device batteries — and a failed one cannot go in the mail

By Michael Obraitis, Managing Partner, and Michael Dale, CEO · Published August 21, 2026 · Last reviewed August 2026 · ADAM Pulse Knowledge Base · Operations Series

Short answer

Three things determine how long the batteries in your deployed devices last, and only one of them is about charging.

• Heat is the dominant factor. Apple's own guidance is that its devices operate between 32° and 95° F, and that exposure above 95° F (35° C) "can permanently damage battery capacity." That is a one-way door. The capacity does not come back when the device cools down.
• Where a device spends its idle hours matters more than how it is charged. A tablet that lives in a vehicle is the clearest example, and the numbers are worse than most people expect.
• A failed battery is a regulated item. Once a lithium cell is damaged or swollen, federal law says it may be transported by highway, rail or vessel only — aircraft is out — and it requires specific packaging and marking. The everyday habit of putting a swollen headset in a padded envelope and mailing it back to the vendor is a hazardous materials violation.

If you deploy phones, headsets, tablets or handsets across sites, the practical version is: control where devices sit when nobody is using them, stop storing spares at full charge, and write down what happens when one swells, because that is the moment when an ordinary shipping decision becomes a compliance problem.

Why device batteries actually degrade

Three separate clocks

A lithium-ion cell loses capacity along three independent paths, and confusing them is why most battery advice is useless.

Most advice concentrates on the first, because it is the one that sounds like a habit you can change. In a business deployment the second and third do far more damage, because devices spend the overwhelming majority of their lives idle — in a drawer, on a cradle, on a dashboard — rather than in someone's hand.

The number that matters

Apple states the operating range plainly:

[Devices] work best from 62° to 72° F (16° to 22° C). It's especially important to avoid exposing your device to ambient temperatures higher than 95° F (35° C), which can permanently damage battery capacity.

And on charging specifically:

...charging the device in high ambient temperatures can damage it further.

95° F is the line. It is not a comfort threshold, it is the point at which the manufacturer says damage becomes permanent. Hold that number, because the next section is about how routinely your equipment crosses it.

Your vehicles are the worst thing you own

This is the section worth acting on, because the arithmetic is genuinely surprising.

The National Weather Service publishes what happens inside a parked vehicle:

The temperature inside a vehicle can rise 20 degrees in as little as 10 minutes... 50 degrees in an hour.

and

...temperatures inside a car can reach 120° F in minutes and approach 150° F in as little as an hour.

Critically, this holds "even when outside air temperatures are in the 70's." And on the workaround everybody assumes works:

Studies have shown that "cracking the windows" provides little (if any) relief.

Put the two sources together and the operational conclusion writes itself:

A technician's tablet left on a passenger seat during a forty-minute site visit, on a pleasant 75-degree day, has spent most of that visit above the temperature at which the manufacturer says damage becomes permanent. Not on a brutal August afternoon — on a nice day in April.

Multiply that by every service call, every day, for two years, and you have the real explanation for why field devices reach end of life so much faster than office devices bought at the same time. It is not that field staff are hard on equipment. It is that the equipment cooks between jobs.

What to do about it

None of these are expensive, which is the point:

• Devices come inside. Into the building on the call, back into the cab — not the bed, not the boot — between calls. The habit is the fix.
• Never charge a hot device. Apple is explicit that charging at high ambient temperature compounds the damage. A device that has been sitting in a hot cab should cool before it goes on a charger.
• Insulated cases are not sun protection. Insulation slows heat transfer in both directions. A device already hot inside an insulated case stays hot longer.
• Mounts matter. A windscreen mount in direct sun is the hottest position in the vehicle. A mount low and out of the sunline is meaningfully cooler.
• Track it by site. If one location burns through device batteries and comparable locations do not, look at where the devices sit between uses before you blame the hardware.

The charging cradle problem

Desk phone handsets and wireless headsets present the opposite problem: they are almost never hot, and almost always full.

A cordless handset or a contact-centre headset typically lives on a cradle whenever it is not in use, which means it sits at or near 100 per cent charge for the great majority of its life. Sitting at high state of charge accelerates calendar ageing — the second clock in the table above — independently of how many cycles the device has been through.

There is a real tension here and it is worth being honest about it rather than pretending the answer is obvious. A headset that is not on its cradle is a headset that is flat when somebody needs it, and an agent unable to take calls costs more than a battery. For a device in daily use, leave it on the cradle. The cradle is doing its job.

The advice bites for devices that are not in daily use, which brings us to the drawer.

The drawer of spares

Almost every multi-site business has one: a drawer, a cupboard or a shelf holding spare handsets, replacement headsets, seasonal tablets, or the devices from a location that closed. They are usually stored the way they came off the shelf — fully charged, powered on, ready to go.

That is the worst possible storage state, and the manufacturer's guidance says so directly. Apple's long-term storage recommendation:

Charge it to around 50%... Power down the device... Place your device in a cool, moisture-free environment that's less than 90° F (32° C).

and, for storage beyond six months:

...charge it to 50% every six months.

So the correct way to hold spare devices is: half charged, powered off, somewhere cool, topped back to half twice a year. Not full, not on a charger, not in a van, not in an un-airconditioned storeroom that hits 100° F in July.

Two failure modes come out of getting this wrong, and both are expensive in ways that do not show up as a battery line item:

• Spares that are dead when needed. A device stored at full charge and left for a year may have lost enough capacity that it no longer holds a shift. You discover this during the outage you bought it for.
• Spares that have swelled in storage. Now it is not a spare, it is a hazardous item sitting in a cupboard, and the next section applies.

Put a recurring calendar entry against the spares cupboard. Twice a year, thirty minutes, check and top to half. It is the cheapest maintenance task in the building.

How to tell a battery is failing

The New York State Division of Homeland Security and Emergency Services publishes five warning signs. These are worth putting on a laminated card wherever devices are stored or charged:

Swelling is the one your staff will actually meet. A handset that no longer sits flat, a headset whose earcup has developed a curve, a tablet whose screen is lifting at one corner. Every one of those is a battery that has vented gas internally, and it is a failed cell, not a cosmetic defect.

The published response is unambiguous:

1. Stop using the device and turn it off
2. Unplug it from power
3. If safe, move it away from anything flammable, using tongs or gloves
4. Leave the area
5. Call 911

That last step reads as dramatic for a swollen headset, and in practice a swollen device that is cool, quiet and not smoking is not an emergency. But the sequence is the right one for anything hot, hissing, smelling or smoking, and the judgement call should not be made for the first time by whoever happens to be holding it. Decide in advance, write it down.

What you cannot legally do with a failed battery

This is the part that catches businesses out, and it is genuinely a compliance exposure rather than a best practice.

Under 49 CFR 173.185(f), lithium cells or batteries that have been damaged, or identified by the manufacturer as defective for safety reasons, "may be transported by highway, rail or vessel only."

Air transport is prohibited. Not discouraged — prohibited. Which means the ordinary reflex of dropping a swollen device into a courier envelope for return to the vendor is very likely putting a forbidden item onto an aircraft, because most expedited courier services move by air.

Ground transport is permitted, but not casually. The regulation requires:

• Individual non-metallic inner packaging that completely encloses the cell or battery
• Surrounded by cushioning material that is non-combustible, electrically non-conductive, and absorbent
• Placed in outer packaging meeting Packing Group I performance standards — metal, wooden or plastic boxes, drums, or rigid large packagings, per 49 CFR Part 178
• Marked "Damaged/defective lithium ion battery" in characters at least 12 mm high

That is a hazardous materials shipment with proper packaging and marking, not a padded envelope.

The FAA's passenger rules run in parallel and matter for anyone travelling with kit:

• Spare batteries and power banks are carry-on only and may never go in checked baggage
• Standard limit is 100 watt hours per battery; 101 to 160 Wh needs airline approval, maximum two spares
• Terminals must be protected against short circuit
• "Damaged or recalled batteries and battery-powered devices, which are likely to create sparks or generate a dangerous evolution of heat must not be carried aboard."

If a technician is flying to a site with spare devices, those rules apply to them personally, and the airline enforces them at the gate.

The practical policy

The workable answer for most businesses is to not ship failed cells at all:

• Quarantine on site. A metal container, away from combustibles, away from exits, not on a charger, not in an occupied room overnight.
• Use a local collection point. Household hazardous waste sites and battery recyclers accept damaged cells and are equipped for them. This avoids the transport question entirely.
• Ask the vendor for their hazmat process. Manufacturers running warranty returns on batteries have a compliant route and will tell you what it is. Ask before you ship, not after.
• Never pass the problem to a courier without declaring it. Undeclared hazmat is the part that carries penalties.

Disposal is regulated too

When a device fleet is retired, the batteries do not go in the bin. The EPA states it plainly:

Lithium-ion batteries and devices containing these batteries should NOT go in household garbage or recycling bins.

They "should be taken to separate recycling or household hazardous waste collection points." EPA's handling guidance is to place each battery or device in a separate plastic bag and put non-conductive tape over the terminals.

The reason is not bureaucratic. Batteries that reach a materials recovery facility get crushed, and crushed lithium cells start fires — which is why EPA notes the industry response to "the increase of fires at recycling and waste facilities across the country."

For a business this lands in two places:

• Device refresh projects. When you replace fifty handsets, you have created fifty regulated items on the same afternoon. Plan the disposal route before the new hardware arrives, not after the old kit is stacked in a corner.
• The e-waste contractor. Confirm they are a certified electronics recycler and that they handle batteries specifically. "We take electronics" is not the same answer.

What to put in your device policy

Most organisations have nothing written down about any of this. A page covers it:

Questions worth asking at procurement

When specifying devices for field or multi-site deployment, these separate the vendors who have thought about it from the ones who have not:

• Is the battery user-replaceable, and what does a replacement cost? A device with a replaceable battery has a materially longer economic life. This is often the single largest factor in fleet cost per year and it is rarely on the comparison sheet.
• What is the rated operating and storage temperature range? Compare it against where the device will actually live, not against room temperature.
• What is the warranty position on battery capacity specifically? Batteries are frequently excluded, or covered only to a stated percentage of original capacity.
• What is your returns process for a swollen or damaged unit? A vendor with a real answer has a compliant route. A vendor without one is going to suggest you post it.
• Is there a ruggedised or extended-temperature variant? For vehicle-mounted deployments this is often worth the premium purely on replacement cycles.

What we would do first

If you do nothing else from this article:

1. Look at where field devices sit between jobs. This one change buys more battery life than every charging habit combined.
2. Open the spares cupboard. Check for swelling, then set them to half charge, powered off, and put a twice-yearly reminder in the calendar.
3. Print the five warning signs and put them where devices are stored and charged.
4. Decide the swollen-device procedure now, including where quarantine happens, so nobody improvises.
5. Agree the disposal route before your next refresh, not during it.

Frequently asked questions

Related articles

• Five inexpensive things you can do today to make your business harder to hack — the same approach applied to security: cheap, unglamorous, effective.
• Grandstream phones dropping off a FortiSwitch — when the hardware problem is really an environment problem.
• Why would hackers target my small business? — another case where the intuitive answer is the wrong one.

References

Manufacturer documentation, the National Weather Service, a state emergency management agency, the Code of Federal Regulations, the FAA and the EPA. Deliberately no product reviews and no model numbers — everything here is either physics or regulation, and neither expires with the next product cycle.

1. Apple — Maximizing battery performance— manufacturer source for the 32 to 95 degree Fahrenheit operating range, the 62 to 72 degree comfort band, the statement that above 95 degrees can permanently damage battery capacity, the warning about charging at high ambient temperature, and the long-term storage guidance of 50 percent charge, powered down, below 90 degrees, topped up every six months.
2. National Weather Service — Beat the Heat, Check the Back Seat— source for a parked vehicle rising 20 degrees Fahrenheit in ten minutes and 50 degrees in an hour, reaching 120 degrees in minutes and approaching 150 degrees within an hour, that this occurs even with outside temperatures in the seventies, and that cracking the windows provides little if any relief.
3. New York State Division of Homeland Security and Emergency Services — Lithium-Ion Battery Fire Warning Signs— source for the five warning signs (heat, swelling or bulging, hissing or cracking or popping sounds, odour, smoke) and the five-step response procedure.
4. 49 CFR 173.185 — Lithium cells and batteries (eCFR)— paragraph (f) is the source for damaged or defective cells being transportable by highway, rail or vessel only, the non-metallic inner packaging and non-combustible, non-conductive, absorbent cushioning requirements, the Packing Group I outer packaging standard, and the 12 millimetre "Damaged/defective lithium ion battery" marking.
5. FAA — Lithium batteries in baggage (PackSafe)— source for spare batteries and power banks being carry-on only, the 100 watt hour limit, the 101 to 160 watt hour allowance with airline approval capped at two spares, terminal protection, and the prohibition on carrying damaged or recalled batteries and devices.
6. US EPA — Used lithium-ion batteries— source for lithium-ion batteries and devices containing them not going in household garbage or recycling bins, the direction to separate recycling or household hazardous waste collection points, the bag-and-tape-the-terminals handling guidance, and the note on rising fires at recycling and waste facilities.
7. PHMSA — Transporting lithium batteries— the Department of Transportation's overview of lithium battery transport obligations, for readers who need the wider regulatory picture rather than the specific damaged-cell paragraph.