12V Low Battery Cutoff

that barest minimum of investment yielded the desired result, the essence of engineering.

That chip is very impressive.  I could see it being one I'd drop on every project involving SD card driven systems like the RPi and BeagleBone boards.

I gathered that with under voltage condition, pin 5 goes low. 

Not the original poster's scenario, but thinking about it for microcontrollers - I imagine you'd need a voltage divider to protect the GPIO on 3.3V logic levels, which would be an additional drain, though.  Is there a better approach to than a voltage divider?

Can anyone see how one sets the "undervoltage" value?  For instance, I'd like a Pi to powerdown softly once a 12V battery dips to 8V that is going through a switching regulator that provides a stable 5V to the pi. 

Thanks,

Sean

From what I understand most 12v standby batteries are considered dead at around 10.6 to 11.2 volts.  There are batteries types capable of deep discharges but they may not hold a charge as well as a standard UPS standby battery.    Here is a good forum discussing it.   https://electronics.stackexchange.com/questions/177032/how-bad-is-it-to-undervoltage-a-12-volt-lead-acid-battery

So if you look at the datasheet that Gene posted you can see the L7712A is designed for use with 12 volt batteries.  The cutoff voltage is 10.8 volts. 

If you need to step down from 12v to 5v just use a voltage regulator.  Most common is a 7805.  Just make sure it is a fixed voltage regulator like the L7805CVL7805CV.

If you do end up using a deep discharge type of battery you could theoretically use a buck boost converter which will output 12v even if the battery is below 12v.  These can destroy batteries though so be careful. Here is a video that goes into buck boost and many other power related things.    https://www.youtube.com/watch?v=IT19dg73nKU

As far as the 8 volts go you should be able to read voltage off the battery from the Ras.  Use a voltage divider to create an analog value that the Pi can read, then activate an output pin when your battery reaches 8v.

You may be able to use a L7709A, which has a cutoff value of 7.6v. 

actually the buck boost would be pointless unless you actually need 12 volts

Sean,

These devices are ordered based on the required "under voltage".

Gene

Thanks, Gene.  At $1 a pop, I'm buying 10.  Wayyy better than a corrupted SD card.

-Sean

While I was ordering, I found TPS3808G30 will cover 3.3V applications.  Could help prevent damaging LiPos.

-Sean

Sean,

For LiPo batteries, the 3.3V limit is likely a little too safe.  These cells are usually safe down to 2.8V, although I have seen commercial products that limit them down as far as 2.1V.

Best of luck!

Gene

The 3.3V was it's nominal with a threshold of 3.01.  Perhaps the TPS3808G30 is better suited for a LiPo.

I guess I was thinking more about the LiPo (safely down to 2.8v or lower) and you were thinking more about the logic (3.3V functional to 3.01).  Either way, these devices are great to make sure you are operating in a safe region within your system.

Keep it safe!