It is difficult to say what the problem exactly is. The charger is completing the charging of the pack (green) as soon as an individual cell is fully charged, regardless of the status of the other cells. So this seems expected in your case.

Please measure the total voltage of the whole battery pack (+ to -). This should correspond to the sum of the individual cell - approx. 12.4 volts. If this is not the case, a cell may not be connected correctly or may have been installed with the wrong polarity. The mower will only start if the total voltage is high enough (>= 10.8V).

As a last option it could also be that one cell is bad or old which results in a high internal resistance. This would result in massiv voltage drop under load and the voltage might not be sufficient to start the mower. Let me know if you need more infos on how to measure this.

I really hope this will help you to solve the issue.

Regards. Joel

Great guide! I just repaired a lamp with the same control circuit at the weekend. It was not turning on or only for a very brief moment. In this case the 47uF capacitor needs to be replaced.

Hi. The charger cannot charge the cells individually but the cells are still monitored individually. This means that if one cell has already reached its final voltage of about 4.1 volts the charging process is considered completed even though the others have little or no charge. This results in a total voltage that is too low to start the mower. In this case this indicates also an imbalance and the method in this guide should also help.

Thanks for the question. It is important that you connect to the terminals of the individual cell and therefore use the tin spots I marked in my picture. If you were to use the terminals that the charger uses only the whole pack (all 3 cells together) could be charged. To solve the problem each cell must be charged individually. Best regards. Joel

I'm glad it helped you. Your approach is fine. Charging with 5V should not do any harm as long as the actual battery voltage is monitored and does not exceed 4.1V (absolute max. 4.2V). There is a good chance that the problem will return in the future as the battery management is poorly designed and does not take into account the individual condition of the cells. As a result the cells tend to drift apart. The only option is to correct them from time to time. It is recommended to charge the battery to around 60 % before storing it for a longer period of time in order to avoid an undervoltage shutdown.

That's cerainly possible since the battery management does not lock the charger after an under discharge event (like some do). Replacement comes with some effort. Here I found a step by step disassembly guide. Challenges I see in finding a specific battery type with similar properties. Replacing the batteries would also require a spot welder. Avoid soldering the batteries to the terminals since the high temperature could permanently damage the cell. Always replace all the cells, not only individual ones.

If you are not firm with the topic consider buying a new one or contact a so called re-cell service.

My battery keeps working so far but in case a cell actually dies I will replace them and create an additional guide for this process.

Good luck and best regards,

Joel