I did some research, but didn't find the exact situation.

I have a device that has a battery inside and is powered by a 5 V 4000 mA USB-C charger. The device requires the battery to be connected at all times and won't turn on if the charge is less than 5%. The device is constantly connected to the charger when in use anyway, but it simply requires the presence of a battery.

The original battery was 3.7 V 6200 mAh 22.92 Wh flat Li-ion pack, made of 2 parallel 3100 mAh pouch cells connected to a protection circuit.

I only found 2 cells of the same size:

  1. 3.7 V 6600 mAh Li-ion battery made of 3 parallel round cells, 2200 mAh each

  2. 3.7 V 3000 mAh Li-ion battery made of 2 pouch cells, 1500 mAh each

Even though the device is connected to the charger, it still drains the battery while in use and when turned off it takes around 20 - 30 minutes to recharge. So I suppose the battery is somehow used alongside the charger. Is it safe to use any of these 2 batteries?

  • \$\begingroup\$ How many connections are there on the (original) battery packs? Some have extra connections for balanced charging, or detection. \$\endgroup\$
    – ocrdu
    Oct 23, 2020 at 10:54
  • \$\begingroup\$ It's connected to the device via a 4 pin connecter, so it's 4 wires coming out of the battery, however 2 (+) wires are soldered to a single spot on the protection board and 2 (-) wires as well. So I'd say it's only + and -, but for some reason divided into 2 pins each. Also the board only says P+ and P- for the 2 pairs of wires. \$\endgroup\$
    – Michael
    Oct 23, 2020 at 11:09

1 Answer 1


Either of the replacement batteries is probably suitable.

A 3.6 V or 3.7 V lithium ion (or Lithium Polymer) battery will be at about 4.2 V when fully charged. Newer chemistries may allow 4.3 V. Charging a 4.3 V capable battery to 4.2 V max will very slightly reduce stored capacity and do no harm. Charging a 4.2 V capable battery to 4.3 V can be a very very very bad idea.

Test: Charge the old battery fully when off and note the maximum voltage reached. This is probably 4.2V but may be 4.3V (newer and less likely).
Look at the datasheets (you do have datasheets available, don't you? :-) ) and note the maximum allowed charge voltage. Again, this is probably 4.2V.

If old and new are 4.2V or new is 4.3V capable then there is a very good chance that either of the replacement batteries are suitable.

If you can supply brands and part numbers for all batteries concerned it will help us to supply a better answer.

  • \$\begingroup\$ Thank you for the explanation. ;) I can't do anything with the old battery as it's damaged, but do not have the datasheet. The old battery model is: YDL754962-2P(1ICP8/49/63-2) manufacturer: Shenzhen Yongdeli New Energy Co. And the 3.7V 6600mAh I mentioned above is brand: Xinlantech and the only number I see on it is: ICR 18650*3 . The voltage seems to be the same, but I'm more concerned if it's gonna be safe in the device when it's gonna be plugged to charger constantly. I read some posts where it says it's important to match the mAh and C rating, but not sure how much it matters. \$\endgroup\$
    – Michael
    Oct 23, 2020 at 12:52
  • \$\begingroup\$ Best to match mAh approximately. Smaller battery will be charged faster than ideal but terminate charging slightly early -which is good. \$\endgroup\$
    – Russell McMahon
    Oct 23, 2020 at 14:02

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