Suppose one uses a poor 18650 (low quality and without a datasheet) to charge a phone and the phone tries to draw 3A-4A (2A @5V) from it. Would this be hazardous even with a protection circuit built in the device (lock out at 2.7V)? Or would the voltage drop low enough even if it's a low-quality 18650?


  • \$\begingroup\$ You can't even be sure that low-quality no-name (or bad name) 18650's actually have protection circuits, despite claims. \$\endgroup\$ – Scott Seidman Jul 15 '15 at 15:52
  • \$\begingroup\$ Sure. Regardless of the protection circuit on the cell, I wonder if it's considered safe to use a protected/unprotected cell in a protected draining device. \$\endgroup\$ – John M. Jul 15 '15 at 19:11

Most bare 18650 cells do NOT have protection circuits built into them. That's because most of those cells are designed to be used in groups as a multi-cell battery. It is that battery that will have protection circuits built in.

You can purchase 18650 cells that DO have a protection circuit built in. There are two basic types of protection: over-current only (polyswitch self-resetting fuse) and under-voltage lockout.

Both types are protection are intended to be used with products that accept a single 18650 cell. There are many such products available - it used to be mostly flashlights but more and more devices are being manufactured that use a single 18650 cell for power.

The under-voltage lockout protection is extremely useful for cells being used in flashlights because those will happily discharge the cell well below the minimum allowed voltage. This results in the cell being damaged or destroyed - either the capacity is reduced significantly or it simply won't accept a charge anymore.

The 18650 cells that have built-in protection are usually very slightly longer than a bare cell. The end contact is often a gold-plated circuit board.

But do be careful when purchasing these cells. In particular, make sure that you are getting the protection circuit that you want. It truly sucks if you think that you are purchasing cells with under-voltage protection and find that what has actually been delivered is cells with only over-current protection.

Been there, done that.

  • \$\begingroup\$ But if the device itself is protected (e.g., with a S-8261), then will it be considered safe to use an unprotected cell? That is, the protection circuit isn't on the cell itself but built in the device. \$\endgroup\$ – John M. Jul 15 '15 at 19:08

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