No output voltage after splitting pack

[Zerogee]

The pack ( http://goldenmotor.com/SMF/index.php?topic=5907.0 )  is split between C9 and C10. I thought things went well until I checked voltages after the split. Need help troubleshooting...

Pack was fully charged and not used since last fall. Before the split the pack was at 53.3V and individual cells resting voltages measured on each cell.

Prior to split.
C1 3.33
C2 3.33
C3 3.33
C4 3.34
C5 3.33
C6 3.34
C7 3.34
C8 3.33
C9 3.33
C10 3.33
C11 3.33
C12 3.32
C13 3.33
C14 3.33
C15 3.33
C16 3.33
This is why I  thought cells were well balanced.

After split during charging.
C1 3.48
C2 3.57
C3 3.68
C4 3.70
C5 3.47
C6 3.79
C7 3.79
C8 3.44
C9 3.59
C10 3.50
C11 3.61
C12 3.42
C13 3.64
C14 3.50
C15 3.61
C16 3.51
Charger went from red to green in like 5 minutes then repeatedly turned on and off for maybe an hour (C6 and C7 hit 3.89V, C3 and C4 hit 3.79 during this period) then stayed green. Left it on for a few hours until bedtime. Not as well balanced as it first appeared.

After charging for a few hour.
C1 3.52
C2 3.57
C3 3.68
C4 3.70
C5 3.47
C6 3.79
C7 3.79
C8 3.44
C9 3.59
C10 3.50
C11 3.61
C12 3.42
C13 3.64
C14 3.50
C15 3.61
C16 3.51

Next day resting.
C1 3.51
C2 3.61
C3 3.68
C4 3.70
C5 3.51
C6 3.74
C7 3.74
C8 3.47
C9 3.61
C10 3.53
C11 3.64
C12 3.44
C13 3.66
C14 3.54
C15 3.65
C16 3.55

After the split I can't measure total pack voltage either on the pack or at the power port with the key on. However, individual cells resting voltage are still around 3.33V. I can measure cumulative voltages from C1 to C5 on the BMS then nothing after that. I measure cumulative voltage on the main pack from C1 to C5 then nothing between C6 to C9. I can also measure the cumulative pack voltage on the minor pack from C10 to C14 and nothing on C15 to C16. I can't measure voltage between the two packs on C9 to C10.

To make the split, I cut the battery tab in half after removing that sense wire then soldered my C9 to C10 wire to the tabs. I have continuity on the added wire between C9 and C10. I also have continuity on the sense wire between the sense wire extension and the splice on the center of the C9 to C10 wire.

I had power OFF. I covered all terminals with tape and only worked one wire at a time to prevent shorting. I unplugged the BMS prior to starting.

Any assistance is appreciated. Thanks.

[Bikemad]

As the BMS is only able to switch the entire pack voltage, if you can't obtain the total pack voltage when measuring on the pack itself, there must be a break somewhere between two adjacent cells. If a break occurs, the BMS can easily become damaged by excessive voltage being placed across the single cell monitoring channel where the break occurs.

Check out this video for a good description of why the damage can occur.

To locate the break in the circuit, simply unplug the balance lead from the BMS and attach your black meter probe to the negative connection on cell 1 and then check the voltage on the positive and negative connection of cells 1 to 16 in sequence.  The voltage should increment by the individual cell voltage of each cell on each positive connection, but it should drop significantly when you measure across the faulty cell interconnect in the circuit.

As your cell voltages seem to be very unbalanced (3.44~3.74V), I suggest that you try and manually balance the cells before attempting to charge the battery again with the standard charger, or it is likely to take a very long time to balance the battery with a low balance current:


If you have access to a R/C type of lithium balance charger (and suitable power source) you might be able to temporarily connect a separate balance lead and balance several cells simultaneously (dependant upon the charger), or just use it to charge one cell at a time starting with the lowest ones (C12, C8 etc.).

If you don't have access to a suitable balance charger, you can alternatively use a 12V 55/60W headlamp bulb to simply discharge the higher cells until their resting voltage is the same as the lowest cell in the pack. This should hopefully bring all of the cells to a similar state of charge.

If the existing BMS is damaged (and not repairable) you will need to replace the BMS unit.

I would also monitor the individual cell voltages during the initial charge to check that the BMS is able to balance the cells correctly at the end of the bulk charging process when the cells reach their optimum charging voltage (3.60~3.65V per cell).

If any cell exceeds 3.7V I would stop the charger and use the headlamp bulb to drain it enough to match the average voltage of the other cells before continuing with the charging/balancing process.

Alan
 

[Zerogee]

Turns out that my work went well with this split afterall. The problem was with my cheap DVM. The high scale on the voltage is fried. The meter can't read past 20V. I borrowed a calibrated Fluke meter from work (used for certifying welding procedures) it has no scale and adjusts accordingly but only accurate to one decimal place. There was an eureka! moment. :sighs relief:

Thanks for the assistance Alan.

I will attempt to balance the pack prior to mounting it.