Hi. I have built a GSM controller to turn on/off my car's Webasto (coolant pre-heater). I'm using a Pro Trinket 5v together with a Adafruit FONA module to do this.
The FONA module requires a Lipo battery to operate, which I have (see att picture of battery).
The controller is constantly powered by the car battery, hence the Lipo battery is always on charge by the built in charging circuit in the FONA.
The controller PCB will be placed in a plastic box, and the controller consumes somewhere between 100 and 150mA.
I'm concerned about low temperatures when it comes to the Lipo charging, as the controller might face as low as -20 C. As a initial safeguard, I have a temperature sensor taped to the battery, that will alert me by SMS if it goes below 0 C.
So, my questions/search for experience:
Will the constant charging help keeping battery temp above 0 C?
If mounted in a plastic box, will heat from the electronics keep box temp above 0 C?
Should I try to isolate the box, to keep the heat from electronics?
I will soon mount the controller in my car, and get some real data on temperaures. Still it would be nice to know if anyone out there has some experience with this issue.
If you know your Li-Ion Battery Cell's model or manufacturer, you can check absolute maximum and minimum requirements for temperatures via its Datasheet.
You can not and should not Constant charge a Lithium Cell, all Electronic Circuits that have Lithium battery charger also have a cut-off voltage when charging the battery even if you want it to over-charge. Constant charging Lithium batteries will cause them wear-out quickly. It's not the best practice, check if you can use Ni-Mh Batteries with the FONA but it's unlikely supports other type of batteries.
When charging Lithium batteries they will get warm depending on charging current, but in your case even always connected battery will not get that warm (considering charging current) (I believe you won't even feel the warmth with your finger tips)
They will not generate enough heat, I believe.
You should consider isolating everything from moisture, heat, cold for general approach.
In general speaking manufacturers provides more sophisticated electronic parts for car industry, for extreme conditions, so implementer be sure they will both work in a desert of ice or sand.
If you can track temperature and the power consumption is not an issue (a car when its engine stopped, is a battery powered vehicle and you should keep this in mind), you can use heat generating resistor for your isolated box to keep your GSM shield and Li-Ion Battery above 0 C if you needed.
I checked the Fona Specs. You can not use Ni-Mh batteries.
Why do you need a battery in your project? For anti-theft reasons? Do you have GPS module on your project? (I am asking because I am also always wanted a pre-heater for a long time)
The controller is constantly powered by the car battery
If you have constant power supply why do you need a secondary battery? Depending how you wire your whole project, you can track your car's main battery health so you can be sure you won't encounter any trouble like depleting your car's battery and you can even cut pre-heating if battery drops certain voltage.
Thanks for reply! I'm currently using the battery as Adafruit clearly states that the FONA will not work without, due to current spikes in consumption. They actually go quite far in their statement on this:
Q: I really want to run my FONA without a Lipoly battery! You can't. We keep getting people who ask "Hey I know its a requirement but I want to build a buck/boost/linear 3.8V regulator and it wil be OK because I said so" and then they realize it doesn't work well The FONA design depends on a Lipoly battery. The battery is not optional. The battery is essential to function. If you do not want a Lipoly battery you will have to use a different cellular module design. All of our FONAs require, depend and will not work without a Lipoly battery You can keep the battery topped up with an external 5V power supply for long term usage but the battery is still required even if plugged into USB
I am however planning to try to replace the battery with a DC-DC converter anyway, and maybe an addidtional capacitor - so I'll guess I have to go that route soon.
I will anyway try the cuircuit as-is and monitor temperatures.
No, I dont have a GPS installed. Currently, the only purpose is to turn Webasto and the car's climate control on/off. Might do more in the future though, such as anti-theft etc!
That was come to my mind. The current spikes (it seems above 2A)so high when GSM device communicating with the 2G cell tower, when FONA is not powered with a battery, it eventually brown-out at some point.
LiPo batteries will operate down to -20C, but do not like to be charged below 0C.
Better quality LiPo chargers have provisions for temperature sensors on the battery (e.g. cell phones) to detect when the battery is outside of the safe range for charging.
It's not clear to me whether this particular module supports a temperature sensor, but if it doesn't, I'd be hesitant to use it in this application without some other safeguard.
