[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.