Better batteries.

4 props died during yesterday’s testing; not because of code bugs but because of battery problems.  The DC-DC converter I use to power the RPi can take an input voltage in the range of 10v – 13v, and convert it to 5v.  This is about the same range as a LiPo battery from empty to fully charged. I think my battery is growing old (or I’ve abused it, or both), and the main symptom seems to be very short charging cycles and very fast discharge cycles (i.e. reduced capacity).  I also suspect that its internal resistance has gone up.  All this means that if the ESCs suddenly delivers a power surge to the motors, especially when the battery is already partially discharged, its voltage drops to under 10v; the DC-DC converter stops putting out 5V, and the RaspberryPi does a hard-shutdown in flight.  Of course once the ESCs are not getting PWM from the RaspberryPi, then they stop powering the motors, the battery voltage rises and the RPi boots again, but sadly no leaving any stats to analyse as these are only written to filetdown (including a flight aborted by me).. So I’ve just ordered a couple of Turnigy Nano-Tech LiPo batteries – 3S, 4000mAh 40 – 80c batteries.  WTF does that all mean?

  • 3S = 3 LiPo cells (nominally 3.7v per cell) connected serially → a nominal 11.1v battery
  • 4000mAh = the battery capacity → 4A supplied for an hour @ 11.1v = 44Wh or 158kJ of power which is a lot, though given the motors may draw 10A each in normal flight, 40A total leads to a 6 minute flight maximum
  • 40 – 80c is the constant and peak discharge rate – 40C = 40 x 4000mAh = 160A constant discharge rate or 320A peak flow. I don’t need anything like this, but what it also implies is that the batteries internal resistance is very low, meaning the RaspberryPi reboot risk is hugely reduced when under load due to much lower voltage drop within the battery itself.
  • Once missing parameter is the charge rate; the default safe setting is acknowledged to be 1C – i.e. charge at no more than 4A in the case of my soon to arrive batteries.  Some can apparently take more, but better safe than have an explosion and fire.

Net: 4 props destroyed (luckily still 12 spares in stock), 2 new batteries on the way, and post all the fixing, the first zero drift full flight Phoebe has ever done.  Sadly no video or pictures 🙁 Another day, maybe.