First DIY battery pack BMS

May 16, 2021
This is going to be my first DIY battery pack. I have ordered a BBSHD from China and 168 Molicel P42A cells.

I plan to build two 14S 51.8V battery packs:

  • 3P 650Wh
  • 9P 1950Wh

I'm splitting the pack into two, as I the majority of my journeys will be short but I will be doing some long distance journeys. That way I don't need to carry the full weight of the 168 cells on short journeys. I plan to have three configurations for the batteries, depending on the range required:

  • Small battery only
  • Large battery only
  • Both batteries

I am comissioning a custom frame bag to hold the batteries as I want to keep the center of gravity central.

Being new to this I am having difficulty finding a suitable BMS. I would like a BMS that offers overcharge protection, over discharge protection, over current protection, short circuit protection and temperature protection. The BBSHD can draw 30A max but the controller can be modified to draw 50A. I don't intend to do this mod at first but I would like a BMS that can handle 50A in case I decide I would like the extra power in the future. I am struggling to find something that can supply just the right charging current to the batteries (4.2A per cell). Something like this looks good (I like the idea of a smart BMS).

However, I am unsure if I will be able to adjust things, like the charging current, using the app, though it seems like I can from what I have read in various forum posts.

I would apprieciate some advice from someone more expereinced on what BMS to use.

I am also considering making a connector, to allow both batteries to be connected in paralell in configuration 3.
It would feature diodes to prevent them trying to charge each other. Is this a worable/sensible idea and is there anything I should be aware of? I understand there will be some voltage drop but this acceptable if it is under 1V.


1 TW
Dec 18, 2018
So long as the balance leads are accessible to be paralleled not just the power pair

then just one BMS will work for all three configurations.

Just ensure they are within say 10V before joining, charge and discharge together after that, no one way diodes needed, let the sub-packs self-balance together in use.

BMS only controls charging on **very** expensive EV setups

A good charger should self terminate, BMS HVC is only backup for when the charger fails.

Also LVC can be redundant to a controller, or rider keeping an eye on the display

again BMS only as last ditch failsafe.

Batteries rarely need temp protection, but controllers may offer that for themselves and maybe the motor.

Just use fusing for shorts and OCP.
May 16, 2021
Thanks for your advice!

I will be using the smaller battery much more meaning it will get much more wear. I will be changing the configuration quite regularly as the range I will need will vary. Will this not make connecting them in parallel an issue?

Do you have any recommendations on what charger to use?