Sunday 11 August 2013

Can I trust my IMAX B6 AC charger?

The IMAX B6AC Clone
The IMAX B6 and B6AC chargers are extremely popular chargers in the RC hobby. They were made to charge all sorts of batteries, including the dangerous yet attractive Li-Po batteries (high power density).
Although I'm sure the original charger is fine, the internet (eBay) is full of Chinese clones with an unknown quality. This post summaries all the weaknesses I could find of my IMAX B6AC charger.

It seems this charger design is fairly common and lots of clones from various brands and counterfeit of original brands can be found on the internet. Here is a list of other chargers seeming to share the same design and firmware (source):
  • RC-Power B6 charger
  • Imax B6
  • Turnigy Accucel-6
  • GT Power A-6
  • Mystery B6
A short while ago, I wrote an article about the dangers of Li-Po batteries. You don't want to charge Li-Po batteries with a charger you cannot trust. You will see hereunder how much you can trust this charger.
Edit: since I wrote this post I received precious input from readers and will try to nuance it whenever seems appropriate

No warning if the balance circuit fails

Balance circuit repaired
The first issue I had with the charger was that the balance circuit broke and I had to repair it. This almost caused a Li-Po fire. Unless you check whether the balance circuit works every time I would not trust the charger.

Look at that cheap (and dangerous) power supply

A cheap power supply

Once you open the B6AC, you see that they made a shortcut and just used a laptop-like power supply inside the casing. 
The ground pin is not even connected to the charger casing ! (this is not mandatory in this configuration but I would feel safer considering this is a cheap Chinese power supply).
If you're reading this, chances are you know cheap Chinese power supplies can be really dangerous. The photo on the right of my B6AC clearly shows I have one of these dangerous ones.
If you're unsure about how dangerous such power supplies can be, you might want to read this story.
The ideal replacement would be a 5.7cm x 13.3cm x 3cm 12 - 15v charger. I recommend the highest power rating possible:
5A x 25.2v = 126W (ignoring conversion inefficiencies -> 150W?) 

Hand soldered (by a monkey?)

Inside the B6AC clone
If you have a closer look at the picture on the left, you will see that resistors are not as professionally soldered as one would expect from an industrial assembly line (not perfectly aligned). This particular meter might have been made by a monkey or a kid in a Chinese factory, I wouldn't know (but I care...).
If it's hand soldered, it is more likely to have a defect and if it's not obvious but can cause the charger to malfunction, you could end up with an exploding battery.

Missing the reverse
polarity protection (?)

No reverse polarity protection

    Or at least that's what the manufacturer says.
    If you look at the picture on the right, you will see the charger misses the BT1 component. It was intended to protect against reversed polarity (?). I don't see how a diode could help in any way.
    One possibility could be a missing capacitor?

    Poor calibration

    To test the precision of the charger, I plugged two Li-Po 3s (3 cells) in series to simulate a 6s battery. The aim is to fully charge them using the 'Balance' program and compare the charger measurements with a precision voltmeter (ISO-TECH IDM 305).

    Test rig: two 3s LiPo batteries in series
    Once fully charged, I measured the battery voltages using the charger, the precision voltmeter and a cheap one (bought at Maplin).

    IDM 305
    Cell 1 4.19 v 4.197 v 4.13 v
    Cell 2 4.19 v 4.227 v 4.16 v
    Cell 3 4.18 v 4.195 v 4.13 v
    Cell 4 4.19 v 4.200 v 4.14 v
    Cell 5 4.19 v 4.206 v 4.14 v
    Cell 6 4.19 v 4.186 v 4.12 v
    Total 25.20 v 25.218 v 24.7 v

    Edit: The critical voltage for Lipo is 4.4V. There however seems to be an empirical consensus in the RC community that any anything beyond 4.2V per cell shortens the Lipo battery life. The point made here is the cell charging voltage precision is around 1% (which is not too bad) but I would prefer if I could keep the max cell charge below 4.2V.

    Here are my conclusions:

    • Don't use a cheap voltmeter to test the voltages: they are inaccurate
    • Although the meter was obviously calibrated, its calibration was a little too high to my taste, since two battery cells were above 4.20v (considered critical) while the charger was reading 4.19v.
    Battery voltage
    Cell voltages
    'Real' voltage measured by the precision meter
    To be below the usual 4.2V, it is my conviction that the cell voltage reading on the charger should be equal or slightly above the 'real' cell voltage (by max 0.1v).

    Next: read my other post on the procedure to re-calibrate the charger.