Help Explain Battery Percentage

ArduCopter
1

Can someone give me a basic 101 overview of LiPo battery percentages and how to interpret (or point me to a resource)? We got our Octoquad flying and I’m doing multiple series of tests to learn about predictable power consumption and flight times possible on a single charge. Here’s what I think I know about LiPo:

  • Nominal, resting voltage of a LiPo cell is 3.7
  • Max, full charge of a LiPo cell is 4.2
  • Storage voltage is +/- 3.8
  • Minimum discharge voltage (don’t go lower or you could damage battery) is 3.0-3.3
  • I understand we use voltage vice capacity to assess how charged a battery is because of difficulty in measuring capacity.

For discussion purposes, I’m using a Tenergy LiPo 5-in-1 tester on Tattu 30,000 6s batteries.

Here’s what I need help understanding:
What does the percentage rating mean ~ specifically, what constitutes 0%?

Presumably, if a tester says your 6s battery is 98-100% charged and voltage is 25.2 then 100% = 25.2 volts. But clearly 0% is NOT 0 volts since a ‘dead’ battery is not necessarily a fully discharged battery. Is 0% represented as the minimum required 3.0-3.3 per cell or 18-19.8 volts?

If yes, then the 0-100% charge range is measuring how much of the 5.4 volts is available for use (25.2 – 19.8)? That appears to be the case. At 39%, volt total measured was 22.88 which is +/- 39% in the ball park. Battery is smart enough to know how much of total voltage is available for use and testers calculate that as percentage.

Thank you for corrections or validation.

JB

2

Capacity and percent consumed/remaining is a measurement of mah not voltage. It’s not difficult to measure.

3

Thank you for the fast reply. I am aware capacity and percent consumed/remaining is a measurement of mah not voltage. I am not aware of a way to measure capacity - specifically mAh remaining. I was under the impression that it wasn’t simple and that’s why so many battery testers and some chargers generally give you a voltage measurement per cell and per battery as opposed to a mAh total.

In any case, my objective is to be able to explain exactly what it means when the battery tester says a battery is at 39% or any arbitrary percentage. How do I explain in lay terms what that percentage means and what it equates to in distance or minutes of flight under standardized conditions?

4

If you have current measurement on the craft, which many (all of mine) do, it’s simply current over time and you start at 100% which is the fully charged pack capacity. The only way is to know what you start with. Attempting to use cell or pack voltage as a measure of capacity will be highly inaccurate.

5

Good to go. Thanks again for fast replies.