Chinese Li-ion BMS Fix

Introduction: Chinese Li-ion BMS Fix

My BMS for my powerwall had an annoying habit of latching off when the battery voltage dropped too low (usually overnight when the power was used up). It's meant to do this to protect the battery, but I had to switch the power to the batteries off and on again to get the charger to start charging the batteries the next day. It's a fairly standard 14S 48V chinese BMS that you can get for about £10 off eBay.


I have included a schematic of the type of BMS that I have used (I had the same issue with another BMS I used for my eBike). This is showing only 4 cells, but the extra cells are just stacked above and are identical circuits to cell 4. The first 3 cells are a bit different as you can see that the 3rd cell provides the bias voltage for the MOSFETs.

I found that adding a 100K ohm 'bleed' resistor (*R25 in schematic) across the charge/discharge port and the battery port allowed the circuit to self reset.

Supplies

  • Your BMS
  • A 100K leaded resistor
  • Heatshrink (optional)

Step 1: Fit Your Resistor!

Please refer to the images - I put heatshrink around my resistor, and it is at the bottom of the photo. Your BMS may be different, but a 100K resistor across these terminals should not do any damage as the current that can flow is tiny (500microamps at 50v). Just be sure you have the correct value first!

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    6 Comments

    0
    username1
    username1

    8 months ago

    What is the name of the 4 integrated in the middle?

    0
    daviddeakin
    daviddeakin

    Question 1 year ago on Introduction

    Hi, How have you dealt with the issue of the charger allowing an external battery to be used without a RS485/Canbus connection with the battery? I have been wanting to use a Solax hybrid inverter but all that I have read online suggests it will not allow allow charging without being sent status signal from a compatible battery. Thanks.

    0
    ollysworkshop
    ollysworkshop

    Answer 1 year ago

    Hi, mine doesn't seem to mind. You just configure the battery type, set the upper and lower voltage limits, and max charge current and it just works. You don't get an accurate state of charge without the Comms, but it makes a reasonable guess based on the battery voltage. That all said, my model is nearly 4 years old. Newer ones may well be more locked down and require the Comms connection.

    1
    tytower
    tytower

    2 years ago

    Indeed only half a milliamp should flow. I get the same problem too with having to do it manually . Thanks . Some of them have all the IC's in one . What IC's does yours have on the BMS board? Have you tried increasing the resistance even further?

    0
    ollysworkshop
    ollysworkshop

    Reply 2 years ago

    Hi. Mine has 14 of the DW01 ICs on it. Thanks for the comment, I didn't label them on the schematic, so I'll update that when I have a chance. I've not tried different values, as I haven't had any issues. If you fancy it, try some and let me know here. I'm a bit tied up with a new baby for the foreseeable! I've got a BMS with the single IC on it, but not tried it yet.