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I’m trying to design a battery protection circuit for this 300mAh 3.7V LiPo battery.

I’m an EE student which is designing his first pcb. Please forgive questions with obvious answers.

My questions:

  1. The seller specifies a battery protection circuit is already present. Should I trust it or I’d be better off providing my own?

  2. The rating of the battery is 0.5C for average discharge current, 1C peak discharge current and 300 mAh. From what I understood, that is a really low rating for LiPO batteries and is not consistent with the 4 to 6 A current limiting protection which they state.

  3. The IC which I would intend to use is TI BQ297x. The design suggested on its datasheet is typical, with 2 FETs to manage charging and discharging. The FETs should be chosen according to their rds. However, the overdischarge voltage threshold of the IC can vary from 90 mV to 200 mV in 5 mV steps and with a max current of 300 mA that would require 2rds = 300 mOhm, that is a quite high. Should I modify the suggested design to meet such a low current? Would it be a good idea to place a resistor between the 2k2 resistor and the FET to increase the voltage drop and avoid high rds FETs?

Edit

I understand that Chinese sellers are not a reliable source of information. However, I was not able to find any LiPO battery from online stores like Digikey, Mouser and so on. Where do you guys recommend to buy to be sure about the product listed?

150 mA would be more than enough for my application. I’ll try to find a better alternative to BQ297x. My final design needs to be as small as possibile because it has to be a smartwatch for health monitoring. I think a ready made protection circuit is not a good way to optimize space. Same holds for old Nokia batteries with integrated protection circuits, I fear.

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  • \$\begingroup\$ Many countries (the EU for example) prohibit the sale of consumer Li+ / LiPo batteries that do not have an integrated protection circuit. \$\endgroup\$ – Peter Smith Jun 20 '18 at 12:10
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    \$\begingroup\$ @PeterSmith I have no reason to believe what you write is untrue, however, I recently was able to buy unprotected Li-Ion based sells in my EU country from a shop in my EU country. Even if it is not allowed, it does not appear to be enforced. \$\endgroup\$ – Bimpelrekkie Jun 20 '18 at 12:16
  • \$\begingroup\$ digikey.com/product-detail/en/adafruit-industries-llc/2750/… \$\endgroup\$ – BeB00 Jun 20 '18 at 13:44
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Note how the seller lists "Nominal Capacity:3000mAh" in the specifications while this clearly is a 300 mAh battery. That 3000mAh is a typo for sure, however I would not rely on anything listed in the offers from most Chinese sellers as they often have no clue what they sell. There are exceptions of course.

From the photo it can be seen that there is some (yellow) kapton tape near the connections and I think I also see an SMD resistor under the tape. That indicates that a protection circuit might be present already. However, there is no guarantee that you will get what is shown in the picture.

If making the protection PCB yourself is not needed for your project you could consider buying a ready made protection module. Or you could buy a battery which is guaranteed to have build-in protection. There are many generic batteries intended for old fashioned (not smartphones) Nokia phones which are cheap and have the protection. These might be larger in size than this 300 mAh battery though.

With such a small battery I would not exceed the 0.5 C discharge current, that is 150 mA. Drawing 4 to 6 A from such a small battery is asking for trouble.

I do not think the TI BQ297x is good choice for such a small battery, it might be better to choose a protection IC designed for small batteries. Such an IC might also have everything built-in so no need for external MOSFETs.

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