Report on failed repaired scooter batteries

We've received 6 batteries of the model TP-BA150H on 15th August.

S/N	Repaired before?	Note	Diagnosis	Current status
DSM2104021284	Yes, C1 was fixed (Batch 2)	E4	Solder pad C1 broke off of substrate	Fixed
DSM2104020912	Yes, C1 was fixed (Batch 1)	E4	Nickel strip broke on C1	Fixed
DSM2104021999	Yes, C1 was fixed (Batch 5)	-	C7 broke	Bad, for recycling
DSM2103311889	Yes, C1 was fixed (Batch 2)	-	Solder pad C1 broke off of substrate	Bad, for recycling
DSM2101240745	No	-	Don't know.	Bad, for recycling
DSM2103291587	No	-	Water damage	Bad, for recycling

DSM2104021284

Solder pad C1 broke off of substrate.

DSM2104020912

Nickel tab broke again, PCB is intact.

DSM2104021999

Tab lifted off of PCB due to poor soldering.

DSM2103311889

Solder pad C1 broke off of substrate.

DSM2101240745

Show this error when pushing the button:

Does not charge. Battery was in balance, no sign of water damage.

DSM2103291587

Shows same error code as previous one. Visible signs of water damage. It was missing one screw - maybe that's why the seal failed and the water got in.

Root cause analysis - E4 error

The reason most of the TP-BA150H batteries fail is that the nickel tabs that connect the connection plates to the PCB are not flexible enough to tolerate the relative movements of the cells to the PCB. These movements occur due to vibration and thermal expansion. The problem manifests itself in the weakest point breaking.

Our experience with repairing 400+ batteries shows that the first point of failure is usually the nickel tab on the C1 pad.  However,  we also seen batteries where the first point of failure was C2, C3, C4, C6, C7, C8, C9.. basically any of the tabs can be the first point of failure except B-, C5, and B+. The tabs B-, C5, and B+ are much wider and therefore more resistant. See photo below.

Since the battery is taken out of service after the first failure, the other potential failure points do not manifests itself. However, if the fault is repaired, a similar failure happens again due to the underlying bad design.

The four repaired batteries that failed in the field again broke due to the same underlying design fault. One failed at a different point, and three failed near the same point where it was repaired. None of them failed where they were repaired, but rather just next to it: every chain has a weakest element, and if that element is reinforced, a different element will became the weakest. This is similar to how a properly welded metal part is extremely unlikely to break at the weld, but rather right next to it.

Analysis - non-E4-errors, no-repair-history

One suffered water damage and impossible to repair, the other had some BMS fault that we could not diagnose.

Proposal to provide additional mechanical flexibility

Following the above root cause analysis, and accepting that widening the nickel tabs is not practically possible, the solution might be to desolder the nickel tabs from the PCB, solder a short piece of thin wire to them, and solder the other end of the wire to the PCB.

The benefit of this solution would be providing mechanical flexibility through the wire.

The problem with this solution is that:

• It would be more work, especially in making sure that the newly added component do not cause short circuits or get caught up in the housing upon reassembly.
• It would have to be done on all 8 thin tabs to reasonably avoid future problems of the same sort.

An in-between solution would be to apply this to only the broken point + the most-sensitive C1.

Repair procedure for ripped PCBs

1. Grind
2. Wet (deposit solder)
3. Attach wire
4. Superglue wire
5. Wait for glue to set
6. Solder other end of wire to tab
7. Tape over it to make it safer.

Conclusion

Future failures are to be expected on these batteries due to their design weakness. The repair solutions can enable the battery to be used again, but it cannot compensate the design weakness.

In those batteries, where the soldering pad has torn off from the substrate, the repair is more difficult, but still possible.

In those batteries where a repaired nickel tab broke again or a not-before-repaired tab broke, the repair can be done similarly as the first one.

Either way, future similar failures are to be expected.