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I'm looking at buying a cheap eBay BMS to prevent overdischarge of a Li-Ion battery, but I'm confused about the specs. Basically every BMS I can see lists the overdischarge cutoff voltage as "2.5-3.0v ± 0.05v". But... what does that mean? Is it 2.5V or 3V?

Is there a reason for such a wide voltage range in the spec?

Clonkex
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    See [what-to-check-for-when-buying-an-electronic-component-or-module](https://electronics.stackexchange.com/questions/504044/what-to-check-for-when-buying-an-electronic-component-or-module). – Transistor Jun 07 '21 at 11:24
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    Probably hysteresis. Overdischarge cutoff is activated at 2.3V (when discharging) and deactivated at 3V (charging) – NMF Jun 07 '21 at 11:30
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    It may also mean the BMS is customisable (probably before purchase) to suit LFP or LiPo chemistries, and if you buy it without negotiating with the supplier, you get whatever they supply by default. We don't know where you're looking, so... –  Jun 07 '21 at 12:09
  • @Transistor That's a good point. – Clonkex Jun 07 '21 at 23:18
  • @NMF Ah of course! That's probably it. – Clonkex Jun 07 '21 at 23:20
  • @user_1818839 Excellent point, I've added a link to the question. – Clonkex Jun 07 '21 at 23:20

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I just spent a couple of hours trying to figure the same thing out. I finally found a clue: https://youtu.be/sbNRaqrar7g?t=160

The link to the cheap ebay BMS you sent is now broken but I guess it was like many of the cheap one that use a HY2120 series chip, emphasis on series. When I looked at the chip on the 2S 3Amp BMS I have under a microscope it had "2120 CB" written on it. If you look at the datasheet for the HY2120, on page 6 you see that different variations of the 2120 chip have different overdischarge detection and release voltages, from 2.25 up to 3.2 with the CB one I have at "2.90±0.08V". I suspect this is where the range of voltages in the descriptions of cheap BMS's comes from and it is really only one value depending on which specific 2120 chip is on the board being sold. I was also wondering if it (re: data sheet page 6) could refer to the "Overdischarge detection voltage" vs. the "Overdischarge release voltage" since that yields a voltage range but I'm not sure that would make as much sense...for the 2120 CB BMS I have those values are 2.9V and 3.0V from the data sheet but the product description on Aliexpress for my board still "Over discharge voltage: 2.3-3.0v±0.05v" which makes me think it's more what I described above since 2.3V isn't shown on the data sheet for the 2120 CB.

As a minor side note since it was hard to find this info: the reason for investigating this was trying to understand if this Li-ion BMS would work for LiPo. Discharge to 2.3V would have certainly killed a LiPo, discharge to 2.9V seems to be debated but at least one person suggests 2.9V is ok.

Clonkex
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infinitesimal kitten
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  • This looks like a mix between comment and nswer. There is enough content to accept it as an answer. Cutting out the "discussion" would make it more readable. – kruemi May 31 '22 at 14:11
  • Your answer is the conclusion I came to while researching this myself. The actual voltage is probably set by a resistor. As a side note, I think the naming system for lithium-ion cells is a mess :| By default, Li-Ion (lithium-ion) typically refers to liquid-electrolyte cells with cobalt-oxide cathodes, but not necessarily. LiPo (lithium-ion polymer) is a type of Li-Ion cell, but with a polymer electrolyte (though the name makes no reference to the cathode material). LiFePO4 is a type of Li-Ion cell, but with an iron-phosphate cathode (though now the name makes no reference to the electrolyte). – Clonkex Jun 01 '22 at 00:08