With the charger provided by DJI, including the accessory that accommodates 3 batteries simultaneously, it takes about 45 minutes to charge a single battery. There is a way to save a little time, and even to charge several batteries simultaneously if you have several chargers or chargers with several outputs. Franck Colombat looked at the battery of the DJI-FPV to try to understand a bit of how it works. The result of his research is very interesting, since it makes it possible to get rid of the DJI charger… Here are the explanations, embellished with Franck's remarks, materialized in italics.
Preliminary and important note
There is always, always, always a risk of starting a fire when charging batteries, even with official chargers and in ideal conditions. Let's remember (see here), any battery charging must imperatively be carried out under active supervision in order to react in the event of an incident. In the case of the tests described in this post, do not embark on experiments unless you are familiar with batteries and charging techniques, and able to react if there is an incident.
The proprietary connector
The DJI FPV battery connector is "proprietary", and its two + and Gnd plugs are accompanied by 3 mini-connectors. There are only 3 for a 6S battery, so this is a first clue: they are not connectors intended for balancing the battery cells.
To check it?
« I used an XT60 socket of which I only kept the 2 plugs, without the plastic that holds them. It allows me to plug them directly into the battery connector of the DJI FPV ". Of course, the polarity must be respected. By slightly pulling the tab of the battery, we see the red (+) and black (Gnd) wires. The + is on the side of the keyed square end, the Gnd on the rounded side.
How to connect the plugs to the DJI connector?
« The plug of an XT60 socket is made up of 4 contact tabs. Just remove 2, in opposition, so that there are only 2 left. By tightening them slightly, they fit perfectly into the connector of the DJI FPV battery. The contact is more than enough to launch a correct charge, even at 3,5 A ».
The battery does not respond...
« When you connect the battery to the charger, it detects nothing! The trick is that you have to turn on the battery with the usual sequence, a short press then a long press on the on/off button. At this time, the charger detects the battery and displays its voltage. I deduce that one of the functions of the 3 mini-connectors of the DJI FPV socket is to "wake up" the battery to charge it ».
A charge without a balance plug
It is essential to use a Lipos charger capable of starting the charge without connecting the balance plug. Most recent chargers are capable of this, but not all. " I charged a battery with an ISDT SC-608 charger capable of initiating a charge without a balance plug, only via the XT60 connector. I carried out my first tests with a charging current of 1A. I got a full charge from 3,60V per cell, which took a little while, but ended with all 6 cells balanced at 4,16V”.
But how to check the balancing of the cells?
Unfortunately, neither the DJI V2 helmet nor the DJI Fly application can display the voltage of each cell. But my secret weapon is the DJI Fly app in Beta version that I used during my tests, before the DJI FPV was announced. Because this version displays a lot of information such as telemetry… and the summary of battery data (voltage per cell, number of cycles, temperature, serial number)! It's a shame, but I can't make this version publicly available, especially since it's linked to an internal account at DJI. Hopefully DJI will reintroduce these functions in DJI Fly, they are important. Even essential! What this version of DJI Fly shows is that the cells are correctly balanced at the end of the charge!
What to remember?
« Balancing is not managed by the charger, but by the internal electronics of the battery ! DJI's charger states a maximum charging current of 3,5A, so I set my charger to 3,5A and got a full charge in 30 minutes instead of 45 minutes with DJI's charger. This means that the latter is probably not using the maximum charging current. I tried with a charging current of 4 A, but it does not work, the battery goes out after a few seconds ". Is it possible to charge the cells at a higher voltage to boost the autonomy a little? It's not recommended, and we didn't try it.
The little bonus?
“The system is reversible, i.e. it is possible to use a DJI FPV battery to recharge other batteries. Perfect for using the DJI FPV battery to charge batteries from smaller devices, like Cinewhoops. It also helps to avoid keeping a fully charged battery too long when not in use”.
Edit: an XT60 adapter to the DJI FPV socket is available for 3D printing on Thingiverse !