Simplest way to disconnect battery if charger is unplugged?
T Chap
Posts: 4,223
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
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
-Phil
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.
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
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.
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
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
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.
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
Download the "Classic" version.
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
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.
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.
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.
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
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
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.