Estimating Phantom & Inspire battery capacity
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Estimating LiPo battery capacity is not as precise as your drone's "fuel gauge" makes you think.

But its not all bad news...
How is capacity estimated
Gauge


We're used to very precise information about our batteries. For years our phones and laptops have reported exact percentage figures. And so do our DJI (and other) drones.

With this false sense of accuracy we're surprised (and sometimes, devastated) when our batteries sometimes go from 20% to 1% and then die within seconds or when our quads drop out of the sky on their way back to base.

This all happens because estimating remaining LiPo capacity is not at all straightforward. These notes aim to explain why this is the case (and what you can do about it)

There are a number of methods for estimating remaining LiPo capacity but we'll limit this discussion to methods used by relatively "normal" consumer devices (and specifically drones). So don't apply this knowledge to your Tesla!

The primary method most commonly used is simply measuring the LiPo's voltage at any point in time. The less remaining capacity a LiPo has the lower the voltage it can deliver. Fully charged cells tend to be 4.2v each while anything close to 3.0v signifies a dead battery.



If only it were that simple...
However, this is complicated by a number of factors:

The voltage delivered by a LiPo drops when the LiPos is under load (i.e. when you're flying)
The voltage delivered drops as a LiPo ages or gets worn
Each make of LiPo can be slightly different
Even things such as ambient temperature have an effect on the voltage being delivered

Time to re-calibrate
These complexities mean that the volt/capacity curve changes over time. As such the calculations that present that tidy x% capacity remaining on your screen need to be re-calibrated occationally.

To re-calibrate, a battery is discharged a lot (more on this in a sec) and the recharged, measuring how much current can be put back into the battery. Doing this regularly is crucial for keeping your readings accurate.

However... LiPos don't like being discharged too much and actually discharing a LiPo to 0% is most likely enough to permanently destroy it. Thankfully DJI batteries auto-cutoff before they get to this state but still discharging fully too frequently will reduce your LiPo's life.

At this point its also worth stating that when your DJI OSD reports remaining capacity, it is actually reporting remaining usable capacity. So when you see 5% it means 5% over the point where the battery will auto-shutdown to protect itself.


So what do you recommend...
There's no right answer here but I'll offer my personal approach:

Never fly to anywhere near close to 0%. Aim to land at around 25%. This keeps your battery in good condition and protects your drone from dropping out of the sky.
Cycle according to DJI's recommendations (i.e. every 20 flights).
Don't cycle (or deep discharge) more frequently than you have to.
When you do cycle, use a dedicated device such as the Phantom Angel to make sure you get the most accurate calibration.
You’re now ready to keep your LiPos happy and performant for hundreds of cycles.
I hope this has helped de-mystify LiPos capacity estimation a bit.

If you have comments or suggests please do write at alex@phantomangel.rocks
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