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Author Topic: Under voltage cutoff / low battery disconnect  (Read 787 times)
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zac
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« on: September 12, 2022, 12:40:21 00:40 »

I'm putting together a backup power supply to run a cell phone repeater, and some small electronics (cell phones powered by usb/5V, 12V for a bluetooth adapter and long range cordless phone, and 18/19V for a wifi extender and the cell phone repeater).  I'm using some surplus [email protected] LiFePo4 batteries.  I looked for an off-the-shelf low voltage cutout relay to prevent the batteries from over discharge, but I couldn't find any that didn't use an electromechanical relay that consumes a couple watts in the shutoff mode.  They all energize the relay when input voltage drops below the set threshold which disconnects the battery.  This would continue to discharge the battery at about 0.1A (at 21V).  I didn't find any with a mechanically latching relay that does not consume power to maintain state.   I could put together a microcontroller with a few lines of c code to provide the desired logic, but that seems like overkill.

I just need the device to provide a pulse to turn off a latching relay when the battery voltage drops below the 95% depth-of-discharge threshold of 21.0V and to provide pulse (on a different line) to turn on the latching relay when the battery voltage goes above a reasonable threshold (maybe 23.5V) indicating the battery has sufficiently recharged to power the load.  Anyone know of an off-the-shelf device or simple circuit to do this without resorting to a microcontroller?  

Convenient latching relay in package with leads:

https://www.functionaldevices.com/downloads/datasheets/RIBL24B.pdf

Posted on: September 12, 2022, 12:31:07 00:31 - Automerged

Some inexpensive off the shelf parts I found for this project:

DC-DC buck converters
https://www.amazon.com/dp/B0B3DDG5QY?psc=1&ref=ppx_yo2ov_dt_b_product_details

https://www.amazon.com/dp/B072BN43P8?psc=1&ref=ppx_yo2ov_dt_b_product_details

USB/5V DC-DC w/voltmeter display
https://www.amazon.com/dp/B08PCLTT5G?psc=1&ref=ppx_yo2ov_dt_b_product_details

25.2V battery charger (suitable for 7 cell in series LiFePo4 battery):

https://www.amazon.com/dp/B08PKHKQC8?psc=1&ref=ppx_yo2ov_dt_b_product_details

low voltage cutoff w/conventional relay:

https://www.amazon.com/dp/B07Y6CM9ZS?psc=1&ref=ppx_yo2ov_dt_b_product_details

notes about this device:

S1 is threshold above which battery is connected to load and disconnected from charging input

S2 is threshold below which relay is activated to disconnect battery from load and connect to charging input

current consumption at 21V, 30 mA with relay off and display connected, 7 mA with relay off and display disconnected, 95 mA with relay on (battery disconnected from load) and display connected

display is 0.2-0.4V negatively offset from actual voltage


---


Found this device which might work though it uses a mosfet instead of a relay.  I supposedly can handle 30A and has a quiescent current of only 2 mA.  

https://www.amazon.com/CZH-LABS-Electronics-Salon-Voltage-Disconnect-Protect/dp/B018TV7NQU/

https://czh-labs.com/products/czh-labs-low-voltage-disconnect-module-lvd-24v-30a-protect-prolong-battery-life

https://czh-labs.com/products/czh-labs-low-voltage-disconnect-module-lvd-24v-30a-protect-prolong-battery-life-1

Here's an off the shelf one with a latching relay, but doesn't have the voltage setpoints I want:

https://czh-labs.com/products/10-to-30v-80amp-low-voltage-disconnect-lvd-module-for-12v-18v-24v-battery

« Last Edit: September 12, 2022, 03:45:13 03:45 by zac » Logged
koky
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« Reply #1 on: September 12, 2022, 08:22:10 08:22 »

you can use bistable relay with double coil
pe

 hxxps://docs.rs-online.com/0ea3/0900766b80027baf.pdf   << coded Link >>

https://docs.rs-online.com/0ea3/0900766b80027baf.pdf     <<one  reason for DO NOT CODE LINKS rule
                                                                                            can you see why ?
« Last Edit: September 13, 2022, 01:00:38 01:00 by pickit2 » Logged
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« Reply #2 on: September 19, 2022, 08:25:30 08:25 »

I thought of the same idea of koki, a bistable relay, the only precaution is if you want the passage not to interrupt the power supply to add a capacitor that keeps the power supply on for the switching time
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