Understanding schematics of Li ion battery gauge

[libuz]

This is a schematics for Li ion Batterry gauge. https://i.imgur.com/0k8r5XZ.png?2 The current flows from battery to pack when discharging and from pack to battery when charging. I understand that current are controlled by the FET which act as switches but don't understand how they work here. Can the FETs pass current in both direction?

 

[barry]

Q2 and Q4 are mirror images. Current flows from Drain to Source in both devices, but in opposite directions, depending on which device is on (only one is on at a time).

 

[libuz]

Thank you for the reply. But my question was, aren't these FET unidirectional? Can current flow from source to drain? Because whether current flows from Battery to Pack or Pack to Battery, current has to flow through both FETs, isn't it? So, if they allow current to flow in only one direction, then current cannot flow because the FETs are in opposite direction. That is my poor understanding.

 

[erikl]

... my question was, aren't these FET unidirectional? Can current flow from source to drain?

These are MOSFETs, and all MOSFETs - due to their construction - have anti-parallel parasitic diodes, see the upper diode in their symbol's circle. So one of them is always "open" for the counter-direction current flow.

 

[libuz]

So one of them is always "open" for the counter-direction current flow.

That is what I am asking. If one of them is always open, then how does the current flow from batter to pack or pack to battery? Current needs to flow through both of the MOSFETs while charging or discharging.

 

[erikl]

If one of them is always open, then how does the current flow from batter to pack or pack to battery? Current needs to flow through both of the MOSFETs while charging or discharging.

Misunderstanding, sorry: I meant open for current flow, like a closed switch: the anti-parallel diode always allows for current flow in the reverse direction.