I want to make an UPS using a LiPo battery for a SIM800l cellular module, I have an SIM800l module attached to a 3.7V wall adapter but I also have connected the SIM800l to a 3.7V LiPo Battery, I want to stop using the battery when there is current from the USB wall charger.

I do not want to use a standard powerbank since these always use the battery.

What about this circuit:

enter image description here

but I would like to know what MOSFET and diode should I choose for a 3.7 Volts input and 3.7 Volts output. Will a BS250P MOSFET work?

  • \$\begingroup\$ And how do you plan to charge your battery then? \$\endgroup\$ Jul 31, 2018 at 23:27
  • \$\begingroup\$ (a) As I said on your last question, your question is inconsistent - it mentions a "3.7V wall adapter", a "3.7 Volt USB wall charger" (what?!) and a "5V USB charger". You need to be really clear, if you want to get accurate answers. (b) Since this is effectively a duplicate of your current edit of that previous question, this might be closed anyway. (c) "I do not want to use a standard powerbank since these always use the battery." That's false. \$\endgroup\$
    – SamGibson
    Jul 31, 2018 at 23:34
  • \$\begingroup\$ Hi Sam Gibson there is no reference now to a 5V USB charger. \$\endgroup\$ Aug 1, 2018 at 2:41
  • \$\begingroup\$ @SiriusFuenmayor - "there is no reference now to a 5V USB charger" Yes, there is! Look at the text on the lower-right of your picture. \$\endgroup\$
    – SamGibson
    Aug 1, 2018 at 10:43

1 Answer 1


No, this won't work the way you want it to. The PMOS transistor won't be conducting until its gate voltage falls (at least) several volts. By the time that happens the voltage on your load is so low that the SIM800 module will not work properly.

The typical way to do this is to use two Schottky diodes. The cathodes of both are tied together and then to the load. The anode of one diode is tied to each of the voltage sources. Current will flow primarily from the source that is about 0.3V higher than the other, and there will never be a time when current does not flow from one of the sources.

  • \$\begingroup\$ Hi - FYI, something similar was already given on the OP's previous question a few hours ago. Then they made several edits to the original question and changed it into the same as this new question. IMHO the question is unclear, as I explained under the question :-( \$\endgroup\$
    – SamGibson
    Jul 31, 2018 at 23:38
  • 1
    \$\begingroup\$ @SamGibson I thought this question looked familiar. Hey, OP, don't do that! Modify your original question or, better yet, take the time to ask a complete question in the first place. \$\endgroup\$ Jul 31, 2018 at 23:44

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