Single 18650 cell polarity protection circuit that allows charging

Thread Starter

SergioS

Joined Apr 8, 2019
8
I have a portable device that's powered with a single 18650 cell, mounted in a Keystone 1043 holder, and draws about 1-2A at most (in short bursts) The battery is charged with a built-in USB charger. Since the battery is removable, and the holder is symmetrical, I want to protect the circuit against reverse polarity. My question/doubt is - if I use a simple N-channel MOSFET, for example, the SI2312CDS-T1-GE3 from Vishay, which has a low G-S threshold, and connect it like on the attached schematics - will it create any issues with the charging of the cell? Am I wrong to assume that if the gate voltage of an N-Mosfet is greater than the source, the channel will be open and allow both normal current flow and charging?
 

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KeithWalker

Joined Jul 10, 2017
2,034
I have a portable device that's powered with a single 18650 cell, mounted in a Keystone 1043 holder, and draws about 1-2A at most (in short bursts) The battery is charged with a built-in USB charger. Since the battery is removable, and the holder is symmetrical, I want to protect the circuit against reverse polarity. My question/doubt is - if I use a simple N-channel MOSFET, for example, the SI2312CDS-T1-GE3 from Vishay, which has a low G-S threshold, and connect it like on the attached schematics - will it create any issues with the charging of the cell? Am I wrong to assume that if the gate voltage of an N-Mosfet is greater than the source, the channel will be open and allow both normal current flow and charging?
That circuit will not protect the load from a reversed polarity battery. The reverse current will flow through the protection diode built into the MOS FET.
 

Thread Starter

SergioS

Joined Apr 8, 2019
8
That circuit will not protect the load from a reversed polarity battery. The reverse current will flow through the protection diode built into the MOS FET.
The schematic shown was taken from here - https://www.electronicdesign.com/power/reverse-polarity-protection-automotive-design,
do you think that it's wrong? It seems that the diode is biased towards normal operation. Or did you mean during charging? As a load, the circuit should be protected, but as a charger...? I think that if the battery is inserted correctly the charging will work, but if it's inserted incorrectly there will be a current surge when the charger is active. I am looking for a simple and economical solution that would allow (and protect) both load and charging modes for a single 18650 cell.
 

KeithWalker

Joined Jul 10, 2017
2,034
The schematic shown was taken from here - https://www.electronicdesign.com/power/reverse-polarity-protection-automotive-design,
do you think that it's wrong? It seems that the diode is biased towards normal operation. Or did you mean during charging? As a load, the circuit should be protected, but as a charger...? I think that if the battery is inserted correctly the charging will work, but if it's inserted incorrectly there will be a current surge when the charger is active. I am looking for a simple and economical solution that would allow (and protect) both load and charging modes for a single 18650 cell.
You are correct. The load will be protected but It depends on the charger. Most will be damaged if the battery is connected in reverse.
 

Thread Starter

SergioS

Joined Apr 8, 2019
8
You are correct. The load will be protected but It depends on the charger. Most will be damaged if the battery is connected in reverse.
Thanks! After some searching, I think I will try out the following circuit (LTC 1998 is just for battery monitoring) The idea was taken from here: https://www.edn.com/design/power-ma...verse-voltage-protection-for-battery-chargers. At least I couldn't come up with a simpler solution. I thought about using a comparator at the battery, but two P-FETs seem like a simpler solution with fewer parts. Anyhow - any thoughts would be greatly appreciated!
 

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