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I am confused with the differences between Li-ion batteries (like 18650) and Li-Po (the flat ones).

I know that basically they are both lithium ion, but when I want to buy I see that LiPos have always undervoltage protection, and they should not be used if voltage is <3.0V, whereas 18650 Li-ions can be with or without protection circuitry and the cut-off is around 2.5V-2.6V (if there is protection circuit.)

Is my statement correct?

In general, can I replace a Li-ion with LiPo in a circuit?

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  • \$\begingroup\$ Radio control models buy Lipos ALL THE TIME (the flat ones that look like bags) without undervoltage protection. It's just a matter of how raw of a product you are buying. I'll bet all the batteries you're looking at with built-in undervoltage protection are very small batteries. \$\endgroup\$
    – DKNguyen
    Apr 1, 2021 at 5:10
  • \$\begingroup\$ i have two big 18650 li-ions and i have bought them with protection..they are more than 3000mAh \$\endgroup\$ Apr 1, 2021 at 15:54
  • \$\begingroup\$ Pretty big. In any case, you can get them easily without built-in protection. \$\endgroup\$
    – DKNguyen
    Apr 1, 2021 at 19:38

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Here's a comparison chart from Electronic Design:

enter image description here

And here's another from Benzo Energy:

enter image description here

These may not be all inclusive, but whether you can replace one with the other depends on which of the attributes are being utilized. For a phone, the non-cylindrical form factor and lack of liquid electrolyte are relatively irreplacable advantages. In addition to this for a device frequently held close to the body and unmonitored, the cells inflate their plastic bag in most failure modes and push the back off the phone, making it clear that maintenance is necessary, typically without a fire.

Generally, you shouldn't use a lithium ion battery without protection, and ones sold without protection are for use in packs with overall protection or devices that provide protection separately. You can also greatly extend their lifespan by not using their full charge range.

It's also worth noting that you cannot replace chemistry or with a smaller cell without evaluating the charging and protection circuits. Switching to a larger cell may also be a risk of overcharging because cut off current is based on capacity, so you have to evaluate the charger circuit in that case as well. Thanks Russel McMahon for the correction.

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  • \$\begingroup\$ May be worth noting that a higher than original capacity battery may be OVER charged by an original charger as acceptable termination current will be a lower fraction of the cell's capacity. \$\endgroup\$
    – Russell McMahon
    Apr 1, 2021 at 11:54
  • \$\begingroup\$ @RussellMcMahon Hmm it's something that I've gotten away with quite a few times, but that does sound intuitively bad. \$\endgroup\$
    – K H
    Apr 1, 2021 at 12:20
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    \$\begingroup\$ @RussellMcMahon: At least from my experience, the battery charge circuit monitors the battery voltage to keep from over charging the battery. When the battery voltage reaches a predetermined value (let's say 4.1 VDC), then the charger switches from constant-current to constant-voltage mode to prevent the battery from being overcharged. A higher than original capacity battery will just take longer to charge. There should not be a risk of over charging if the battery protection circuitry is working properly. digikey.com/en/articles/… \$\endgroup\$
    – Grace
    Apr 1, 2021 at 15:07
  • \$\begingroup\$ @Grace the battery can be overcharged because the constant voltage charge mode cut off happens when the charge current reaches 1/10th of the C (mAh capacity) of the battery. If for example, you double the capacity of the battery but don't modify the charger circuit, the circuit will keep charging until charge current reaches 1/20th of C because it's still set to the cut out current of the smaller cell, which will result in an overcharge. I suspect the damage done is more a reduction in MTBF than a guaranteed fireball as long as you don't increase capacity too much. \$\endgroup\$
    – K H
    Apr 1, 2021 at 23:18
  • \$\begingroup\$ @RussellMcMahon - Hi, Just FYI Grace originally posted as an answer - but since it wasn't an answer, I've converted it to a comment, as it was clearly addressed in response to your comment. However, due to only having 1 point currently, Grace cannot currently write further comments in reply. \$\endgroup\$
    – SamGibson
    Apr 1, 2021 at 23:27

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