I have a LiPo battery protection subcircuit in my project using a DW01A protection IC and a FS8205A mosfet.
Normally, the battery positive goes through a 100ohms resistor, bypasses a capacitor and enters the protection IC on pin 5. Once the voltage of the battery drops below 2.4V, the ground connection of the battery gets shut off by the mosfet and the battery cannot be discharged any further.
I found 2.4 volt much too low and tried to include a voltage divider to adjust the threshold where the mosfet shuts off the battery connection.
Since I wanted it to be shut off at 3.6V instead of 2.4V, I used a 360k as R1 and 680k as R2.
So: if the real battery voltage goes down to 3.6, the divider outputs 2.4V and the battery gets shut off.
It was pefect in theory but once the PCB arrived, it did not work at all.
Even if I put in much higher voltages so the output of the divider is 4.0V, the battery is shut off.
I then rebuilt the protection circuit on a breadboard, and measured at many different points.
The circuit works perfectly fine if the battery positive only goes through the 100ohms resistor.
The voltage divider also works perfectly fine, putting out the voltages just as intended.
But if I switch the input of the protection IC from the normal connected to the output of the voltage divider, the battery is not put on.
I definitely have the right voltage output of the voltage divider that the protection IC should put on the battery but it does not give me the results I want.
Does anyone have a clue what I did wrong here?
I don't know if the DW01 works that way. And I've never heard of anyone being able to adjust the overdischarge voltage to a higher value. But looking at the datasheet, it seems more likely that just increasing the value of R24, without any divider, might raise the overdischarge voltage, as well as the overvoltage level, unfortunately.
Typically I think people just use a different protection chip that triggers at 2.9V or 3V, such as the FS312F-G. But be aware that the release voltage would still be about 3V, so the FS312F-G would be more likely to oscillate than the DW01. It would shut off at 2.9V under load, but then the voltage with no load could easily go back above 3V.
I will solder a 1k resistor tomorrow and test what happens. But what would be the background of how that would work? The voltage entering the IC would be the same, wouldnt it?
A LiPo batterys state of charge is at 0% at 3.3V. 2.4 would be too far below that I thought.
Furthermore, many tests show that LiPo batteries last much longer if you only use them between 20% and 80% of charge, so never emptying them completely.
That is a completely different approach than relying on an emergency discharge cutoff, which is what the battery protection IC provides.
The relationship between battery state of charge and voltage varies with many factors, such as temperature, age and discharge current. To monitor that, you need a "fuel gauge" chip.
Having looked at so many projects and having watched so many youtube videos of battery measuring with an MCU during the last months, this is the first time I am reading about fuel gauge ICs.
First of all, thank you, this might be very useful for my future projects, that will probably not be used with a voltage divider anymore. I am really wondering why everyone is using voltage dividers if there is something much better.
Can you maybe recommend a specific IC that is available on the typical sites like lcsc and can be eaily connected to an MCU for example with i2c?
Still, I am wondering why my modification above does not work. It should adjust the cutoff voltage but it does not
They are actual coulomb counters. That term has appeared here many times.
Check the battery's data sheet to confirm this: The usable voltage range for a standard LiPo battery cell is 3.2v to 4.2v. Any lower than 3.2v and the battery may be permanently damaged. Any higher than 4.2v and you significantly increase the risk of a battery bursting into flames. Read the data sheet to be sure.
That value is a matter of opinion, and far from universally agreed upon.
LiPo battery management is still in the realm where professional design skills are required, primarily for commercial applications. For amateurs, lead acid, NiCD or NiMH batteries are vastly superior, for several compelling reasons.
I was thinking more like adding a 22R resistor in series to bring it up to 122R in total. This is just based on the Functional Block Diagram in the datasheet. It shows resistor dividers inside the chip to feed the opamps. So adding a little more to the external 100R resistor might lower the voltage at the non-inverting inputs to the opamps. Maybe. But as I said before, I've never seen a report of this kind of adjustment being successful. So the circuit may be too complex for it to work.
An alternative would be to measure the battery voltage at an MCU analog pin, and do some kind of shutdown when it goes below the level you like. If you need a divider to do that, this circuit can do that without wasting hardly any current at all.
I think @jremington is right that you really don't ever want any of the protection levels to actually trigger. On the other hand, there must be hundreds of millions of lithium batteries out there that are protected by 2.4V DW01 chips, and they apparently do ok. So maybe going that low isn't really a problem.
Protection voltage of the DW01A is in most cases 3.0volt, not 2.4volt.
When a battery is almost flat, it's internal resistance increases. That causes the voltage to drop lower than what the battery voltage actually is when a load is switched on.
The DW01A switches off at 2.4volt, but battery voltage bounces back.
It doesn't switch on again unless it bounces back above 3volt.
So the battery drops to 3volt worst case, which is not a big problem.
Leo..
This is embarassing, I did not notice the voltage divider inside the IC. I will try it out today!
I actually built different switchable dividers in the past with n channel and p channel aswell as a combined mosfet. It did work fine. In my last project however I built a divider with two 1MOhms resistors and a capacitor. It drains only 2.5 μA so a 1000mA battery would take 45 years to drain from the divider and I would save space on the PCB and a pin on the MCU.
The issue why I am so interested in the fuel gauge is, because, if I understand correctly, it compensates for the spikes while draining the battery and factors such as temperature while I could use the i2c lines my display uses. My voltage divider can't do that without further circuitry.
It would require a quite big shunt resistor though.
I know, but in my project I work with motors and the battery would drain completely quite often because it is intended to not be monitored very often. So I thought about designing the project so the protection IC cuts off the battery quite regularly, while it is intended to do so only in a emergency as I understood now.
But I noticed that the best solution is, as you said, measure the state of charge and turn off the whole device with the MCU.
So I am not planning to use the protection IC to shut off the device anymore, but I still want to get it to work because of curiosity.
Oh I see, that makes a lot of sense. This is what ShermanP meant by warning about the other IC here:
Well if you get a chance to experiment with increasing the series resistance going into Vcc, I hope you will test all of the related voltage levels - not just the trip points for overdischarge and overvoltage, but also the release points. Even if you get overdischarge to be higher than 2.4V, I suspect the effect on the other levels will not be to your liking. But that's just a guess. Anyway, a lot of people have a problem with the 2.4V, so if there's a way to raise it, that would be good to know.
Edit: I've seen the FS312F-G recommended a good bit, but I've always had my doubts. There just isn't enough hysteresis between the shutoff point and the release point, so if there's any load to speak of, it could oscillate like crazy. It wouldn't oscillate fast, but I think it might keep going a long time. Even the DW01 will turn back on under certain circumstances, but it usually only does that a few times before the battery doesn't have enough left to get it back to 3V. Well, basically it depends on the load.
so it did not make any difference changing the resistor from 100ohms to 220 ohms, unfortunately.
It makes me even more curious. There must be a way to manipulate that threshold. Now where I'm writing this, I might want to use a very small voltage divider at the VCC input, taking maybe only 0.1V off. Or are there any other ideas how to make this work?
So apparently the Functional Block Diagram bears little resemblance to the actual circuit.
The XB7608A is another version of a protection chip which includes the mosfet pair inside the chip, so there are only three pins. You might look at the datasheet and see if anything there helps you. But it calls for the same 1K resistor into VDD, and the functional diagram has the same opamp setup for overdischarge. As with the DW01, there's no indication of how the innards are actually powered. Still 2.4V, unfortunately.
What I was just thinking:
The quitsense of the DW01A is only 3μA.
So either the internal resistors are in the 100k range, or it only takes a measurment every so seconds to have such a low power draw.
So replacing the 100 Ohms resistor with a 220 might not have a big enough impact to influence the beviour, maybe I went from 100100 (100.1k) ohm to 100220 (100.2k) ohm
I might try it again tomorrow with a 10k and 100k resistor to see if it will behave differently
If those two dividers shown in the diagram actually exist, you could get an idea of what they might be by using your meter to measure the resistance between the VDD and GND pins. There are two dividers shown, so the total resistance in one of them would be roughly twice what you measure. Or at least that would be a minimum for what it could be. The other path to ground would be through all the other circuits of the chip, so you might get a low resistance measurement that wouldn't tell you much.
So it looks like the external resistor is only there as part of the bypass filter, and you're right that it would have to be a lot higher to affect the trigger points.
Why are you actually doing this.
That battery protection is a safety net, in case the charger fails or the load doesn't switch off in time. You should have your 20%-80% charge levels controlled by the charger and the load, not with the battery protection circuit.
Leo..
