External Powering the APM from a 3S LiPO

I had a browse and found many conflicting information on how to power the APM board from the external external power connectors while unsoldering the jumper.
MY ESC's are only rated to 500mA and I am afraid to fry them as they do get very hot just from powering the APM. Given the conflicting messages online, I had a look at the Schematic of the APM and Looked at the circuit. Looking at the LM317 specs, powering the board with max 12-13V from the Lipo should not be an issue as its input and input/output delta voltage is well rated above.

I tried to power the board from the 3S flight Lipo, however the the board starts behaving weird and keeps either rebooting or never comes out of calibration.

I am at my wits end. Is it the IMU that has additonal circuitry that is interfering ? Could not find PDF schematic of the IMU to trace the power management. The other thing I was thinking about is the resetable fuse playing up ? However, its after the LM317 so, the current draw should be any different between a 2S or 3S LIPO.



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  • Hi Bob, i did use 7.2V tap on the Charging connector to start with, but then got worried about any bad effect off the LIPO getting un-balanced that you described. I am not a battery specialist and was not sure if that would be undesirable for the LiPO and/or the effect on flight times.



  • One other thought. If your 3S LIPO is your main pack (supplies motor current too) and it has a charge balancing connector, you can tap the 2S LIPO voltage and use that to power your board. Of course, that will unbalance your pack during use, but if you use a cell balancing charger, you won't do any harm to the pack.

    Typically the motor current will be many multiples of the 210 ma (I have an XBee 900 Pro, so your current may be less) that the APM+IMU combo takes. That means that the imbalance will not be large.
  • Thanks guys! I guess i will get an external regulator then to get it down to 6-7V before feeding it into the APM. Just wanted to avoid this not to waste more  loss power on the additonal reg. But so be it :)
  • My APM+IMU draws about 210 ma.  With a 6.5 volt drop across the regulator, nearly 1.4 watts will be dissipated in the regulator.


    So, how hot is it getting?   I'm guessing pretty hot.  Maybe hot enough that the regulator is shutting itself down to avoid self destruction.

  • Ollie, Sounds to me like the regulator is overheating on the 3 cell. Voltage may be glitching and you can't see it in a DVM. The schematic shows a different regulator than the LM317, a MIC5219. What ever yours is, it can't handle the voltage input. Check out this place. I use their products and have never had a problem. They make switching regulators that don't wast the extra voltage as heat like a linear regulator does. The cost more, but with a linear reg you are wasting more battery power than you are using going from 12 V down to 5 V.

  • Russel, a 2S LIPO works just fine. So it seems to have to do with the Voltage rather than the LiPO not being "dirty".



  • Bob, That's the strange thing. It is a nice 4.98V on the IMU 5V rail. So I really don;t get why the boards keep rebooting or not starting up properly. Sometimes the rx/tx orange leds keep blinking or it just does a full reset on the 3S, 2S works like a charm.
  • You can use this Regulator to test different input voltages with APM.  It also make a good power supply for the APM.  1000mA output,  I think the APM dosen't like dirty power straight from the battery.   


  • What Vcc voltages are you seeing when running with the 3S LIPOs?
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Aug 25