12 volt temperature controlled warming box

[lasvegasmale31]

Just my .02, but why not leave the phones on a cell charger overnight? If memory serves, they monitor battery voltage and if it begins to drop, the phone starts charging. If my logic is correct, and if my phone isn't the oddball of the cellular industry, phone batteries get pretty warm as they charge.

 

[dknguyen]

There maybe a problem with lithium battery chemistry (something like the minimum/maximimum voltages) where the limits are different enough at temperature extremes so that a charger not compensating for temperature can damage the battery.

I think some RC pilots have run into this problem.

 

[Diver300]

The batteries are certainly the weak link.

Leaving them on charge won't help much. Lithium-Ion or Lithium polymer batteries don't heat themselves up when being charged, and they can't be charged when colder than about 0 C (32 F)

I guess that it won't take much power from the vehicle battery to keep the cellphone above about 0 C.

You definitely need a temperature controlled heater, but it does not need very precise temperature control. The circuit shown, with an NTC thermistor will work. If you squeeze that into a mobile, or the mount for it, that should work. The resistor values depend on the supply voltage.

Alternatively you could invest in car phones, rather than hand-held mobiles. One such phone is M930 GSM Car Phone - Motorola USA
The advantage is that they can't be accidentally taken from the vehicle, and they don't have a battery at all. They run straight from the vehicle battery.

Handheld mobiles only charge from their external supply, they cannot run from the external supply if the battery is dead or too cold.

 

[Andy1845c]

I put pennies in with the phone last night and today the phone was actually on

What do pennies have to do with anything? Are you adding handfulls of them simply for mass?

 

[MrAl]

Hello there,


Newton's law of cooling looks something like this:

T3=(T1-T2)*e^(-a*t)+T2

where
T3 is the final temperature
T1 is the initial temperature
T2 is the surrounding air temperature
a is a constant
t is time.

We can not change this fundamental property of nature, but what we
can do is change the constant 'a'.
This is actually pretty easy to do too, as 'a' depends on the amount of
insulation surrounding the closed space of say a small rectangular box
built just large enough to house the cell phone and a small light bulb
or other heating device like a resistor.
With such a box, surrounded by a single layer of thick styrofoam with
the inside heated to +30 degrees C, the temperature T2 after say 1 hour
might be around +15 degrees C. Well with twice the original thickness,
the temperature after 1 hour would be +21.8 degrees, so by doubling
the insulation we effectively increase the time that a given amount of
heat keeps the inside of the box warm.
To keep that same box up to about 12 degrees C after 24 hours would
require 20 times the original thickness, but it all depends on the type
of insulation used.

To get an idea what would be required, a box could be built and lined
carefully with some insulation, then the inside heated to say +50 deg
C, and left outside or in the freezer for 1 hour, then measure the
inside temperature again. Using those two measurements and knowing
the time t and the freezer temperature the constant 'a' can be
calculated. Once this is done, the time to keep 'warm' for 24 hours
can be calculated and if the minimum temperature doesnt stay high
enough more insulation can be added. The amount to be added could
be calculated by assuming that for every additional layer of the
same thickness the constant 'a' decreases by one half. Thus, if 'a'
first calculated to 0.00001 then for two layers a would be 0.000005.
T3 can then be calculated again and if that doesnt come up high enough
add another layer, etc., etc.

All this can be calculated from that one measurement.

Alternately, try it with a thermos that has a mount big enough so that
the cell phone fits in it. Heat it up, make a measurement, calculate 'a',
calculate the new T3 for the 12 or 24 hour period. Go from there.

Note that all this is assuming that the inside of the box is heated only once.
If the heater can run off the battery, the situation gets even better. The
more insulation the less power required to keep the inside of the box above
say 10 degrees C. If there is enough room the box can be quite large meaning
lots of insulation and that would require very little power to keep warm.

 

[delgriffith]

What do pennies have to do with anything? Are you adding handfulls of them simply for mass?

My vacuum insulated jar has a 2-7/8" mouth, just big enough for my rather large i355 phone. Vacuum insulation is going to be vastly superior to any other insulation, so I don't think there is any point in messing with anything else or building a box of any sort. The pennies I put inside were at room temperature, so the additional stored energy in them must dissipate through the vacuum flask also. It's similar to filling the flask with warm coffee, but they don't make the phone soggy. The pennies would warm back up to room temp during the day when the vehicle is warm, so they'd "recharge" easily as long as they were out in the open and not sealed in the flask. I think this solution is going to work for us since my phone did survive a -20F night and I don't think I quite had things optimized with flask model and amount/type of energy storage material (pennies).

The other thing is that M930 phone. That could be helpful - I was going to write it off since we need GPS locator enabled phones, but I see that it is GPS enabled and might actually work with the cell provider we have. Thanks!