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I have a project where I'm planning to use a unprotected Li-ion pouch cell with a capacity of 10 mAh. I will be providing my own external protection circuitry.

All of the overcharge/overdischarge protection chips I can find have current draw on the order of 2-3 uA. While this is fine for a 1 Ah battery, it will deplete my cell after only a couple months on the shelf. Some of these protection chips have a "sleep mode" which typically go down to 10-100 nA, but the only way to exit it is by plugging the system into a charger, which is not ideal. I would like my system to be able to turn back on with a button press.

Does anyone have advice on how to deal with this problem? Do I just need to bite the bullet with the offered sleep modes, or is there another way?

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  • \$\begingroup\$ your system is still not clear but from what I understood you will need to cut off the supply from your LiPo battery with a mechanical switch. you can use a dual position switch (position for ON, and another one for OFF) this way you will disconnect the battery from you circuit and when you need to operate (charging or using it normally) you will need to turn it ON manually. \$\endgroup\$ Commented Jul 4, 2023 at 19:22
  • \$\begingroup\$ This will not work. LiPo protection chips often have volatile state are not designed to be disconnected from the battery. \$\endgroup\$
    – John M
    Commented Jul 4, 2023 at 19:40

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If you are looking for long shelf-life you could put a NC Hall effect switch in the product and put a magnet in the packaging. Once the product is removed from the package the switch shuts. The issue is then you have current draw of the protection IC and the power loss of the Hall effect. Do you have a battery capacity run time budget? It really depends on application and requirements. I was given unreasonable requirements in a battery application and I told them unless they wanted to increase the size of the device to accommodate a bigger battery they were out of luck. This was a wearable so they backed off the off-the-shelf charge requirement. The other option would be like a pull tab depending on your Li-ion cell format. If it’s prismatic that doesn’t really help you unless you get creative with the contacts which would require more than likely a high volume manufacturing. Custom stamping starts to save money at around 125k units. Good luck

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a couple months on the shelf.

I assume you mean the time between manufacture and first use by the end user. If so, that's what the "sleep mode" is for. It perfectly acceptable to tell the end user: "Fully charge before the first use". You have seen that in many consumer products. That first charge will wake-up the protector BMS and will replenish the charge lost during shipping and storage.

If, instead, you mean 2 months on the end-user's shelf, then the instructions are: "Fully charge after long storage periods".

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You could do a very simple over discharge only protection circuit with a logic level N channel MOSFET, a PNP transistor, 2 10M ohm resistors, and 3 diodes or a zener combined with the diode drop from the PNP to make it start to turn off around 2 Volts.

MOSFET source would be connected to the lithium cell negative. The MOSFET diode would allow charging until enough Voltage exists to turn the MOSFET on. MOSFET gate goes to PNP collector. It should be easy to figure out the rest. You might add a 3rd 10M resistor from the PNP base to emitter and make the resistor that is in series with the diodes/zener smaller.

Current draw when turned on would be a little over V/10Mohm.

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