Turn Off Those Batteries With Their Protection Chip

It should be a feature of every device powered by a lithium-ion battery, that it has a protection chip on board that automatically disconnects it should it go out of its safe voltage range. A chip most often used for this purpose in single-cell applications is the Fortune Semiconductor DW01, and [Oliver] shares a tip for using this chip to power down the battery. The DW01 has a CS, or current sense pin, which if taken high momentarily will put the chip into an off state until the battery is disconnected.

Looking at the DW01 datasheet we can see that this would work, but we can’t help having a few questions. The CS pin is a safety sensor pin, providing over current, short circuit, and reverse polarity detection. It’s the kind of pin one might mess with only when one is absolutely certain it’s not likely to trigger a dangerous fault condition, so a bit of care should be required. However, we can see that leaving its resistor in place and supplying it a momentary logic level through another resistor should work. We’d be interested in the views of any readers with more experience in the world of lithium battery protection on this hack.

Meanwhile, a good read for any reader should be our look last year at lithium-ion safety.

22 thoughts on “Turn Off Those Batteries With Their Protection Chip

  1. There should be a physical switch to disconnect battery when a device is stored for a long time. I’ve modified some devices like this, and batteries in them is still working great after 10 years or more. While other devices have greatly degraded batteries because they are over discharged each time you leave them stored too long.

    1. I have found that the high quality memory backup Lithium Ion batteries are not subject to the issues of leakage that other batteries are.

      However, not all memory backup batteries are this type of battery. I have seen many devices where the manufacturers try to save a few bucks by using alkaline or even acid-electrolyte battery types instead. These batteries are far more prone to leak and damage the holding board.

      Additionally, sometimes you need to hard-reset a battery backed device, especially to wipe it’s memory. In this case forcing the power-down circuit then resetting said circuit can fix this issue.

  2. Soooo, it must be something like this that kicks in when an old Palm Tungsten E2 quits working, until one opens it up, disconnects and reconnects the battery, then reassembles it.

    Then it will usually stay working unless the battery is allowed to go below ~10% or so. Then you repeat the process until you get fed up with it and replace the battery with a new one of slightly higher capacity. They make such good MP3 players. When released in 2005, one reviewer gave up on waiting for it to die after 12 hours of MP3 playing (with the screen off). The aftermarket battery just made it even better. ;) Got any other player? You just wish it had that kind of runtime.

    1. I’ve got two tiny iPod Shuffle v4’s. Apple advertised 15 hours, but at least one reviewer saw 16 hours. I don’t tend to listen that long at a single sitting (use them for podcasts while running), so I’m not sure what the battery life of mine is now.

    2. 2 used Nintendo DS Lite LiPo cells in parallel, taped to a Sony Walkman NW-E003F, ran for ten days straight the last time I checked before I gave it away. The original cell was dinky, like half of a stick of chewing gum.

  3. One might be able to do the switching through a 100 nF capacitor. That way there should be less risk of messing with the over current protection over longer time intervals, as the capacitor would only allow a short pulse to go through.

  4. It seems to me that pulling the CS pin high will cause some current to flow through the DW10 chip, defeating the purpose of turning the chip off. It will disconnect the battery negative terminal from the rest of the circuit, but the chip will continue to draw power and never enter the low quiescent current mode because you have to keep pulling the CS pin high to keep it from resetting the current protection after a while.

    1. Are you sure it works that way? Sounds like it just takes a single pulse and it cuts the circuit until you disconnect and reconnect the battery. Is that not correct?

      1. Both are correct… When the DW01 cuts power to your circuit, the CS pin would *normally* go high, and if your circuit drives it high, it ends up at the same logic level it would normally be in when the load is turned off.

  5. When I go to charge the batteries in my DeWalt 20v drill, sometimes the charger won’t charge them or will only charge one of them. I saw a neat video on youtube where you can jumper across the terminals to momentarily bring the low battery up to a voltage that will allow it to accept a charge. Works great! Of course I did this in the middle of a corn field in a building made out of concrete and bricks while wearing a face shield, fire department on stand-by, and with plenty of ABC rated fire extinguishers around. So far my drill keeps working great and has survived numerous explosions.

    1. There’s a similar trick with a USB lead for single-cell 3.7V lithium cells. You just stick 5V onto the battery for a few seconds. If the battery has shut itself down from low-voltage protection this usually lets it reconnect and work in a proper charger. Done it a couple of times.

      1. I’ve also used USB chargers to charge lithium batteries in emergencies. One occasion was a set walkie-talkies with 7.4V batteries, this needed two chargers in series. I connected them to the probes of a multimeter and used the probes to connect to the battery. That way I could monitor the battery voltage as it charged; the charger output would “kneel” a little when the current was high and combined with the drop in thin wires it worked surprisingly well.

        Not ideal at all but got us through a couple of weeks of work in a remote area in Africa after the radio charger had crapped out. The biggest issue was that I had to sit there holding the probes to the terminals with both hands. If there was nothing on TV (or the TV didn’t work) it made for a very boring evening.

  6. I’d be accessing that pin via a diode, (Schottky if a lower voltage drop is necessary), or using MOSFETs, a relay, or some other means to ensure that there is no additional current path to that pin during normal use. I don’t trust that 5-to-1 resistor ratio – when ‘Your Project’ isn’t attempting to modify the DW01’s behaviour, the DW01 shouldn’t even be aware of the existence of ‘Your Project’.

    1. There are two possibilities for your project modifying behaviour.

      If it’s injecting current, the DW01 will turn off more easily. That isn’t a safety concern.

      If it’s taking current (ie. your circuit is driving it’s output pin to 0v or below), it prevents the circuit turning off.

      The 5-1 ratio means the current limit and the short-circuit current are increased by 20%. Thats a potential concern, although component variation in the mosfet Rdson is far more than 20%.

      BUT WAIT… The GND of the thip and the GND of your circuit aren’t connected. In fact, if you had no resistor at all, and pulled the logic line directly to your projects ground line, the DW01 would still turn off at the correct current.

      The only way you could do something unsafe is to take the signal line below zero – ie. have some kind of inductor and switching circuit to deliberately pull current from this signal pin. Thats pretty hard to do by accident.

      Why have the resistor there at all then? Well you could still put a capacitor in your circuit to Vcc, which would prevent the signal line rising as quickly in the case of a sudden overload. the 5k resistor means you can only affect the dynamic response by 20%.

  7. It’s something i want to experiment more with as well. With the common TP4056 (DW01) boards i sometimes had the issue that it won’t turn until i give it a little charge or reconnect the battery. I might be wrong, but my guess would be that the inrush current of the device behind it is too high so the voltage drops and the DW01 triggers.

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