Simplest way to disconnect battery if charger is unplugged?

I have a 12V lithium battery that has a smart charger. The battery is always connected to a 5V regulator and if the power goes out, after several days that battery goes to near 0V. I need to inject a circuit between the battery and the other parts to stop the battery drain if it gets below some value ie maybe 9V so that the battery does not approach 0. Any suggestions on an off the shelf or easily hacked up method?

Comments

  • Here's a circuit that should work, albeit with a caveat:

    battery_disconnect.gif

    The supervisor is a voltage detector. Pick one with an open-drain output, active low when the input voltage is over its threshold. Choose the divider resistors so that its Vdd input is at threshold when the battery voltage is at 9V. Make sure that that its output pin can withstand the full battery voltage. Also, pick a device that has a wide hysteresis band and/or a built-in time delay.

    That last item addresses the caveat, in that it helps to prevent chattering: i.e. when the load shuts off, the battery voltage will rebound, causing the load to be reconnected, ad infinitum.

    -Phil
    340 x 233 - 3K
  • You can buy a under voltage module on Amazon for $12 bucks. Has turn off and turn on adjustments
  • DigitalBob wrote:
    You can buy a under voltage module on Amazon for $12 bucks. Has turn off and turn on adjustments
    Sounds great! Can you provide a link?

    -Phil
  • Thanks guys. In this case I'd love to find something that will just plug in off the shelf.
  • That seems perfect! Thanks for the link. Will order it now.
  • The electromechanical relay SRD-12V draws 30mA to keep the circuit alive. Is that okay in your power budget?
    The description assumes you have an SLA battery, not lithium.
    If it is a 3.7V nominal lithium, for best service life I’d want to set the cutoff threshold at no less than 3V per cell, 12V for the pack. That said, I wonder, most Li 12V battery packs already have a circuit built in under the shrink wrap that cuts out the battery at ~2.7V per cell, as well as protection for overcharging and overloading.
  • tritoniumtritonium Posts: 317
    edited 2020-07-24 - 11:08:04
    Hi
    Don't know if this is any use. Used it many years ago to recharge nicads after first discharging.
    The input went to the ni-cad with a 50ma feed to recharge after discharging, and the output to a load resistor-maybe 10 ohms. Press the momentary switch and the discharge started and only stopped when the output voltage fell below the threshold set by the potentiometer. An led across the output (via series resistor) illuminated when discharging.
    I thought it might do what you needed.
    Once it switches off it wont switch on again until the switch is pressed- its a sort of flip flop action.
    You may want to adjust the resistor values for your purpose.
    The circuit is ultra simple and cheap.
    Dave

    Edit- circuit corrected- thanks Yanomani

    895 x 598 - 39K
  • ercoerco Posts: 19,737
    Marginally related:

    I use these cheap Chinese (yeah, I know Ken... bad erco) modules to charge single Lipos and Li-Ions built into projects. They have built-in battery protection to disconnect the battery once voltage drops below a certain voltage, as Sir Tracy advised above. There are seperate BAT and OUT terminals. Very handy, I try to keep a few (AHEM) on hand.

    s-l500%20%282%29.jpg
    500 x 500 - 22K
  • tritonium wrote: »
    Hi
    Don't know if this is any use. Used it many years ago to recharge nicads after first discharging.
    The input went to the ni-cad with a 50ma feed to recharge after discharging, and the output to a load resistor-maybe 10 ohms. Press the momentary switch and the discharge started and only stopped when the output voltage fell below the threshold set by the potentiometer. An led across the output (via series resistor) illuminated when discharging.
    I thought it might do what you needed.
    Once it switches off it wont switch on again until the switch is pressed- its a sort of flip flop action.
    You may want to adjust the resistor values for your purpose.
    The circuit is ultra simple and cheap.
    Dave


    IMHO, there is a typo at the lower left corner of the schematic: (OUT(+VE)) is displayed, in place of (OUT(-VE)) which seems to be the right label.

    Henrique
  • @Yanomani

    Yes thanks for catching that!

    I had to find and download a free circuit drawing program to make an image I was able to present here.
    In my normal life everything these days is drawn by hand for my own use.
    In DOS days I had a cad package for circuits and pcb's that was accurate, fast, easy to use etc etc- but wont run on a windows pc without dosbox.
    This free cad is called tinycad. It works, is reasonably easy to use, but I don't like the rather low res output images it exports as. I had to zoom in and 'snip' the area of screen I wanted using the snip tool in windows. Is there something better for this kind of thing?

    Dave
  • Hi tritonium

    Old eyes (mine) are worse as ever, so I'm furiously relying on Ctrl key + scroll the mouse wheel, in order to zoom in/out almost everything that appears on the screen. :lol:

    I use to depend on freely available, web based Digikey Scheme-It. (link ahead); now at version 3.0, it got me rid of all that paper&pencil-(& rubber)-work (almost, since old habits tend to persist (till grave, at least)).

    Just some few cliks-and-typing, in order to create a free account/login, and you are ready to go.

    Not the ultimate do-it-all solution, but plenty of good stuff, right on your preferred browser (Firefox, in my case). The learning curve isn't that steepy. I'm sure it'll worth your time.

    Hope it could serve you as have been serving me.

    Henrique

    https://digikey.com/en/resources/design-tools/schemeit
  • ExpressPCB has a pretty good free package that has a good schematic section.
    Download the "Classic" version.
  • Dittoing Expresspcb free cad. Simple and easy to learn. Downside closed source and proprietary, but it works well for what it does and the price is right.
  • Hi
    Thanks all for your suggestions.
    Not keen on browser apps.

    Tried to install expressPCB
    They wanted my email- I gave it them- they emailed me with 'press to download button'- did that-downloaded- ran and..... got the attached result.
    So
    I sent them an email with screen shot and am awaiting a response.
    The file is where its expected and there are no restrictions on it from the details in explorer.
    Ho Hum
    Dave
    503 x 384 - 41K
  • I tried out several modules. One was kind my ed above. I saw one with readout which was neat. On the non display it drops out by default at 10. At 12.4 it draws 37mA. When the really drops at 10 it draws 4.6mA. 2mA at 5. 1.18mA at 2.5v. On the display version the draw is 3x. 105 at 12. 20 at 5. 8 at 2.5.

    So the drain never stops. I see a value in reducing the load below 10 and slowing the decay, but these don’t save the battery draining down. I did this type project once, it was just a 12v relay! It dropped out at some lover value below the turn on value. ie off at 5. On at 9 Just guessing but that’s the gist. The power from the battery to the relay gets cut when the relay drops out. When the charger gets AC again and brings battery back up it can turn the relay back on. So it’s automatic. Zero batt drain when the relay drops.
  • YanomaniYanomani Posts: 1,005
    edited 2020-07-26 - 02:36:46
    Hi @tritonium

    Now, even .msi files are facing problems when one tries to install them. Has MS kicked its own Dragon Ball Z?

    Sometimes, my W8.1 Pro system don't let me install some freshly downloaded .msi stuff, such as Libre Office updates, even when logged at an account with administrative privileges.

    There is a fix to that behaviour I'd found way ago, but it's burried inside tons of saved links, and it's in portuguese, which is a no-go for most of you, but, the trick can be "shortly" described as:

    - After downloading, bee sure to where you did saved the .msi package (I have a specific folder at Root C:, where I put anything I want to install in my system (and it's not the Download folder, which is user-specific);

    (W8.1 Pro procedure, other flavors could vary a bit, but the basic idea still remains)

    - Logged to an account with administrative privileges, CTRL+ALT+DEL and entrer the Task Manager;

    - in Processes, find Windows Explorer: right click, End Task;

    - your desktop will almost vanish, in the background; don't give it a damm;

    - File menu, Execute New Task:

    - In the pop-up, type EXPLORER.EXE, check the box "Create Task with Administrative Privileges", then click "OK";

    - your desktop will magically return, in the background (now, with Windows Explorer running with Administrative Rights, as it have ever been to be running, when logged at an account with administrative privileges: DOOHH);

    - Exit Task Manager;

    - Right click bottom left screen Windows Logo, then RUN, at the pop-up;

    - In the "Open" BOX, browse and find the .msi file you downloaded and click, to fill the box with the appropriate path and name;

    - be ensured that "This Task will run with Administrative Privileges" is properly activated, then hit "OK";

    - step thru all the items within the installation process;

    - be happy. :lol:

    Hope it helps

    Henrique

    P.S.: I always logoff and reboot/restart my system, after any install process has been executed, either succesful, or not.

    Hint: I EVER use CCleaner (up-to-date) after installing and restarting, just in case the install proccess did some mess/mistake, or left traces of any kind, before running the just-installed package for the first time.
  • Hi Yanomani

    Thanks for going to the trouble of typing those instructions.
    Before trying I will await instructions from the PCBexpress people whom I have emailed.
    I wonder if they are suddenly getting a lot of complaints?

    Dave
  • A followup on Phil's suggestion about using a supervisor IC to establish the threshold... A more versatile chip for that is something like the LT6703-x comparator+reference (SOT23-5, quiescent current 6.5µA). You haven't said how much power the load draws, but a mosfet load switch such a the FDC6324 can handle 1 amp (SOT-6). The easiest arrangement would have the threshold detector on the charger side, but with a bypass pushbutton or bit of clever circuitry, the threshold detector could go on the load side and achieve the zero power goal. Watch out for back current into the charger when it is ostensibly off.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 22,691
    edited 2020-07-27 - 17:59:03
    The easiest arrangement would have the threshold detector on the charger side, but with a bypass pushbutton or bit of clever circuitry, the threshold detector could go on the load side and achieve the zero power goal.
    After I saw Tritonium's post above, I realized that's what I should've done. That also take s care of the "chattering" issue I alluded to.
    Watch out for back current into the charger when it is ostensibly off.
    Might the reverse-current issue be solved by using two pMOSFETs back-to-back in series, with common gating? That way the body diodes would be in opposition.

    -Phil
  • Tracy AllenTracy Allen Posts: 6,504
    edited 2020-07-28 - 00:28:36
    The ICs that are packaged under the shrink wrap for LiPo, LiIon, and LiFe cells and packs usually do have FETs back to back, to forestall reverse leakage and overcharging as well as to cut off power to the load before the battery drops below the point of no return. There is also a poly fuse to protect against over-current. Those are dumb dedicated circuits.

    On the other end are the smart battery charger ICs that have connections for multiple external FETs and a plethora of internal registers, thereby to program what you can hope are the best cutoff voltages, currents and joules for the battery chemistry and application.
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