DJI Inspire-1 TB47 Battery - Broken Cell Fix
Introduction
For no obvious reason two of my Inspire-1 TB47 batteries would no longer charge until completely full but start to flash the center LED on the battery and stops charging at about 70%.
The DJI App shows the following error message:
Further inspection showed that for two cells the voltage was considerably off (cell one and cell four)
[PHOTO label="cellvoltages.png" id="670"]Despite the warning I’ve put this battery into my Inspire-1 and hovered in 1 m height until its capacity was down at 20%. Except for the lower flight time the battery did not behave erratic in any way. More interestingly the “Battery Cell Broken” error disappeared at about 40% capacity. Recharging the battery brought the error and cell voltage difference back though.
As this also happened to a second battery a few weeks later I came up with the idea to unsolder the “damaged” cells and replace them with two good ones from the second TB47. (I am aware that cells within a LiPo battery are but together selectively in order to match – but I thought at least give it a try…)
Analysis
It was clear to me that the “Battery Cell Broken” error was just due to the cell voltage variation being too big and for some reason the balancing function of the “intelligent” battery firmware was not able correct it while charging.
My next idea was that before I start to solder out cells I could give it a try and balance the cells manually. It turned out that for the time being this manual balance did the trick. After manually balancing both batteries I was able to charge them to a 100% without getting the “Cell Broken” error. I took my batteries into my Inspire-1 and did a test flight without any problems and I was also able to fully recharge them to 100% without the cell error reappearing.
This might be only a temporary fix but for now it seems to have saved me some money by not having to buy now batteries.
Manually Balance
DISCLAIMER:
LiPo batteries can explode, cause fire or injury if they are treated in a wrong way. I do not encourage anyone to try the following method of manually balance LiPo cells. If you do so it’s on your own risk!
Remove plastic cap
The first step is to remove the plastic housing of the TB47 – search Youtube for instruction videos on how to open the battery.
Unplug balancing cable and remove insolation paper
Now carefully unplug the balancing cable and remove the protective paper. From now on you have to be very careful not to shorten any cell contacts.
Remove outer plastic housing
In order to clip cables onto the contacts I also removed the outer plastic housing.
Identify individual cell connectors and measure voltages
The next step is to identify the individual six cells and measure each voltage level in order to identify those cells with the highest voltage variation.
The following diagram shows how all cells are interconnected and the corresponding voltage levels:
Corresponding to the cell voltage display at the DJI app, it shows that cell one (connector 1 > 2) and cell four (connector 4 > 5) voltage levels being considerably off.
Balance individual cells
There are two possible ways to balance voltage levels of all cells – discharge those cells that have higher voltage levels or charge cells with lower voltages. As charging LiPo cells should not be done by just connecting any power source to the cell, I decided to go for option one and discharge those cells with higher voltages.
I wanted to stay below one Ampere and simply added eight 47 Ω resistors in parallel (5,87 Ω). As I = U / R (current is equal to voltage divided by resistance) ˜4 V / 5,87 Ω equals to 0,69 Amperes. Each of my eight resistors has to deal with a current of 86 mA and should therefore support 0.5 W (P = 0,086 A * 4,0 V = 0,35 W).
In order to keep track of the current I included my meter and discharged all four cells until their voltage levels were close to cell one and four (3,95 V). With this setup I takes about 45 minutes to lower cell voltage by 0,1 V – I checked the voltage level every 30 minutes but it really took about one and a half hour to have one cell down at 3,95 V.
Finish
Carefully put the battery back into its housing and charge the battery using the stock charger. Stay next to the battery while charging in case you damaged the battery while it was out of the housing!