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I made a circuit for charging a 40mah lipo battery (381018), my problem appears when a new uncharged battery is connected. The battery measures 0V at the begining, when it's conected the VBAT pin only goes to 1.58V, so the battery is not getting charged at all.

The problem doesn't appear if the battery have some voltage on it, in that case the battery charges normally.

I have a 25k (0402) resistor connected to Rprog, if I change that resistor to 10K the problem dissapears but I'm charging a 40mah battery with 100mA, it's not safe.

1) Could the problem be the size of the resistor (0402)?

2) Should I put an special array for the 0V uncharged batteries?

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    \$\begingroup\$ If a Lipo battery shows 0V, it means that it is likely damaged beyond recovery, depending for how long it was in that state. A normally discharged battery should show at least 2.5 - 2.7 V. Normal shipping state of a good "uncharged" battery should be at 3.5- 3.6 V, "0V" batteries should be discarded. \$\endgroup\$ – Ale..chenski Aug 29 '18 at 23:58
  • \$\begingroup\$ The battery cell is at 1.7v and after the small cicuit they have it's 0V, Could the 1.7v is to low for a discharged battery cell? \$\endgroup\$ – Joc Aug 30 '18 at 18:02
  • \$\begingroup\$ Yes, 1.7 V is considered as "deep overdischarge". Sitting long time in this state will cause irreversible damage to electrode structures, separators, and electrolyte. The 1.7 might be not that bad however. \$\endgroup\$ – Ale..chenski Aug 30 '18 at 18:36
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This is most likely self-discharging of the defective battery, or - hopefully - a feature of the charging IC to prevent the battery from catching fire. LiPo batteries should be disposed of if they're discharged below 3 volts or so because they get damaged when they're over-discharged.

Don't continue using this battery. It is a fire hazard. Get a new one.

Also, don't throw the battery in the trash - put it into a battery recycling container.

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  • \$\begingroup\$ Hi Jonathan, thanks for helping me. The battery cell is at 1.7v and after the small circuit they have integrated is 0v. The problem is the first time it's connected, the mcp73831 seems to drop the voltage with the 25k Rprog, if a change the Rprog to 10k resistor the battery starts to charge and then I can switch back to 25k Rprog and everything works great. Do you think the battery demands more current at the begining to break because of the low 1.7v voltage, and that's why the MCP73831 drops the voltage when there is not enough current? \$\endgroup\$ – Joc Aug 30 '18 at 18:09
  • \$\begingroup\$ @Joc: Again, 1.7V is absolutely not a safe voltage for a LiPo. Don't continue using this battery, get a new one. And don't let the new one's voltage drop below 3 volts. If you get the battery with such a low voltage, contact the seller because it's defective and request a new, working one. You don't need to change the circuit if it works with good batteries (>3V). \$\endgroup\$ – Jonathan S. Aug 30 '18 at 23:58

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