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Bidirectional FET to switch on/off single cell Li-Ion battery

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p75052

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I need to come up with a circuit that sits between the single cell Li-Ion charger and the battery so the uC can cut the battery off if the voltage gets lower than 3.0V. I heard of the bidirectional FET (since the battery will be charged as well as discharged) but could not find any reference design. Any help would be appreciated. Please see attached picture for details.
 

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Why not just have the load connected to the battery constently then have a simple low voltage monitor that does the job of disconnecting the battery. This would be much simpler as it would eliminate the need for a bi-directional FET. All you would need is a single power transistor of some sort to disconnect the battery. For a monitor, try looking at the ICL7665.
 
For Li-Ion battery, you should not connect the load directly to the battery as the battery in this case won't get charged properly, thus shorten the life of it.
 
That sounds a lot like an issue with the charger not the battery. If it is a good charger it should be able to deliver more than enough current to charge the battery and power the load at the same time.
 
The charger or the battery work fine. I just need to find a way to isolate the battery when the voltage is too low.
 
I'm still convinced that it's an issue with the charger itself but I'll entertain the thought that it is working fine. The easiest way to have the battery disconnect from the load would be a NO relay that simply trips at a voltage because there is not enough to keep the coil energized. That would still create the problem though that it is using power just to keep itself closed, and as soon as the battery is set to charge again, it would re-close which, as you said, is not wanted. The next best way to do this is to have a comparator to monitor the voltage and trip at a voltage that is too low. I already gave you a part number, the ICL7665 is good for 1.6V up to 18V so it can easily handle the job. As for the FET that you want, I am unaware of any FETs that are bi-directional. You can make a bi-directional FET switch by simply connecting two FETs in opposite polarity of each other. Try to avoid using a power MOSFET though since most of them have an internal diode to protect them from high voltage spikes. This diode would create a short across the opposite FET, and your switch would be completely useless.
 
OK, thank you Dragon, I think I came up with something - two P-FETs with the drain and source tied together. Two sources will connect to charger and battery respectively. I will ask a friend to run SPICE to confirm.
 
Your circuit has the load connected to the charger but not to the battery. Is that intended?
 
Yes, that way the battery charger can manage the battery and the load independently. In case the main source power is not enough to provide power to the load, the battery will supplement the power.
 
In case the main source power is not enough to provide power to the load, the battery will supplement the power.
Does that mean the charger will accept power from the battery and route it to the load? I've not come across one of those. Which charger are you using?
 
Interesting. Thanks for the links.
 
Hi,

Most MOSFET switches have their sources tied together.
 
I wish I could. I looked at the list of article titles and nothing stood out. My issues are scattered and a few CD's of the issues are misplaced. It would take a huge effort to find it. It would be in an issue from 2009 to 2011. It doesn't appear to be 2010. I'll have to look for my 2009 CD.
 
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