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ZDNET's key takeaways

• Excessive temperature is a key factor in premature battery wear.
• Car charging is the perfect storm when it comes to overheating.
• You can still charge in the car, but you have to take some precautions.

I've been tracking the battery health of my daily driver iPhones for so long now that it's practically second nature. It's like a weird hobby/habit that I've picked up. Every few days since I got my iPhone 17 Pro Max, I've been firing up the Settings app and taking a look at my battery stats. I'm particularly curious because, with this handset, I decided to throw caution (and Apple's battery care tips) to the wind.

Also: I spent 2 years charging my phone exactly how Apple suggests - here's where it's at now

Why? Well, I followed all the rules with my last iPhone. I turned on Optimized Battery Charging, was careful not to keep the phone on charge for hours on end, and even did my best to keep it cool. Despite doing all this, the battery barely lasted two years before it became so bad it was frustrating to use, and a handset I'd planned on keeping for three years got replaced after only two.

So, this time around I stopped caring. No charge limitation, no fancy optimizations. I just threw it on whatever charger or used any power bank that was close to hand.

There was one thing I changed.

The car charging dilemma

I spend quite a bit of time in cars. And putting my phone on charge is just as much of a ritual as putting on my seatbelt.

But here's something that I've noticed has become an increasing problem, especially over the past few years. No matter how I charge my phone, whether it's wired, wireless in a dock, or using the car's built-in charging pad, my phone always gets hot. And I don't just mean a little warm, I mean hot.

Also: After testing this Anker, I wish every wireless charger had a thermoelectric cooler

Cars are an odd thing. In summer they're like a greenhouse on wheels, and in winter we're pouring heat into them to warm them up. And that cooling and heating is aimed at keeping the humans comfortable. The phone is normally on the dash, in the window, in front of a vent, or in a crevice.

It's a tough life for a phone.

Something else that I noticed was that the only time I have ever seen my iPhone enter "Charging on Hold" mode (a mode where it pauses charging in order to cool down) is when it's on charge in the car. I've not seen this happen when it's attached to a power bank, or when it's charging on my nightstand.

It's only when it's in the car.

Heat is the enemy of batteries

Heat is bad for batteries, and excessive heat is known to accelerate chemical degradation, leading to premature failure.

Apple has a lot to say on the subject of heat.

Also: iPhone charging slowly? 6 quick fixes to try before blaming your battery

The iPhone (and iPad, iPod, and Apple Watch) all operate best at 32° to 95° F (0° to 35° C), and the company warns that it is "especially important to avoid exposing your device to ambient temperatures higher than 95° F (35° C), which can permanently damage battery capacity."

The interior of a car can easily get to the point where it's bad for batteries (and humans!) very fast. Here's a calculator based on a Stanford University School of Medicine study. Even on a mild 70°F (21°C) day, the interior of a car can hit 104°F (40°C) in just 30 minutes and 113°F (45°C) after an hour. Bump the temperature to 75°F (24°C), and the internal temperature will break 118°F (48°C) in an hour. And a Texas summer high of 98°F (37°C) will push the interior temperature to a sweltering 123°F (52°C) in half an hour.

Also: 12+ iPhone settings you can change to noticeably improve its battery life (iOS 26 and older)

Batteries are going to die -- you can't change the laws of physics -- but you can give them a fighting chance. Apple itself warns that high temperatures can permanently reduce battery capacity (this is why iPhones go into a "Charging on Hold" mode when things get too hot).

So, I decided to stop charging my phone in the car altogether. Well, almost. I'll get to the exception in a moment.

The results so far

This one small change seems to be making an appreciable difference. My iPhone is currently at 196 recharge cycles, and the battery health is still at 100%. That's impressive, considering Apple claims newer iPhone batteries are designed to retain up to 80% of their original capacity after 1,000 full charge cycles.

At nearly 200 cycles, I'm already 20% into the 1,000-cycle lifespan Apple estimates, and yet my battery health hasn't budged.

Back in February, I wrote about how amazed I was that my phone had hit 122 recharge cycles without dropping to 99% battery health. Now, at 216 days old and almost 200 cycles, I'm even more impressed. It's almost like my iPhone is defying the laws of battery physics -- it's not, it's just proof that listening to the science and avoiding heat really does help.

The exceptions

Now, I'd be lying if I said I never charge my phone in the car anymore. Sometimes, life happens, and I need a quick top-up while I'm on the move. But when I do, I take a few precautions. I make sure to keep the phone out of direct sunlight and away from the heater. I'll even do my best to position my phone near an air vent with the AC on if it's a particularly hot day.

Also: Why this MagSafe battery pack is our readers' favorite model right now - especially at its price

Also, I've replaced all my in-car chargers with ones that make use of thermoelectric cooling to actively cool the iPhone while it's charging (more information on thermoelectric coolers here).

None of this is perfect, but I'm not perfect either, so it's a compromise that I'm happy to live with.

What about those built-in charging pads?

Bottom line, they're awful.

Partly this is down to the fact that my iPhone 17 Pro Max is too big to fit most charging pads, and because I have it in a big, thick case, but also because they're just so poorly designed most phones -- especially ones that have a "camera bump" -- don't interface with the charger well enough.

I rent a lot of vehicles for travel, and I always try the charging pad if there's one available, and I can't think of one I've had that, in the words of the late Apple CEO Steve Jobs, "just works." They seem to work better with a smaller handset, but even then the experience is far from great.

Also: Setting a MagSafe charger on my nightstand was the iPhone upgrade I didn't know I needed

Every handset I've tried on every in-car charging pad I've had access to has warmed up unacceptably.

It's shocking that companies that can build cars can't build a decent in-car charging experience.