Introduction: Xiaomi M365 Electric Scooter Battery Repair

By Well Done Tips

In this video I’ll show you - how to diagnose and repair Xiaomi M365 electric scooter battery.

How I did it - you can check by looking DIY video or you can follow up instructions below.

For this project you will need:


Materials:


Tools:


Step 1: The Faulty Electric Scooter

If you are familiar with electric scooters - you well know what I got - yes, this is Xiaomi M365. I bought it quite cheap as faulty. I already took off dashboard cover to check maybe some issues is there, but nothing suspicious was found. All I know about this scooter - that battery was tried to repair and now scooter beeps, won’t take a charge and don’t run. There’s nothing what can’t be fixed - so let’s take battery out and take a look what’s wrong with it.

As we could see battery was touched before, external sealant was cut and glued back with kapton tape. Removed four screws - and took battery out.

Step 2: Battery Inspection

Voltage check - 23.6V, that’s definitely not good. It should be between 30 and 42V.

Removed all insulation to get better access to 18650 cells.

Here is visible, that balance tabs already was soldered. Originally they be spot welded.


Removed more insulation and found first loose connection. In other side one balance tab was loose too. This is often problem caused by soccer vibrations riding bumpy roads. All other tabs looks good.

Step 3: What About Voltage?

Let’s check voltages of each cells group connected in series. This is 10s3p battery, so it will have 10 groups of 18650 cells connected in series and 3 parallelly connected cells in each group.

This one good, first bad one, second bad, third bad… aaaaaannnddd looks that all of rest groups are ok.

Step 4: Troubleshoot Iteration

So I found three dead cells groups connected in series. Let’s move forward and proceed testing by replacing dead cells with good ones. I disconnected bad cells from battery pack and temporarily replaced them with this 3s1p 18650 battery pack. Soldered main and balance wires. Connected balance wires to BMS and checked voltage. It should show us about 36 volts. Great, battery have nominal voltage again, let’s check it on scooter.

Sadly scooter don’t work and beeping 21st error code. Long beeps indicates first and shorts - second number of error code. Regarding my found information this could be related to faulty BMS. 

Step 5: More Findings

BMS LEDs wont show any life, plus while I was testing battery with scooter, I noticed that BMS got extremely hot, especially this part filled with resistors. First thought - maybe BMS is balancing cells, so I left battery for about 6 hours. Sadly - it wasn't that case. It turns out, that BMS was draining only this 3S group of 18650 cells connected in series. I wrote each cell voltage when balancing wires was connected to BMS. Take a look how much voltage left after 6 hours. All cells was drained down by 0.3V. At the same time all of rest cells voltages stayed in place. So, there nothing strange, why those 3 groups of cells are dead flat.

Step 6: BMS - Battery Management System

All those signs shows that this BMS is faulty and must be replaced. I ordered online this BMS replacement for Xiaomi M365 battery for 33 euros (that’s around 40 bucks). I leave a link in video description to this product.


Desoldered all wires from bad BMS and soldered to the new one. BMS looks identically, except different labeling.

Step 7: New BMS Works

After balance wires was connected, BMS LED started flashing blue light - that’s a good sign. Connected battery to scooter to check does new BMS solved my problem. And looks it did. No error codes beeping and scooter is running again.

Step 8: Bad Cells

Time to replace bad cells inside the battery.

Desoldered BMS from battery to prevent any possible damage, while bad 18650 cells will changed to good ones. Removed 18650 test pack and old nickel tabs from dead battery cells.

Step 9: Old Vs New 18650 Lithium Cells

This plastic battery body is design in such way - that 18650 cells could be removed only by splitting battery case in two parts. In my situation this is impossible, because cells are welded with nickel tabs. So I took rotary tool and carefully removed small pieces of plastic from few plastic clips. I cut only plastic, cells wasn't touched. This allowed me to remove one by one 18650 cells.


Originally this battery pack is made from LG M26 cells. They are rated for 2600mAh of capacity and could hold 10A of continuous discharge. I found, that the best option to replace dead cells, will be Panasonic NCR18650PF cells. Those cells have close discharge characteristics as LGs. They have 2900mAh of capacity and same maximum discharge current of 10A.

Step 10: Installing New Cells

Installed cells in place and spot welded with 0.15mm pure nickel tabs. To solder cells with soldering iron is possible too, but not recommended. At the end, spot welded tabs for cells balancing connections. All cells was spot welded and it’s time to confirm that was done properly by checking voltage of battery pack. Looks good.

Step 11: Connecting BMS

Soldered BMS back and connected balance wire. This time BMS LED flash red light, so I reset BMS by pressing reset button in corner. 

Before finally assemble I fully charged each individual cell group connected in series to 4.2V with Imax B6 chargers. By the way - those iMax B6 chargers are the best value for the money what you could get back in a day. Also I recently purchased a new small but very capable smart charger ToolKit RC M7AC charger Link to product! Highly recommended!

Step 12: Reassembling Back

Battery was fully charged and it’s time to wrap it back with insulation and install in the scooter. Connected charger, but battery was full so no action there. Secured battery cover and scooter is ready to roll.

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