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Discuss the Xiaomi mijia mi M365 original and pro versions in this forum. Topics include hardware, software, hacking, riding, and everything in between.
By danyro
#17526
Hi folks,

i just finished the upgrade of an external battery pack to my m365.
Now comes the question that i did not find an answer even high level to it.

How should be charged the external and the internal batteries in order to be fully charged an in the same time balanced across all cells?

i am using the Y method.

any advice is extremely appreciated.

thanks
By Caidyn
#17534
If it’s wired up properly I believe you should be using the same charge port used to charge the internal battery. The batteries should be wired in parallel to add the capacities together. If it’s wired in series it would add the voltages together which you don’t want.
By danyro
#17610
They are set in paralel through Y method.
But the external battery shouldnt be charged via charging port in order to benefit of an even charging via BMS?
Will make a difference if the charging port of external battery is soldered to the factory charging port? Im this.way both packs.will be loaded via their BMS

Any thoughts?
By IamPurple
#17612
Dont quote me on this, but I think the best, right after charging separately, is to charge via the second batteries charging port, that way the external will charge trough BMS and the internal, even though charging trough the wrong port, will still have some protections and balancing going on because it has more fancy BMS.
By IamPurple
#17613
This is what Rollerplausch has to say (translated from German): ABSOLUTELY IMPORTANT!
It becomes dangerous when trying to brake electrically with 42V and more! The scooter with all eventualities denied any electric brake assistance and one is forced to come to a halt with the one weak disc brake, which is intended only for slow holding! The voltage buffer between 41.5V and 42.25V is thus mentioned for a good reason so sharp and is ALWAYS up! At the beginning, regenerative braking (= KERS or electric braking) is possible downhill for at least a certain amount of time!

Separate charging - charging time 4-6 hours (RECOMMENDED)
In the meantime, the balances for both batteries may occur and the voltages need only be checked shortly before the journey in order to assemble both batteries. This at best only shortly before the journey, so that balancing flows or losses are saved - which can arise due to the different voltages (between 41.5 and 42.2V). Still harmless compared to coming!

Charging by additional battery charger - 8-12 hours (possibly OK)
Charging via the additional battery charging port would be the second safest method for the additional battery, but also charges the internal battery to 42V. This is determined by the other power supply or its set charge end voltage - therefore, the voltage should be guarded to save the M365 battery!

Charging via Xiaomi charger - charging time 8-12 hours (not recommended)
This charges both batteries to the power supply end set voltage of 41.5V. It should be noted that the additional battery is charged via the discharge port and the BMS of the additional battery in the case of an out of control cell NOT turn off nor can properly balances , since the cell voltages just never reach 4.2V or even exceed!

The following exception has already been explained for the BMS ports : The BMS unloading port (-P) was re-soldered to the loading port (-C) of the BMS. Then, of course, this method seems ideal for lazy people ... Attention, the additional battery will not balance itself!

Charging by both chargers - charging time 4-6 hours (negligent)
Here the BMS can not work as intended! Depending on the combination of the batteries , the following occurs in addition to an uncertain final voltage :

Y-Method : The BMS of the auxiliary battery can not switch off due to the Xiaomi charger, which contributes via the discharge port!
Parallel soldering to the internal battery : Both BMS can not shut down properly or work !!!
By danyro
#17654
thanks for explanations and reference.

so the best method without too much hassle, would be the 2nd option.

But maybe someone with electricity skills/know how can say if charging port of external battery would be connected to the original internal battery port charging and in the same time, it will charge both batteries via their designated charging port, at the same voltage...

dont know if this would be a feasible scenario ..
By IamPurple
#17681
danyro wrote:
Mon Nov 25, 2019 12:02 pm
thanks for explanations and reference.

so the best method without too much hassle, would be the 2nd option.

But maybe someone with electricity skills/know how can say if charging port of external battery would be connected to the original internal battery port charging and in the same time, it will charge both batteries via their designated charging port, at the same voltage...

dont know if this would be a feasible scenario ..
That setup you describe sounds like the worst scenario mentioned by Rollerplaush:
"Charging by both chargers - charging time 4-6 hours (negligent)
Here the BMS can not work as intended! Depending on the combination of the batteries , the following occurs in addition to an uncertain final voltage :

Y-Method : The BMS of the auxiliary battery can not switch off due to the Xiaomi charger, which contributes via the discharge port! "

And it doesnt accomplish anything that charging via the second batteries charging port wouldnt, it just adds extra danger. Both batteries will still charge to 42V, because they are connected in parallel and will equalize no matter what. If you dont want them to charge to 42V to prolong charge cycles, you can limit the charger to 41.5V or 41.0V, but that might be problematic: if you use the cheep aliexpress batteries, they will require to charge to close to 42V for cell balancing.
I woulnt worry much about reduced charge cycles, because you will have two batteries and each battery will be used less, equalizing the loss. Might be even better because the batteries wont heat up as much as less current is drawn from each one, prolonging their charge cycles/ life.

The part about regen braking not working when battery is at 42V is omething you want to test - does it really not work and how much you have to drain the battery for it to start working again. Shouldnt be a big deal once you know it.
By m365b
#18154
I believe danyro's stated option is the "Charging via Xiaomi charger - charging time 8-12 hours (not recommended)" option except the additional battery's charging port has been soldered to the original scooter's charging plug. This means when charging, the additional battery's output port has to be disconnected from the original scooter battery. Now the xiaomi charger is charging the original and additional battery at their intended charging ports. I guess if you forget to disconnect the two batteries at their output port, then you would be charging similar to the fourth option ("The BMS unloading port (-P) was re-soldered to the loading port (-C) of the BMS").

But I believe you still answered his question. As you stated, the typical cheap batteries require the voltage to be at least 42v for the BMS to operate and the original battery only charges to 41.5v at best (don't want to reconnect the batteries with a voltage difference greater than 0.5v). If you have a BMS that starts to operate when an individual cell reaches 4.2v then danyro's stated option would seem to be the safest, correct? For me, the third option is ok (Charging via Xiaomi charger - charging time 8-12 hours) as the original battery is charged as intended and the additional battery can be charged separately once in a while to balance the individual cells.

Bottom line, I believe from what I have read, the best charging option (when using a cheap battery with a BMS that operates at 42v) is the following:

1) disconnect the batteries from one another
2) individually charge each battery at their intended BMS ports using their own charger
3) discharge the additional battery until it reaches a voltage within 0.5v of the original battery
4) reconnect the batteries
5) use scooter as soon as possible. overall battery life is longest when kept as close to 36v as possible.
By danyro
#18170
thanks IamPurple amd m365b for your feedback.

i've ended up by charging both of them via external battery charging port to 42V.
I really dont care that the number of life cycles will be reduced :) for sure its purpose was already achieved.
too much hassle for charging them separately, discharge the external one to 41,5V and then connect them..

again, thanks!!

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