So, I want to know if I can hook up a 6V 2 watt solar panel to the Vchg pin on a SIM908 gsm/gps module and trickle charge my 6Ah LiPo battery that way. A little background...
So, I have completed the first prototype for a new product I'm developing that is essentially a SIM900 GSM modem, and a separate GPS receiving module, all hooked up to a 6 Ah battery that gets trickle charged using a 2 watt 6V solar panel hooked into the Adafruit Solar Li-Ion charging module (which is based on the MCP73871 charging chip). This charging chip features something called "Voltage Proportional Charge Control", which allows you to set the chip to charge at the max current possible without collapsing the input voltage field below a certain value (the Adafruit board uses 4.5V as this value). This essentially means that in any light condition, the charger will draw as much current from the solar panel as possible without causing the panel's voltage to collapse below 4.5 V.
Seems to work okay.
For the next iteration of the prototype (version 2.0, if you will) I would like to switch to the SIM908 module, which from what I can tell is essentially a SIM900 module with a GPS unit incorporated. I should mention that I'm also going to be just getting the SMD breakout board version (not a shield), so that I will have access to all of the pins on the SIM908.
In reading the "Hardware Design" guide that SimCom produced, I noticed that the SIM908 also has a Li-Ion charging function designed into it as well. Sweet! That means that I won't need to buy another Adafruit Solar charger (which costs me about $20 plus shipping). Right?
Adafruit seems to think this won't work very well, and say that setting up the charging circuit this way would result in the charger turning on and off repeatedly as it collapses the voltage coming from the solar panel by trying to draw too much current. Here is their discussion on this where they state:
Now you can see what happens if you connect a 6V solar panel to a lipoly charger. As long >as the current being drawn by the charger is less than the panel's short circuit current >at that light condition, everything is peachy. The moment the light changes even a >little, and the current the lipo charger wants is higher than the short circuit current, >the charger becomes unstable: it will draw too much current, which will cause the voltage >to collapse, which causes the charger to turn off, which reduces the current draw, which >makes the panel voltage recover, which turns on the charger again, which then draws too >much current, and the cycle repeats.
But would I really lose that much efficiency? Is there a downside to having the charger repeatedly turn on an off? Does anyone know how to find out if there is a Voltage Proportional Control Charge function built into the SIM908? I'm guessing I'm going to have to stick to a solar-specific charger, but I thought I'd ask here in case I'm wrong.
Here is what I'm thinking the SIM908 charging circuit would look like:
simulate this circuit – Schematic created using CircuitLab