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I am trying to make a PCB for a drone run off of Li-ion batteries for a project of mine. I wanted to protect the drone and cells from an Undervoltage situation and cut off the Li-ion battery from the motors. I had some ideas that I wanted to ask about:

  1. This was fairly easy to implement on my PCB. I would use an external low-voltage alarm module such as this one Li-ion Battery Low Voltage Buzzer Alarm on Amazon. This video on a battery protection circuit is the basis for my cutoff part, I would just need something like the voltage alarm to activate the relay. Should I just buy the modules since I don't know if this would be the right call as I wanted to integrate the voltage alarm into my PCB, as I feel that would be cheaper and more reliable than soldering an external module.

  2. Using a battery protection IC such as the TI BQ77216. This would help with the integration and reliability factor, however as I am inexperienced with such ICs (and PCB stuff in general), I was confused on a certain aspect of these ICs. I was planning on having a 1-10S connector to plug the BMS of a Li-ion battery into. The problem arises when I look through the BQ77216 Datasheet, as it shows that I need to connect the various channel pins together when using underneath 16S. BQ77216 14 Cell Example on Page 15
    (Found on Page 15)

    I wanted to just have an 11-pin connector and just basically plug it in for various cells, but I don't know if I have to connect those pins or put jumper caps between each pin such as the schematic found in the BQ77216EVM Datasheet on pages 14 and 15. There is also the problem with the pack+ and pack- as if I have differing BMS, these pins would move around, such as in the schematic below.

schematic

simulate this circuit – Schematic created using CircuitLab

  1. (Continued) I was planning on using a simple 1x11 male header pin to attach the BMS, but it seems as if the pack+ or pack- and the cells would have to be switched, and I don't want a lot of jumper caps. Can I just connect the pack+ trace to each of the pins so that the pack+ trace would vary on where the BMS is connected and account for the varying amount of cells? Would simply using 10 of these TI BQ2973 ICs also achieve the same effect? I'm just worried that two or more ICs would simultaneously read a cell, such as cell 1 and cell 2 at the same time, and cause inaccuracies, even if there is a delay included on the IC.

  2. Should I just use multiple comparators or voltage references like the TI TLV431 with diodes so that the voltage and current of one cell doesn't reverse into the adjacent cell (I don't know if that is correct or even possible, I'm just worried if that is a possibility). Something like this (Diodes are just examples, also I'm fairly new to CircuitLab so I don't know if the symbol for the TLV431 would be right in this schematic).

schematic

simulate this circuit

So far these are what I have in mind. Sorry for the long post, I just want to make sure what direction I should be heading in.

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  • \$\begingroup\$ Is this a "if any one (in series) presents a voltage below some threshold then the entire series chain is to be removed from the circuit" kind of thing? (I thought I'd ask before trying to read everything.) \$\endgroup\$ Jun 24, 2023 at 2:40
  • \$\begingroup\$ @periblepsis Yes. Similar to the video linked, a relay will cut off power to the motors through the power connector of the LiPo. That relay will receive voltage from a component that checks to see if any one of the series batteries has a voltage underneath the under-voltage threshold. It is mainly going to be used to cut off the LiPo's power connector as the threshold will be above the actual voltage the Lipo should be cut off from. \$\endgroup\$
    – Texas-72
    Jun 24, 2023 at 17:45

1 Answer 1

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I would like to suggest that you'll be much happier buying one of the inexpensive protector BMS modules that are readily available on Amazon, eBay and AliExpress: they have already solved problems that you haven't even realized they are problems yet, they are guaranteed to work, they are well designed, and they are cheaper than even just your part cost.

For 3 cells in series: https://www.batteryspace.com/pcbfor108v111vpack.aspx

For 4 cells in series: https://www.batteryspace.com/pcbfor144v148vpack.aspx

These are so cheap that you can easily buy one of each, rather than jury rig a way to use the same BMS for 3 or 4 cells in series.

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