DIY DJI Inspire 1 Battery "Charger"

"DIY" is one of the watchwords here at EastBay RC, so you can imagine it's going to be an interesting experience to deal with a real store-bought system like the Inspire 1.

Our first project: a power supply for charging the Inspire batteries.  Here's the prototype, which is working well.

The DJI power supply has these features: rated output of 26.3V, 3.83A (100W), proprietary connectors for battery and radio.

In order to get the 26.3V, I'm using a boost converter.  It will take input from 4V to 25V and convert it to 26.3V. I got it on Amazon for $12.  You can get it for about $8 if you don't mind waiting for China shipping.  Specs on my unit are

• 9A max input, 6A max output.

Double check the ratings for the unit you get.

We'll go into more details in a followup post about how to configure the boost converter.  Here's my initial test connectors.  They are two prongs from an old extension cord, and fit perfectly into the battery.  Positive is on the left.  I'll do a video with some more construction details as well.

 Here's a second connector.  I glued the prongs onto a piece of scrap wood and insulted with liquid tape.  I double and tripled checked the polarity was correct.

Here's the final connector.  It works really well!

I can now charge the battery using any 12V-24V power supply, including my regular charger on Pb mode.  Charging from the car should be OK as well, but I'll double check the amperage requirements.

Anyways, hope this quick note helped, and if you're interested in seeing more DIY Inspire projects let me know!


Power Equivalence.  The table below shows the required amperage at several voltage levels in order to provide the equivalent power.  The first amps column is the straight calculated equivalent, and the second amps2 column take into account the approximate 10% power loss from the boost converter.

Watts Voltage  Amps Amps2
100      26.3  3.83
100      24    3.3    4.5
100      20    5.0    5.5
100      18    5.5    6.1
100      12    8.3    9.1

Note that the 12V amperage is just barely within the maximum amperage specified for my unit, mentioned above.

If you have a standard RC 4-button charger, you can set it to Pb (lead-acid), 20V, 5A and be just a little shy of the maximum equivalent charging capacity.

There's a rumor that the built-in charger of the smart battery won't current-limit. Sadly, too much of the DJI world runs on superstition, so until a battery dies and we get to do an autopsy, we won't be able to determine if this is true.  In the meantime, if you don't exceed the  Amps2 number for your power supply voltage that won't be an issue. (update: see below)

(Update) Some people have asked why "Charger" is in quotes in the title.

• The technically correct terms are "power supply" for the thing that plugs into the wall and "charger" for the circuitry that supplies electricity to the cells.  This is also the common industry usage.  A typical RC charger includes the circuitry to properly charge "dumb" LiPo batteries.
• DJI uses the arguably "technically incorrect but more commonly used" term "charger" for the power supply.
• Displaying EastBay RC's famously pedantic nature, we therefore put "charger" in quotes.

(Update) DJI has some good information on their wiki.  Specifically, this datum:

7. Over Current Protection: Battery stops charging when high amperage (more than 10A) is detected.

If this situation is detected, the document indicates that LED2 will blink twice per second.

So the total power provided to the voltage converter should be under 260 watts.

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