Author Topic: 3.7 mod battery cut off (Read 5373 times)

methos · « **on:** January 27, 2014, 09:52:28 AM »

I am building a 3.7 mod for the wife and I want to put some kind of battery over discharge protection in it. I am using a lipo cell and I would like shut it down around 3 volts to keep from killing the battery. could this be done with a simple diode or would it take more components? TIA

kortt · « **Reply #1 on:** January 27, 2014, 03:07:08 PM »

I'd be interested in learning how this could be done as well.

CraigHB · « **Reply #2 on:** January 29, 2014, 04:43:48 AM »

Easiest way to do it is with a MOSFET and a voltage detector. Just need one resistor so three components. It's getting late here so I'm not up for drawing a schematic right now, later if you want to see it.

It would be possible to use a zener instead of a voltage detector, but a detector will work a lot better and it's only 3 pins to wire.

methos · « **Reply #3 on:** January 29, 2014, 06:12:03 AM »

Thanks Craig. When you have time a schematic would be great. Gonna be ordering some parts soon so I could grab all the parts for it.

CraigHB · « **Reply #4 on:** January 29, 2014, 04:08:10 PM »

Here you go;

CraigHB · « **Reply #5 on:** January 29, 2014, 05:06:36 PM »

The detector brand is On Semi and the MOSFET brand is Vishay Siliconix. Any P-channel MOSFET with a low on resistance and low threshold voltage is fine. That one is a new part so the availablity is not that great on it, but you could order free samples from Vishay if you want.

The way that detector works is the reset pin is pulled down to ground when voltage on the input pin is above 3V. The reset pin is pulled up to input voltage when voltage on the input pin is below 3V.

The power consumption on that detector is very low. It draws about 150nA through the input pin. That's plenty low enough. It would take quite some time for an over-discharge to occur. 1 mAh will power the detector for about 9 months. When a LiPo has been discharged to 3V it has quite a bit more than that left before any damage occurs due to over-discharge.

The trigger switch on the reset input allows the MOSFET to do double duty and elimates the idle draw that would be present from the 10k resistor if the trigger switch was not there. Current draw is the essentially the same through the detector input pin regardless of state.

The 1u cap provides some glitch reduction for the detector so it doesn't react to spurious voltage spikes. It should be the MLCC type and should be located as close to the input pin as possible. It can be left out if preferred, but it's good practice to put one on there.

methos · « **Reply #6 on:** January 29, 2014, 06:23:09 PM »

Thanks Craig!!! Exactly what I was looking for. I will have to dust off eagle and wip up a small board for this.

CraigHB · « **Reply #7 on:** January 29, 2014, 06:32:28 PM »

Shouldn'be too hard, the detector is an SOT package and the MOSFET is SOIC package, those are relatively large packages and easy to solder by hand.

BTW, I edited the crap out that post since I was thinking about stuff after I initially made it. So, make sure you viewed the latest version and refresh your browser window to get the latest drawing. Sorry about that. Happens a lot, I'll make a post then keep thinking about it and want to change something. I was unusually bad about this one.