I recently received a Li-ion BMS which has a checkbox called:

Balance function: with / without

Apparently the BMS does NOT have a balance function. I want to use it with 18650 cells, the specifications are here: https://www.aliexpress.com/item/14S-Lithium-Battery-BMS-for-58-8V-18650-Battery-or-Lipo-Battery-PCB-with-Max-40A/32806164694.html?spm=a2g0s.9042311.0.0.jfSUrh

What does a BMS do when it does not have a balance function? is it ok for 18650s ?

  • \$\begingroup\$ The one in the photo has "with" marked, looke like they sent you the wrong part. \$\endgroup\$ Aug 7, 2017 at 0:45
  • \$\begingroup\$ Thanks, indeed i've put a complaint on AliExpress. For the moment i have an message: roasted geng de hybrid rubber cover hong meng lian xie three!, zhe bai cai zhe zhuan xing de bai fen plum plum qian de ... it will be difficult i can tell. \$\endgroup\$ Aug 7, 2017 at 5:57
  • 1
    \$\begingroup\$ Seeing that message, tagged "comprehensible", first thing in the morning has made my day! \$\endgroup\$
    – user16324
    Aug 7, 2017 at 9:12

1 Answer 1


It protects against over-charge, over-discharge, and over-current. Another name for this function is Protection Circuit Module (PCM). According its specifications that module is suitable for 18650's.

It should cut when any cell voltage goes too high or low, but it can't equalize the cell voltages. This isn't a problem if the cells are initially balanced and have very low self-discharge rates.

However it might be worth providing a facility for manually checking and balancing the cells. On Lipos this is commonly done with a 'balance connector' which taps into the connections between the cells.

  • \$\begingroup\$ Thanks Bruce. I still don't understand how it can fail to balance cells. Without balance means that the the entire pack stops charging when a single group reaches 4.2V? If the BMS can independently charge the cells, then they will be balanced. The bms has 14 sense wires used for charging, what is the difference with a balance connector which taps the connection in between the groups/cells? \$\endgroup\$ Aug 7, 2017 at 6:08
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    \$\begingroup\$ The difference between sensing and balancing is that sensors just read the cell voltages (and disconnect the entire battery if any cell voltage is out of range) while a balancer discharges cells whose voltages are too high. Some balancers only become active when the cells reaches peak voltage (4.2V). Others continuously compare the cell voltages and discharge any that are higher even before they reach peak voltage. \$\endgroup\$ Aug 7, 2017 at 8:14

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