is it bad for my iphone 8 plus battery to be hooked to my car radio for long periods via usb cable

What you read was nonsense. The actual charging is managed by the phone. It stops charging when it reaches 100%, and if you leave it plugged in, the external power source will power the phone. This reduces the number of charge cycles; a charge cycle is defined as charging from zero to full. So if you charge the phone from 50% to 100% that is 1/2 of a cycle, and doing it twice would be 1 cycle. But leaving it connected is zero cycles.

If you turn off the ignition and the phone's battery drops to 90%, then you turn it on again it will charge to 100%, using a tenth of a charge cycle. 10 times would be one cycle.

After all of that, the answer is just leave it plugged in. It won't hurt the phone or its battery.

mdesignffm wrote:

there are different chargers around, with different methods of charging (slow or fast, constant or pulse). the algorithms that prevent overcharging are different, some are ineffective under certain circumstances and therefore do not prevent overcharging. if the current and voltage feeding the charger are not stable (which may be the case in a car), the charging process itself may vary, too - which may strain the battery.

I had to get back to this specific comment of yours. Yeah I'm piling on, but it needs to be said, because what you wrote is just all sorts of wrong.

That thing with the USB port or maybe a captive Lightning cable providing DC power? Again - not a charger. It's not charging a NiMH AA/HR6 battery where it's implementing some sort of negative delta V charging algorithm, where it has no idea what the battery has been through. It's a 5V DC power supply. Provided it can provide a well-regulated, flat supply within ±0.2V you're golden. That's all it has to do. If it doesn't, the battery management system inside an iPhone will complain (This accessory may not be supported.). I've even seen that with a genuine Apple Lightning cable that had been bent way too many times at the connector tip (but the boot hadn't frayed/torn) by my wife, connected to a genuine Apple 12W adapter (which Apple says is fine).

All the brains (the battery management) is inside the phone. That applies the algorithms. That reads/updates the battery stats data stored on a small board in the battery pack. That determines when to start, when to stop, and when to reduce the charging rate. It always backs off from the maximum charge rate as it gets closer to 100%. That's why you can typically charge it from 0% to 80% in less than an hour, but it might take an hour to get from there to 100%. Before smarter fast charging systems were used, you'd end up with battery fires from a fast linear charging rate. There's also that data in the battery's little board that's helping the battery management in getting better estimates of how much capacity (ultimate and immediate) the battery still has.

I believe that Apple has engineers who are really good, and probably better than you. And probably better than me, also, but, as an engineer with 50 years experience myself, "belief" doesn't mean much without actual controlled experiments. So please describe the controlled experiments that you have performed that validate that leaving the phone plugged in overnight reduces its life. I have not done experiments, but I have always charged every iOS device I have owned overnight, every night, for over 10 years. None of the batteries showed any signs of degradation over the life of the device. What degrades the battery is accumulated charge-discharge cycles. When you leave the phone plugged in it actually reduces the number of charge-discharge cycles, because the external power source runs the phone rather than the battery. Consider two scenarios:

1. Charge overnight - once the phone reaches full charge it stops charging. Suppose it started at 50%; that's 1/2 of a full charge cycle
2. Again charge from 50% to 100%, then disconnect. When you need the phone again it's at 50%, so you charge it again. That's a full charge cycle rather than a half charge cycle

You can play with the numbers, but no matter how you do it charging for a longer period of time is less wear on the battery overall than more discharge/charge cycles.

The flaw in your thinking is your statement "my personal experience is that keeping a device charging over longer periods of time regularly (eg over night) will reduce battery life." The flaw in your logic is that keeping the phone plugged in is NOT keeping the device charging...overnight. The charger is not the thing that plugs into the wall. The charger is not the cable that connects the thing that plugs into the wall to the phone. The charger is a microcircuit (U2 Tristar) in the phone that regulates charge. And it stops charging when the phone reaches 100% on the display (which is NOT the full capacity of the battery, BTW; Apple leaves a gap between 100% and full charge, and also between zero and a dead battery).

Note that your experience may have been with other battery technologies or inferior products, but all Apple products and most other current generation manufacturer's products that use Lithium technology batteries employ similar "smart" charging circuits. My 2 Macbooks are connected to power for weeks on end, yet the still provide full spec'd battery life when I use them mobile on battery power. The older one is a 2010 Macbook Air. 7 years out of a battery with no loss of capacity is pretty impressive.

mdesignffm wrote:

a battery is an analogue component and its specifics vary to a certain degree. the way how a battery charges depend on these specifics, but also on temperature and the workload (how much power the phone is using) while charging.

there are different chargers around, with different methods of charging (slow or fast, constant or pulse). the algorithms that prevent overcharging are different, some are ineffective under certain circumstances and therefore do not prevent overcharging. if the current and voltage feeding the charger are not stable (which may be the case in a car), the charging process itself may vary, too - which may strain the battery.

using an original apple charger indoor while not using your phone too much will probably not harm your battery. for any other setup, i would not guarantee anything. in this case: using a low power usb fed by an instable voltage in a maybe hot drawer of a truck while streaming music via a weak cellular signal may be a different story.

That's where you're incorrect. That thing you plug into is not the "charger". It's a regulated power supply. The charging circuits are completely within the phone itself, and currently Apple regulates it to a max 2100 mA. You could provide it with a supply capable of 100 A and it wouldn't matter. If the supply is unstable, the charging circuits will tell the thing to stop charging - maybe a message that "this accessory may not be supported".

I don't recommend using a poorly designed power adapter, but most people can figure out how to get a quality one.

a battery is an analogue component and its specifics vary to a certain degree. the way how a battery charges depend on these specifics, but also on temperature and the workload (how much power the phone is using) while charging.

there are different chargers around, with different methods of charging (slow or fast, constant or pulse). the algorithms that prevent overcharging are different, some are ineffective under certain circumstances and therefore do not prevent overcharging. if the current and voltage feeding the charger are not stable (which may be the case in a car), the charging process itself may vary, too - which may strain the battery.

using an original apple charger indoor while not using your phone too much will probably not harm your battery. for any other setup, i would not guarantee anything. in this case: using a low power usb fed by an instable voltage in a maybe hot drawer of a truck while streaming music via a weak cellular signal may be a different story.

mdesignffm wrote:

my personal experience is that keeping a device charging over longer periods of time regularly (eg over night) will reduce battery life. so i believe this is a fact, regardless of what most user manuals say.

but anyway, in your case you do not have a choice. working your phone on battery all the time is no option either. so just go for usb and see what happens. maybe you can prove me wrong?

I leave my iPhone 7 plugged in pretty much every day, including overnight or sometimes even all day. After about a year and over 175 complete cycles, it's registering around 97% of the original design capacity.

"100%" is not really the maximum capacity of the battery. It might even be safe to go a little bit above that, but that's a ceiling that's been chosen to achieve greater longevity and safety. During the early days of lithium rechargeable batteries, there was a push to squeeze out more capacity by charging further along, and there were often fires. Most of the world's lithium-ion battery supply in the mid-90s was made at one Sony factory in Japan, and that place caught on fire - creating a massive shortage.

If anyone really needs to use the battery, it's only a matter of wearing it down and replacing the battery. However, there are many people who have them hooked up to battery packs or even battery cases all day without any substantial loss of battery capacity.

No, that will not hurt it but do try to work in a full charging cycle about once per month. Don't run the iPhone down to 0% though.

Apple has a standard - Made for iPhone or MFI. If the power source (it is NOT a charger) meets this standard it is safe to use. In the OP's situation the truck's sound system is providing the power, and I think it is reasonable to assume that it meets all standards for power stability. This would not be the case if it was a cheap cigarette lighter power source, but I'm sure the truck's power source is stable.

There are cases where inexpensive, uncertified power adapters have damaged the phone - usually blowing the U2 chip - but that hurts the chip, not the battery. As Apple will offer only a phone replacement if that happens there are a number of 3rd party repair facilities that will replace just that chip - and yell at you for using a cheap power adapter. Note that I am NOT advocating 3rd party repairs, just reporting reality.

my personal experience is that keeping a device charging over longer periods of time regularly (eg over night) will reduce battery life. so i believe this is a fact, regardless of what most user manuals say.

but anyway, in your case you do not have a choice. working your phone on battery all the time is no option either. so just go for usb and see what happens. maybe you can prove me wrong?

I use coconutBattery running on a Mac. There may be other apps out there that do it directly on an iPhone/iPad.

https://www.coconut-flavour.com/coconutbattery/

Apple has promised battery stats will be readily available in a future update (probably iOS 11.3). The last update doesn't have it yet.

Lawrence Finch wrote:

There are cases where inexpensive, uncertified power adapters have damaged the phone - usually blowing the U2 chip - but that hurts the chip, not the battery.

It would need to be something wholly unsuitable to do that, such as a really lousy, unregulated power supply putting a ridiculously high voltage that damages the power IC before it can react. I wouldn't worry about a UL or CSA certified power supply damaging an iPhone unless it were defective. And in that case people have had genuine Apple power adapters that were defective.

Lawrence Finch wrote:

There are cases where inexpensive, uncertified power adapters have damaged the phone - usually blowing the U2 chip - but that hurts the chip, not the battery. As Apple will offer only a phone replacement if that happens there are a number of 3rd party repair facilities that will replace just that chip - and yell at you for using a cheap power adapter. Note that I am NOT advocating 3rd party repairs, just reporting reality.

I've seen all sorts of things. Apple isn't immune to battery issues. They essentially buy batteries as commodities from the leading manufacturers, even though they're made to spec. I've had genuine Apple batteries do crazy things, such alternate reporting negative capacity and normal status, or swelling. The brand new battery in my MBP was reporting 95% new capacity as soon as I got it.

However, there is one thing that Apple does differently with notebook computers compared to iOS devices. They have that 95% thing, where it can only charge from 95% to 100% if it's continuously connected to the power adapter when starting at under 95%. I've had it at 100% where it slowly drops down to 95% while continuously connected to the adapter. Once it hits 94% it just starts charging again up to 100% and the cycle continues. I've heard this is meant to reduce the number of cycles at the top of the charge range.

mdesignffm wrote:

so you believe in the manuals. nothing wrong about that.

being an engineer myself, i also trust in my own experience.

no offence.

Except your spreading incorrect and misleading information. That I do take some umbrage at. I trust actual science rather than one person's (even my own) report.

mdesignffm wrote:

there are different chargers around, with different methods of charging (slow or fast, constant or pulse).

You may be an engineer but, that sentence indicates that rechargeable batteries and small electronics are not your field of engineering.

so you believe in the manuals. nothing wrong about that.

being an engineer myself, i also trust in my own experience.

no offence.