I am powering a SIM800C from a 3.7V Li-ion battery. If the battery charge drop below 85% (it still has 4.1V measured without load), the module keeps restarting during startup. This probably happens because of a high load (up to 2A) that causes a voltage drop. According to the SIM800C manual, this could be avoided by using capacitors. I have tried to interpret the reference circuit from Simcom and I use the recommended capacitors but this does not help. Am I connecting the capacitors the right way?

This is the reference circuit from the SIM800C manual:

Reference circuit

And this is how I have made the connections:

My connections

CA is a 100uF tantalum capacitor and CB is a 10 uF ceramics capacitor.

  • \$\begingroup\$ This depends on layout as well - the caps need to be as close as possible to the module. What are the values of Ca and Cb? \$\endgroup\$ – pjc50 Apr 19 '16 at 9:05
  • \$\begingroup\$ Ca is 100uF tantalum and Cb is 10 uF ceramic. The module is mounted on a breakout board and the capacitors are mounted on a separate breadboard. Do you think I need to make a PCB with everything on the same board to make it work? \$\endgroup\$ – Nelson Apr 19 '16 at 9:17
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    \$\begingroup\$ The 100uF capacitor is for decoupling purposes, as it is stated in the datasheet. They are recommended to place as close to the module's VBAT pins as possible. To avoid voltage drop during transmit burst you probably need an additional Buffer capacitor. Here is an other EE.SE thread about choosing the right one. \$\endgroup\$ – Bence Kaulics Apr 19 '16 at 9:21
  • \$\begingroup\$ @BenceKaulics Thank you. That thread looks very interesting. \$\endgroup\$ – Nelson Apr 19 '16 at 9:24
  • \$\begingroup\$ The length of the transmit burst is indicated in the datasheet, probably it is about 577 μS. \$\endgroup\$ – Bence Kaulics Apr 19 '16 at 9:26