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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:

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ It's a good question. I took a quick look at the datasheet for the charger IC adafruit favors. It seems like a good choice for this application, although I only scanned it. This is not the only charger IC that does this. I have seen several TI chargers that have this feature. They back off on charge current when the USB rail starts to collapse. I would look into what the maximum input voltage of the charger IC is, and the maximum open circuit voltage of the solar panel. Make sure you are not going to blow anything up. \$\endgroup\$ – mkeith Dec 16 '14 at 4:41
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    \$\begingroup\$ Oh to answer your other question, it is not a good idea to let the charger repeatedly crash and start over. For one thing, it will never get a chance to terminate charging. For another thing, there will probably be a certain amount of dead time in between. I don't think it is safe or reasonable to allow that to happen intentionally. \$\endgroup\$ – mkeith Dec 16 '14 at 4:45
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The answer is to use a charging chip that has the ability to back off on charging current when the input voltage from the solar panel begins to collapse.

While using a regular Li-Ion charging IC will technically "work" with a solar panel, it will take forever to charge in practice, and, as mkeith pointed out: "it will never get a chance to terminate charging".

The discussion about this on the Adafruit website is the best explanation of this phenomenon that I can recommend to others with this question:

Most people try to plug a solar panel directly into a lipo charger and while it sort of works, the battery takes forever to charge because the efficiency is terrible! That's because most lipo chargers are meant to plug into a USB port or wall, and are very simple in their design. USB ports supply 5V at up to 500mA and they're pretty solid - the voltage doesn't change much even at the max current draw. So when you plug a charger into a computer with a USB port, they just draw 500mA or so and happily chug away. Same goes for wall adapters. The voltage and current limits are kept steady. Solar panels are a little different, the voltage and current vary constantly depending on sunlight available. They are unstable! That instability confuses battery chargers, which causes them to do one of two things: rapidly turn on and off as they try to draw more current from the panel than possible and/or draw much less current than they can, to keep the voltage from collapsing

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