dchislop wrote: ↑Sun Nov 04, 2018 8:12 pm

I've been trying to work portable as well, I had no luck with 4x1.2Volt rechargeables or even 4x1.5Volt AA batteries. They didn't have enough power to even boot up the Pi 3. I even tried a 5.1Volt Zener Diode circuit but didn't work either. I now trying a 6Volt sealed lead acid battery from Toolstation priced £6.38, it's rated at 1.2Ah and small too, just 97x24x58mm. It drops to 5.7 Volt once up and running but the voltage soon falls away only giving me a operating time of 15 minutes before the low voltage indicator appears and another 5 mins before conking out completely. I know I should really supply a steady 5.1Volt so I would interested to hear of other solutions.

1.2Ah * 6v = 7.2Wh. On the Pi side, 2A * 5V = 10W, so even at best (100% conversion, batter is able to supply total capacity, etc.), you would only expect to get 45 mins. Since a Pi3-class device might draw more than that, the battery rating may be...fictional, you can't drain a battery all the way down, and no voltage converter is going to be 100% efficient, 15 to 20 minutes is reasonable.

I have to restrain myself... The analysis needed here is basic electrical theory. It isn't rocket science (which is good, because I'm not a rocket scientist). Just convert all activity to Watts (W) and capacities to Watt-hours (Wh) and the math becomes simple. Allow for a generous dollop of losses and you'll be reasonably close. Things like this are why engineers are conservative and allow "margin for error" in specifications.

And, for those watching at home, the bit about giving LiPo battery "capacities" in Ampere-hour (Ah) or milliAmp-hour (mAh) is *advertising*. It gives the impression that the battery is more capable than it really is. The same is true of commercial USPes, where the rating is given in Volt-Amperes (VA) or Kilo-Volt-Amperes (KVA). In AC systems, you have to take the time average of voltage and current, which are sine waves. The true power out (assuming 100% conversion efficiency) is the Root Mean Square (RMS value of the peak-to-peak value. So to find out how much power a UPS will deliver, multiply the VA or KVA by (1/2)*sqrt(2), or close enough to 70% to use that. Thus, a "1 KVA" UPS should provide 700 W.