18650 polarity protection that can also charge? - Page 1
Forum section: electronics / projects

author: sebmadgwick
author_rank: Regular Contributor
timestamp: November 06, 2017, 02:02:37 pm
content: I am designing a device to use a removable unprotected 18650 cell. There is a risk that the user inserts the cell at an incorrect orientation and so polarity protection is required. The device can charge the cell (max 1 A) so the polarity protection must be compatible with this.I am struggling to think how this can be achieved. For example, aMOSFETwould protect the device from the battery but it would not protect the battery from the charging device.Any suggestions?

author: ebclr
author_rank: Super Contributor
timestamp: November 06, 2017, 02:16:02 pm
content: Charger onlyhttps://www.ebay.com/itm/USB-Li-Po-18650-5V-Lithium-Battery-Protection-Boost-Charger-Board-For-Power-Bank/232396843120?epid=20004597947&hash=item361bee6870:g:coYAAOSwPK1ZW2Iihttps://datasheet.lcsc.com/szlcsc/TP5400_C24154.pdf

author: sebmadgwick
author_rank: Regular Contributor
timestamp: November 06, 2017, 03:08:58 pm
content: The part you have linked does not feature polarity protection. I would like to protect from the cell being inserted with the wrong polarity.

author: Nusa
author_rank: Super Contributor
timestamp: November 06, 2017, 04:49:37 pm
content: If you specify 18650's with button tops, you can solve the problem mechanically. Simply make it so the positive contact on the device is in a depression that cannot touch the battery if there is no button.

author: maleetronic
author_rank: Newbie
timestamp: November 06, 2017, 10:45:41 pm
content: What if you use the DFBR030U3LP from Diodes? You always have the right polarity, even if you swap the input pins. This part has a very small voltage drop, you have to consider this when charging the cell.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: November 06, 2017, 11:55:48 pm
content: The DFBR030U3LP is an interesting device. It would not allow the battery to charge though. Perhaps there is not electrical solution and I must either use protected cells or button-top cells and implement mechanical polarity.

author: forrestc
author_rank: Unknown
timestamp: November 07, 2017, 01:09:15 pm
content: Quote from: sebmadgwick on November 06, 2017, 02:02:37 pmOne option:Separate your protection for the non-charging electronics from the protection for the charging circuitry, and use a charge controller which can handle reverse polarity correctly.Another option:Using back to back fets, you should be able to arrange a circuit which only turns on the fets when voltage is available on the battery side. The back to back fets will act as a switch for both directions. The control of the fets will need to be some other circuit (microcontroller, etc).A combination of the above:If you have a microcontroller in circuit already, separate as per the first suggestion, and then use the microcontroller to enable a pair of fets to connect your charging circuitry if a battery is detected.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: November 07, 2017, 03:47:37 pm
content: Quote from: forrestc on November 07, 2017, 01:09:15 pmThe charger part of the device is already designed and cannot be modified. I am hoping to create a polarity protection circuit that sits between the device and the battery.Quote from: forrestc on November 07, 2017, 01:09:15 pmI have considered some kind of back-to-back FET arrangement but how would these been controlled? Any circuit I think off will fail when the charger is active. When the charger is active, it will apply a voltage across the battery and so it will appear that the battery is at the correct orientation.

author: forrestc
author_rank: Unknown
timestamp: November 08, 2017, 07:58:40 am
content: Quote from: sebmadgwick on November 07, 2017, 03:47:37 pmWithout knowing the operation of your charger, it's hard to suggest a specific solution.At the bare minimum, using a circuit which will read the battery voltage (using something like a 8 pin microchip PIC, or maybe an analog equivalent with an opamp or two), and only enabling the fet once it detects voltage of the proper orientation would go a long ways. This would ensure that the fet is not turned on without a battery in place.Once the fet is on then yes, if the charger provides a continual voltage then you might have a problem. You may want to check to see if this is the case. Many chargers do not provide a continous voltage, especially since most chargers do not float lithium ion batteries. If this is the case, then your circuit can detect the battery loss and reset itself.One other thought is that as long as you're measuring battery terminal voltage, you might find that inserting a battery the wrong way will result in a dip below the threshold, and permit your circuit to reset as well.