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| 18650 series discharge protection |
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| BravoV:
--- Quote from: alxptt on May 01, 2018, 03:48:41 am ---Well, for one thing, I want to be able to understand how the components work. Simplifying my design as much as possible makes that more feasible, since I'm a beginner. Just slapping a generic module on might work, but honestly I'd rather use something I actually understand rather than a black box. --- End quote --- As many recommendations, these are not blackbox, all you need is to read their datasheet to understand how it works. As you admitted your self you're a beginner, not to rain your parade, I doubt you can improve much the current design, price wise and size wise. Most sold are rounded to be placed on top of battery, or separated board like this below. Just look at the size compared to a TO-220 sized body, imo, its already quite small. |
| alxptt:
@nathanpc Wow, thank you for all that detail! That's fantastic and I really appreciate it! :) Totally addressed all the initial operation questions I had. I'll definitely be playing with this project as op amp practice. One question though -- since there is a 25k combined resistance in the resistor divider... it will have 0.24 mA of quiescent current draw, right? By my math, if I had a 3Ah cell, it would drain it in about 500 days. Is this correct? I am wondering (not for this project, but in the long term) if there is any way of creating a discharge protection that doesn't have a quiescent current draw, e.g. for an emergency system that's charged once and then left in my emergency backpack, or somesuch. How difficult would that be? (I'm assuming here that the Li Ion cells themselves don't have any significant self-discharge rate, since I'm told by the 18650 people that they don't. let me know if this is wrong!) |
| KL27x:
Easy way to do this is use something like 3 pin voltage detector which are specifically made for something like this. There is in/power pin, ground pin, and output pin (which you can get in open collector or push/pull logic), and they come in voltage set point in every increment between 2.5-3.3ish V and more, to cover the cutoff voltage you are interested in using, drawing a gnat fart of current. Take the output and connect that to the gate of a logic level Nfet. Connect the source to the ground terminal of the battery, and seal that over. The drain is now the negative terminal of the protected battery. You can put these cells in series and each cell is individually protected. The battery will cut out when the weakest cell hits the limit. But you have to deal with the hysteresis or bounce with your main circuit, somehow. This is two tiny 3 pin components. You technically don't need anything else but maybe a pulldown resistor on the FET gate. 3 components. Add pullup on the output pin of the voltage detector if you use the open collector variant. 4 components, max. |
| alxptt:
--- Quote from: KL27x on May 11, 2018, 10:17:26 pm ---Easy way to do this is use something like 3 pin voltage detector which are specifically made for something like this. --- End quote --- What type of voltage detector chip families should I be looking at here? --- Quote from: KL27x on May 11, 2018, 10:17:26 pm ---But you have to deal with the hysteresis or bounce with your main circuit, somehow. --- End quote --- What do you mean, bounce? --- Quote from: KL27x on May 11, 2018, 10:17:26 pm ---gnat fart of current --- End quote --- That is my favoritest thing ever and I'm stealing it now. :D |
| KL27x:
--- Quote ---What type of voltage detector chip families should I be looking at here? --- End quote --- I don't care about brands, but the one I have used is Microchip TC54VC/N xxxx. The VC is logic output. The VN is the open drain. I didn't care too much about cost, so I didn't look at a lot of other options. I just happened to find it one day while looking under "battery management" IC's on the Microchip website. Read the datasheet and as far as I can tell you would need to spend a lot of time and space and components cost/count to make something to compete with this and still end up drawing more current. As you can imagine, you are not the first to identify this application. There are many companies who have developed products, such as this, specifically to tackle the problem you are describing. --- Quote ---That is my favoritest thing ever and I'm stealing it now. :D --- End quote --- This is a common phrase around here. You would not be stealing it from me. :) --- Quote ---What do you mean, bounce? --- End quote --- The cell will rebound and the voltage will rise when the FET switches off. So this protects the cell from damage. It doesn't protect your device from browning out and turning back on/off over and over. |
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