Need a low voltage detector/shutoff circuit for SLA battery  - Page 1
Forum section: electronics / projects

author: Bassblaster505
author_rank: Contributor
timestamp: September 15, 2019, 03:55:27 am
content: Greetings, first time posting in a while. Ive been in the works building a big Bluetooth speaker (original right) with the heart being a nice SLA battery i got for free that is in great health, takes and holds a charge great. i need some kind of a low voltage protection circuit. my original idea was to have the main VCC, straight off the battery, go through a relay, and have the low voltage cutoff dis engage the relay when a voltage threshold is hit. for SLA i think its around 11.5V. But because my SLA is fairly small at 4.5Ah i dont want a relay to always be drawing current.EDIT: latching relay?my current idea is use the same low voltage detector and power a 555 LED flasher for a "low battery" type thing, but then i have to manually shut it off to avoid over discharging.For the circuit i need i THINK i would need a basic comparator and some kind of voltage divider, but i dont know how to do thatCan someone sketch me a circuit that will "turn on" when a voltage is reached? Can i have that output of said comparator power the 555 flasher or should i stay with the preferable relay hard shutoff method? BUT ill need a circuit for that too. i have the amplifiers, charging, fusing and bluetooth taken care of im just stuck on this part.

author: ledtester
author_rank: Super Contributor
timestamp: September 15, 2019, 06:44:19 am
content: What kind of current handling capability do you need?There are ready made modules for this on Amazon and ebay - e.g.:https://www.amazon.com/dp/B07929Y5SZhttps://www.amazon.com/Electronics-Salon-Voltage-Disconnect-Protect-Prolong/dp/B018TW0KN2/ref=sr_1_1_sspa(Search for "Low Voltage battery protection")but read the reviews. They aren't perfect, but they are going to be competitive in price to any DIY solution - especially if you have to order any parts.If you are really concerned about power consumption I would use a microcontroller and a MOSFET/latching relay. A micro also makes it trivial to add more features like a visual or audible alarm. You can even use an off-the-shelf Arduino. Here's some good info on lowering the power consumption of a Arduino Pro Mini:http://www.home-automation-community.com/arduino-low-power-how-to-run-atmega328p-for-a-year-on-coin-cell-battery/

author: tggzzz
author_rank: Super Contributor
timestamp: September 15, 2019, 10:31:42 am
content: It might be worth looking at compressor controllers for 12V fridges and freezers. They can have a cutoff voltage elected by a resistor.

author: Bassblaster505
author_rank: Contributor
timestamp: September 15, 2019, 04:36:31 pm
content: Current of 5A would be fine. that Electronics-Salon one on amazon looks good, just non latching. Seller says it draws 40mA the entire time its latched. the ICStation one has too much going on.the main reason im concerned about power draw is the TPA3116d2 amplifiers are going to be driving 2 Ohm speakers so with 4.5Ah play time already won't be that great at full volume.

author: JeanF
author_rank: Regular Contributor
timestamp: September 15, 2019, 08:30:11 pm
content: Hi,For a few days last year I was interested in low-voltage cutoffs as well. I've made myself a small memo, I'll attach it here so maybe you can get some inspiration for this project or for a future one.Trying and breadboarding circuits takes time, depending on what you want to focus on and the time you have for this build, a commercial unit may make sense. Maybethis onecould fit ?You could as well decide to use a commercial unit to switch reliably and preserve your battery, and add a little home-made circuit for a low-battery indicator if you want

author: floobydust
author_rank: Super Contributor
timestamp: September 15, 2019, 09:13:47 pm
content: A proper LVD needs a precision low voltage detector with hysteresis, a timer and a latch.With audio as a load, anything less will false trigger during a loud portion of a song/decent kick drum, and then restart during the silence between songs as the battery voltage comes back up. You don't want it to chatter and cut in and out.For SLA batteries, consistent low voltage for a minute or two is what I use. Once tripped, wait for partial charging to happen before resetting the cutoff.A small MCU like ATtiny85 does fine for a two chip solution, and some firmware.

author: Bassblaster505
author_rank: Contributor
timestamp: September 15, 2019, 10:19:45 pm
content: Oh i never thought of that transient loads from audio. ATTiny with the timer would probably be exactly what i need, and i could have built in LED light and not a 555. but i have a handful of those things so yay 555

author: mariush
author_rank: Super Contributor
timestamp: September 15, 2019, 11:00:51 pm
content: Easiest would probably be a microcontroller with a latching relay.You can usually connect multiple pins together for higher current ... for example some PIC16F microcontrollers can do 15-20mA per pin and ~150mA in total - so you could join 3-4 output pins together to have enough current for those 10-20ms it takes to switch a mechanical relay.So a tiny 50-100mA ldo, a 8-16 pin micro, a 5v latching relay... a couple resistors as a voltage divider for the ADC of the micro...

author: nfmax
author_rank: Super Contributor
timestamp: September 16, 2019, 07:33:56 pm
content: I made a battery undervoltage protection circuit for my home intruder alarm system using an LM10 precision reference+OPAMP and a power MOSFET. I added a CMOS logic device to reduce the current consumption, when the protection is activated, to just a bit more than the drain leakage current of the MOSFET. Described here:https://www.eevblog.com/forum/projects/battery-undervoltage-lockout/msg1060148/#msg1060148Since that was written, I have tweaked the resistors to increase the shutdown voltage, as the alarm panel doesn't take much current. The values in the schematic would be appropriate for higher current drains.