Tuesday, March 26, 2013

48v Bike battery pack investigations

As I think I mentioned a few posts ago, my 48v bike battery seems to have a problem with cutting out under full throttle.  This is the case even immediately after charging.  I haven't known much about the battery pack, since I bought it used and it came with no documentation, but after reviewing lots of eBay auctions selling similar items, I have a pretty good guess at what I'm dealing with.  I think I've even figured out what happened, and what to do if there's any chance of fixing it.

So I believe I'm dealing with a 16S LiFePo4 pack with a nominal Vcell of 3.2v, (or with all 16 cells in series, 51.2v Vpack).  I disassembled my BMS (battery management system) so I could get a look at how it was configured. The battery has a main output (2 wires), as well as 16 wires (one for each cell junction) that are all connected to the BMS.  This allows the BMS to measure and even balance the cell voltages within the overall pack.  The BMS can shut down the output or charge if any of the individual cells are out of safe range.  Incidentally, for a LiFePO4 cell, this is quoted as ~2.2v (discharge) and ~3.9v (charge).

My pack voltage settled out around 49v after full charging cycle.  This is low, about 3-5 volts lower than it usually is.  I then measured the individual cell voltages using the 16 wires.

B1 = 0.0v
B2-B16 were all somewhere between 3.1v to 3.7v, with quite a bit of variation.

As you can see, B1 is a big problem.  I didn't really know what to do.  I know that no cell should ever be lower than 1v no matter how dead it is.  It'd basically have to have been shorted.  After thinking and researching about it, I realized my mistake.  I stored the battery for the winter months with the BMS attached.  I read somewhere that a lot of BMS will use the first cell to power its circuitry.  If this is true in my case, the BMS killed the first cell down to nothing.

Further intuiting has lead me to the conclusion that the BMS can't balance the pack when its power ain't on.  So that explains 0v on the first cell, and no balance on the pack.  The BMS at least threw the pack way out of balance, and at worst, destroyed the B1 cell.

Since I don't have a 16S cell balancer, or an individual cell charger, the first thing I tried was to leave it on charge for a long time.  I noticed the charger did briefly cycle on and off about every minute for about an hour or two.  I then unplugged the BMS and left it overnight.  I then measured the following:

B1 = 2.8v
B2 = 3.3v
B3 = 3.3v
B4 = 3.3v
B5 = 3.3v
B6 = 3.3v
B7 = 3.3v
B8 = 3.3v
B9 = 3.3v
B10 = 3.3v
B11 = 3.3v
B12 = 3.3v
B13 = 3.3v
B14 = 3.3v
B15 = 3.3v
B16 = 3.3v

This is a very good sign.  My B1 cell is not destroyed (though it may be damaged, but it's still alive).  It's clearly still badly out of balance.  The solution seems to be one or a combination of the following:

-Somehow kill batteries B2-B16 until they are around the same level as B1.
-Somehow charge B1 individually to 3.3v.

Whatever happens, it needs to be done through the dinky little 20 gauge wires that are attached to the cell junctions. At this point, it's possible the BMS will have enough power to do its job and start balancing the cells, but the big question will be, if I leave the BMS plugged in, will the first cell go dead again.  As far as I know, from some basic research, the BMS doesn't charge individual cells - it only shunts the other ones in an effort to drain them to the lowest cell voltage (I believe I read it has a max of 70ma).  It would seem to me that it would have a lot of work to bleed off that much power, since the lowest cell is so drastically different than the others.

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