Author Topic: Using protected 18650 if circuit already has protection  (Read 857 times)

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Offline PeabodyTopic starter

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Using protected 18650 if circuit already has protection
« on: May 25, 2020, 03:08:51 pm »
I'll be using one of these:

https://www.aliexpress.com/item/32870411748.html

and it already has the usual DW01 protection circuit.  I just wondered what difference it would make if the 18650 used in this module is a protected cell.  I assume at low current it wouldn't make any difference, but if  your load is something like an amp, it seems you would be going through four mosfets, and that might make a noticeable difference.  I don't have an unprotected cell at the moment to experiment with, but wondered if anyone has already looked into this.  Basically, if you had to buy a battery for this device, would you make a point of buying an unprotected one, or does it just not matter?

A related question.  If I wanted to use a protected cell, but didn't  want double protection, it seems I could bypass the protection on the module by just connecting the negative battery terminal directly to ground.   Is that right?

 

Offline tunk

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Re: Using protected 18650 if circuit already has protection
« Reply #1 on: May 25, 2020, 03:35:52 pm »
I don't know about the protection, but a protected cell
may not fit as it is 3-4mm longer than an unprotected cell.
 

Offline KL27x

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Re: Using protected 18650 if circuit already has protection
« Reply #2 on: May 25, 2020, 09:43:55 pm »
I think almost all 18650 cells have a fuse in there. If you burn the fuse, the cell is dead. Some sellers might call this fuse "protection."

I have used polymer batteries with an internal circuit that cuts out at overcurrent and have to be reset. That's annoying, because you have to put 4V on it to wake it up, again, and the protection acts like a fast fuse, and it should really be a slow-blow, IMO. I haven't seen that yet, on an 18650, but apparently they exist (^ and it adds 3-4 mm). Yeah, you will lose a tiny bit of power to the internal FET, but that will be very little. I wouldn't worry about it. 

All li ion chargers regulate charge current and max voltage. The circuit you linked also prevents overdischarge. The protection on the cell itself is often just for max current (for discharge. I dunno if it also works for charging, but the charger should take care of that). If your cell has extra goodies on the protection, then it might be redundant. That would be unusual, though, to have undervoltage protection on a cell, itself. The cutout voltage depends on the load, so it's not typical to add that to a generic cell. The board you linked decided that for you, but the optimal cutout will depend on the draw at the time, and is typically up to the designer/engineer to decide where that should cutout.
« Last Edit: May 25, 2020, 09:55:26 pm by KL27x »
 


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