[bt848]

The bit about the cell phone discharge seems pretty unlikely. A charged car battery has at least 2MJ of stored energy. GSM in its highest-power mobile station state draws 800mW. Add maybe another 150mW for the GPS, call it 1W total. The battery should power such a phone for weeks.

You should be able to smell-test this fact by considering that your mobile weighs nothing and runs all day whereas a car battery weighs 20kg.

[zaarn]

Car batteries are generally a starter battery (minus EV ones). They don't like being closed circuit for extended periods of time.

A low current is usually allowed (200mW is the top end in my experience), but anything over will, over time, sulfinate the plates in the battery and cause it too loose charge and capacity rapidly.

Generally if you want to run a low current device on a lead acid battery, you should use a deep cycle battery with plates designed for it. Starter battery have large, flat plates to carry a lot of current for only a few seconds tops, then be charged up again.

[ash]

It looks like Zipcar should have a separate smartphone-like battery to power GSM and GPS.

[mcguire]

How long does it take to damage the car battery?

How long does it take to discharge the car battery?

[zaarn]

Depends on the load.

[msbarnett]

Your smell test is a bit misleading in that most car batteries have a drastically lower energy density than Lithium Ion cells. There’s still a lot more energy in the car battery, of course, but not by nearly as much as a naive comparison of the weights would suggest. If it had the energy density of your cell phone, the lead acid battery in your car would only weigh 5 or so kgs.

Your cellphone will also last a lot less than “all day” if you take it somewhere where it has to run the radio at full power to try to maintain a connection. I’ve seen otherwise reliable and long lasting phones easily end up burning 1% a minute in areas of really bad signal.

[dTal]

I also seem to recall that standard car batteries respond poorly and inefficiently to trickle-discharge, being designed mainly for providing a lot of amps over a short period. Providing long term off-grid power is what deep cycle marine batteries are for, and they are a separate product.

[makomk]

As far as I know standard car batteries are fine with this kind of trickle discharge. Where they fall short compared to deep cycle batteries is providing a more substantial current (think tens of amps) over a prolonged period of time.

[zaarn]

Trickle discharge up to a few dozen milliwatts is generally fine for car batteries. They don't like being closed circuit in general but if you draw more than about half a watt for a longer time period, the battery plates will start to desintegrate.

[bt848]

OK, here's another test. A common way to end up with a dead battery is to leave the lights on overnight. The lights on a car draw over 100W (standard halogen headlights and incandescent tail and marker lights). But you can leave your lights on for hours and still start the car. Therefore it stands to reason that the same battery could power a GSM radio for hundreds of hours.

[zaarn]

That assumes the battery behaves the same for low and high current (they don't).

Even lithium cells already don't; high currents causes them to warm up, reducing capacity. So a device that draws 1 Watt will last longer than one that draws 10 Watts from a small lithium cell.

Lead Acid responds even worse, especially starter batteries, less so on deep cycle, and it's capacity rapidly changes, without temperature influence. A 1 Watt device might only last a day while it's fine starting up an engine with 10 times the total power consumption

[karlp]

This assumes the battery is fully charged, if it's cold or if the battery is slightly defective, it could be a lot less than full charge.

[2rsf]

Searching for signal uses some kind of exponential backoff algorithm (somekind because of patent wars between Qualcomm Broadcom and others), the phone doesn't simply sends the maximum power every fraction of a second. 1% per minute means something is wrong with your phone, or a weird edge case.

[bt848]

GSM phones don’t send anything at all in the first phase of registration.

[PetahNZ]

Anecdotal, but if I leave a USB charger plugged into the cigarette socket, without even anything plugged into the usb, it drains my battery overnight.

[bt848]

Get a refund.

I leave a phone plugged into my car's USB port 24x7 and the car has its own GSM modem too and obviously it starts right up every time.

[dharmab]

The difference is that you're using a USB port directly built into the car (which is "smart" about not draining the battery) while the other user is using a cigarette lighter to USB adapter (the cigarette lighter is a "dumb" circuit).

[allannienhuis]

getting your battery tested might be useful too.

[oh_sigh]

If it's not a car from the 80s there may be a fuse you can add/remove which converts your cigarette lighter from always on to only when engine is on. Check the back of your car manual.