Continued: Lipo batteries are extremely dangerous ... and Lipo chargers even more (IMAX B6AC especially)

LiPo fire: dangerous

Last month I posted about a LiPo fire I barely avoided in my appartment in Brussels. Well it seems the incident was caused by two things:

• A deffective LiPo battery
• A deffective charfer (the chineese IMAX B6AC)

The IMAX B6AC LiPo charger

This post is about explaining what failed in the charger and how to fix it.

So... what happened?

Well the IMAX B6AC charges batteries through the main connector (see the banana plugs at the bottom-right of the picture) and controls the cells voltage and adjusts the charge of each cell through the balance connector (middle-right).



If one of the pins of the balance connector is disconnected, the charger will not be able to get the voltage two cells. Since cells are connected in series, each pin of the balance connector is used to measure the voltage between one cell and the next.

It seems the charger has a couple of safety features and we will see how they all failed here:

• Max voltage (12.6v for 3s Lipo): since one of the cells was damaged, it was impossible we never reached the maximum voltage
• Max duration (in this case set to 4h): the fire would have occurred before this timeout.
• Max voltage individually for each cell: since 2 of the 4 wires of the balance connector were disconnected (inside the charger, we will see that later), the charger could not see that we exceeded the max voltage on at least one cell.
• Temperature threshold: this functionality is optional, and you have to purchase it separately. I did not (in this case, I failed !)

Diagram made by LBMiller5 from rcgroups.com
He made a brilliant post on lipo and balance connectors

The charger apparently uses the balance connector to automatically detect the number of cells the battery has. It seems it actually searches for the highest connected pin on the balance connector. This is also the check it does to make sure the balance connector is plugged ! In other words, as long as the last wire of the balance connector is plugged, the charger will accept charging a LiPo battery ...

If the last paragraph was unclear and you would like more info on how multi-cell Lipo and their balance connector work, I strongly recommend reading LBMiller5's post on rcgroups.com

If you have a look at the picture above. While charging a 3s lipo, when the 3rd button is pressed (right arrow button), all individual cell voltages read 0v ! I never thought this would mean that a disaster was bound to happen !!!

So here is a summary what happened: the charger was unable to measure the individual cell voltage and therefore did not notice the defective cell (or an out of balance battery) and over-charged one cell.

So, how do we solve this issue?

Here is how I opened the charger, diagnosed and solved this dangerous problem.

First we need to open the charger:

• Pull the four buttons out (use pliers)
• Remove the 5 screw from one side (screwdriver PH1). Only one side is needed

• Slide the bottom aluminium plate. All the contents of the charger will come out:

• In my case the disconnection came from two burned traces next to the balance connectors. The reason why these two traces burned remains a mystery to me.
• While looking at the inside, I also noticed that the quality of the power supply (the black box) inside is quite poor. It looks like they tried to save money with it and I'm not happy about it. Cheap Chinese power adapters are unstable, dangerous and inefficient (tons of experts already wrote about this on the internet, here is my favourite one about usb chargers).

• Re-wire the burned traces using small pieces of wire, solder and an iron as shown in the picture below.
  The attentive will notice that I mentioned earlier two pins of the balance charger disconnected and that there are three pieces of wire below. That's just because I made a short circuit in my first attempt to fix my charger... and burned yet another trace.

• That's it, check for short circuits (not like me)
• Close the charger and check the cell voltage:

Conclusion: if you have a balance charger, DO check frequently if your charger can (still) measure the voltage of all your cells. If not, you are at risk (and not a small one: I almost got a lipo fire home !)