Is it possible a cell phone in use with the charger plugged in could drain a healthy car battery if the phone is used (with the engine off) for an hour? Two hours? If it matters, the car probably had to have the key turned to the accessory mode in order for the charger to charge. I’ve also read a phone charging from a car battery while the phone is in use draws more than it charges. (Aside: I noticed this when driving around using Siri for directions while charging the phone…it never gained much charge, if any.)
Is it possible? Sure, anything is possible, but it seems unlikely, unless the car battery wasn’t as “healthy” as you may have thought it was.
A charged, healthy battery? No. A typical car battery has a capacity of about 45-60 Amp Hours. A cell phone charger draws up to 3 amps. Something else is drawing down the battery or it isn’t nearly as charged or healthy as it should be.
Unlikely for a healthy battery. Some cars draw a lot of current for electronics and electrical accessories and may be difficult to start if the battery is drained by enough hours of cell phone use or charging. If that happens it’s more likely a sign of an old battery or something else draining the battery like a short somewhere that no one can find until you sell the car and the new owner discovers it immediately and never has a problem.
That’s 3 amps on the downstream end of the charger, which is 5 volts. Assuming a 100% efficient power supply, the amps coming from the car battery would be 3 * 5/12 = 1.25 amps. Efficiency is probably ~90%, so figure about 1.4 amps from the car battery.
That seems doubtful. Most phone manufacturers claim you can talk for several hours on a single full charge, whereas you can rapid-charge a phone in about an hour with a decent charger. If your charger is so underpowered that it can’t keep up with the power requirement of a phone call, that’s not the car battery’s fault, it’s the charger.
Even using other features (instead of making a call) takes many hours to drain the phone’s battery:
This matters.
The phone itself wouldn’t kill the battery in an hour or two. But all of the other things that are turned on by the key being in the accessory position could. This also varies, depending on the car.
This also varies, depending on the phone, and the charger. A phone with a fairly high power draw can overwhelm the capability of a generic charger mounted in a car. The same thing can happen if the phone is plugged into a generic charger at home. You probably won’t be asking Siri for directions around your house (at least I hope not), but any app with a high CPU load that keeps the screen on, like games for example, could cause the phone to draw more current than the charger can replenish.
One important difference is that the phone usually blanks the screen while you are talking. The OP was using a mapping app that was constantly updating its location via GPS, which is fairly CPU intensive. The app was also keeping the screen on the entire time, which draws a lot more power.
A car battery that dies while charging a phone for an hour or two was never going to start the car, anyway.
Some cars will draw maybe an amp or two when the key is in the accessory position. Others will draw significantly more, maybe as high as somewhere around 20 amps. Again, it depends on the car. On the low end, the battery could last a day or two. But a car that is on the high end of that range can easily kill a perfectly good battery in a couple of hours. The fact that the phone killed it after a couple of hours doesn’t mean it was a bad battery. Again, the important thing is that the ignition was switched to the accessory position.
One of my cars turns on a lot of crap when you switch the ignition to accessory. But it also shuts all of that back off after about 10 or 15 minutes. If you switch it to accessory and turn on the radio, then switch the key back to off, almost everything turns back off but the radio will keep playing. But again, only for about 10 or 15 minutes (I haven’t timed it but it’s somewhere in that range).
Not all chargers are created equal. “Standard” USB is 5 volts and 500mA; only 2.5 watts, which is not much power. A phone with the screen and GPS on could easily be pulling near that much from the battery. A phone playing an intensive game is most likely using more energy than provided by the charger.
Most phones, when paired with the appropriate charger, will be able to use much more power. For example, a modern iPhone connected to a USB-C power delivery (PD) charger will consume 2000mA at 9 volts, so 18 watts (assuming the battery is low enough to accept that much charging). Even the hardest use is unlikely to use 18 watts, so the phone should still charge at a reasonable rate.