Installing additional battery upgrade on Xiaomi Mijia M365

I wanted to install an additional battery on my M365 since I bought it, but I couldn’t find any info regarding the process written in English.

Later I found a video with the good description of the process. I liked pretty much everything, except the soldering directly to the original battery.

Then in the comments I found a link with another process description, but it was written on an image and it was in German.

I went ahead and used the similar setup as was shown on an image, photographed everything and decided to share it with you, English-speaking folks.

The plan:

1. Add a connector to the original battery, without soldering on it.
2. Make an extension cable for the external battery.
3. Mount the external battery.

Parts needed:

• 36V Li-Ion battery (use 8Ah or more)
• 1 x XT60 connector
• 2 x XT30 connectors
• 1 m of red & black 14AWG wire
• Heat shrink tubing
• Car fuse holder on a wire
• 25A car fuse
• Quality solder and flux

The process:

1. I started by soldering two short wires to an XT30 male connector:

Use a lot of flux and make sure there is no cold soldering.

I set my soldering iron to 400°C for the whole process, I think it’s a good temperature to solder wires this thick. I also used a wide soldering tip.

2. I cut a piece of insulation in the middle of each wire and placed two small heat shrink tubes on each of the wires:

3. I soldered a female XT30 on the other side and used two of four heat shrink tubes to seal the connections:

4. I made a female XT60 connector with short wires:

5. I soldered the XT60 cable to the points on the XT30 cable I cut before:

Again, use as much flux and solder as needed to make quality joints. These wires are going to pass high currents. Badly soldered joints can heat up and desolder or cause fire.

6. Shrink the tubes we attached before to finish this Y connector:

7. Attach the connector to the battery and the controller:

The scooter should be able to turn on after you attach this. I ran the scooter for some time to test if nothing heats up. My test went smooth and nothing was heating up. If it did, I’d re-solder the joint that causes it.

8. Make an extension cable with a male XT60 on one end and pass it through the scooter frame:

Mine turned out to be around 70 cm, which is more than enough.
I forgot to make a photo before inserting the cable, sorry :)

9. Add a car fuse holder and another male XT60 connector to the extension cable:

Use long pieces of heat shrink tubing to prevent it from ever slipping, as this part will be moved and rubbed against much more than the internal parts. I used a 25A fuse, as was shown in the other guides.

10. Connect the extension cable to the Y connector:

Use a multimeter to test if you have voltage on the outer end of the cable. If not — check the joints and if you forgot to install the fuse. Also the cable might have been damaged while you were inserting it, check for that too.

11. Mount the battery housing with the battery:

In my case, it’s a phone bag that I bought a while ago, and it just happens that both guides I mentioned before recommend to use one like this. I just used the original three straps to attach it as I didn’t want to drill into the scooter frame. The 8Ah battery fits very tight in it.

EDIT:
I made a 3D model for mounting the battery bag in a much sturdier way.
I completely cut off all the straps on the bag and mounted it using this model and a piece of MDF board on the inside of the bag. It’s also a much better alternative than drilling into the scooter frame to mount the bag.
If you add two pieces of rubber under the mounts — it will hold really nice and tight.

12. Solder an XT60 connector to the external battery:

13. Charge/discharge the batteries to the same voltage level. If you don’t have a multimeter — then fully charge the stock battery and the external battery separately, with the original M365 charger. If you connect two batteries with different voltage — one will start charging the other one very fast and nothing good is going to happen.

14. Connect the external battery to the extension cable and hide the wires:

Be sure to check again if nothing heats up. Run the scooter for a while to somewhat discharge the batteries and plug in the charger. Check everything once more.

15. Close the deck and go for a test ride. Check if nothing heats up on high loads.

16. (Optional) Create a custom firmware using this site.

I configured mine like this, to get more power in normal mode, and to get a stock-like mode in the Eco mode. The first one gives the ability to climb steep hills and accelerate better. The Eco mode should give an ability to drive like a stock, but with double the range.

If you like stock — you can enable only “Remove charging mode”, to avoid charging mode turning on while you drive, in case the external battery starts charging the stock one.

I hope the guide turned out clear and you like it. I’m planning to update it with some range tests on stock and custom firmwares later on.

Have fun!