Go Down

Topic: low voltage cutout switch/relay to prevent killing batteries?? (Read 832 times) previous topic - next topic

skelectrician

I'm looking to buy or build a device that would prevent my in car arduino Project from killing my battery. This is especially important to me as I live in part of the world with some very brutal winters. A dead car battery is pretty much ruined at -40c.

So I'm looking for something that could disconnect my project when battery voltage dips below 12v, and automatically reconnects it when the battery is well charged at 13.4v.

Any ideas folks? I've seen  industrial cutoff relays that are designed for 24v systems but they look expensive and im not sure if they make a 12v equivalent.

Thanks a bunch! I'm pretty much a rookie with this type of stuff so you may have to explain things like I'm 5 if you wish to help.
Take 'er easy cause there's two miles of ditch for every mile of road.

calvingloster

Use an arduino stand alone atmega328 chip. They are cheap. Then use the chip to monitor the battery voltage via a voltage dividing circuit in parallel with the battery. Then connect an n chanel mosfet to a digital pin and connect the n chanel mosfet to a 12v relay that can handle the current that connects positive wire to battery. Once the voltage drops switch the n chanel mosfet on which switches the relay off.


I'm just not sure what type of relay can handle that type of current

calvingloster

Oh ok I see you want to cut the power to your project. That's kewl then u can use just a regular small 12v relay

Coding Badly

I'm looking to buy or build a device that would prevent my in car arduino Project from killing my battery.


Is it something that would be actively consuming power when the key switch is turned off?

lemming

#4
Jun 09, 2014, 10:37 am Last Edit: Jun 09, 2014, 01:34 pm by lemming Reason: 1
We can get these German units in Australia.  See if you can get them where you are.

http://www.kemo-electronic.de/en/Car/Modules/M148A-Battery-guard-12-V-DC.php

They have a low latent current draw and I have found them reliable.  Cost about $35AUD .

To make it smaller and cheaper you could use a low current voltage detector like the microchip TC54VC2102EZB and feed the output into an N chanel mosfet to switch the power from the battery. To use another 328 will merely drain the battery even though the main circuit is switched off. You need to make sure your detection circuit does not drain your battery while switching off the main device.  The TC54VC2102EZB above draws an operating current of 1 uA.

cdacunha71


To make it smaller and cheaper you could use a low current voltage detector like the microchip TC54VC2102EZB and feed the output into an N chanel mosfet to switch the power from the battery. To use another 328 will merely drain the battery even though the main circuit is switched off. You need to make sure your detection circuit does not drain your battery while switching off the main device.  The TC54VC2102EZB above draws an operating current of 1 uA.


Fully agree, just use some standard IC out there to detect the voltage drop, this will save you a lot on power consumption.  I'm a bit worried about the temperature it has to operate in. For instance the datasheet for the TC54VC2102EZB states -40C/+85 as operating temperature. I wasn't able to quickly find an alternative that will operate at lower temperatures.  Whatever you choose, please make sure you take into account the low temp effects on all electronic components

skelectrician

That TC54VC2102EZB idea sounds good! How do I get everything wired up and working properly? Do I need a divider to get the voltage low enough to use as an input to the ic? Will the mosfet be able to handle the total project current? (Likely no more than an amp at peak). A schematic would be great if at all possible. The project will be running 24/7 even with the key out of the ignition.

Thanks!
Take 'er easy cause there's two miles of ditch for every mile of road.

MarkT


Once the voltage drops switch the n chanel mosfet on which switches the relay off.


That should be "switch the n channel MOSFET _off_ which switches the relay off.

An active relay winding will drain your battery too....
[ I won't respond to messages, use the forum please ]

lemming

#8
Jun 10, 2014, 02:39 am Last Edit: Jun 10, 2014, 03:31 am by lemming Reason: 1
Try this setup (anyone else feel free to chip in with improvements).

Get the nearest value resistors for the divider as these are theoretical values.
You need to minimise the current draw through the divider but still maintain enough curreent for the TC54. See the datasheet.
I would put a trimmer pot in instead (although this is not considered kosher) with the wiper pin to Vin of the TC54 and the outer pins to the + and - and then you can have infinite control of the divider.

Check the temperature on the arduino linear regulator as at 13+ volts it may be getting too hot. You may want to put in another regulator/DC-DC converter prior to the arduino.  

Edit:
Just noticed your hysteresis requirement (12v cutout, 13.4v startup). You can lookup LM393 circuits with "hysteresis" in the search. Substitute this for the TC54 although it pulls 0.4 ma which is much more than the TC54.

skelectrician

#9
Jun 10, 2014, 03:28 am Last Edit: Jun 10, 2014, 03:31 am by skelectrician Reason: 1
Here, I added a couple things to the circuit that hopefully sheds light on what I want to accomplish. Will it work? Can someone give me a dummied down theory of ops as to how the circuit works? Will I need to add caps for my voltage regulators? If so, where, and what values/sizes?

Thanks a bunch guys!
Take 'er easy cause there's two miles of ditch for every mile of road.

lemming

Explain how the relay shield , Fob, Arduino, and cellular shield are meant to interact and what you are trying to achieve.  Do they only activate when the ignition is on or do you need them always active (listening)?

skelectrician


Try this setup (anyone else feel free to chip in with improvements).

Get the nearest value resistors for the divider as these are theoretical values.
You need to minimise the current draw through the divider but still maintain enough curreent for the TC54. See the datasheet.
I would put a trimmer pot in instead (although this is not considered kosher) with the wiper pin to Vin of the TC54 and the outer pins to the + and - and then you can have infinite control of the divider.

Check the temperature on the arduino linear regulator as at 13+ volts it may be getting too hot. You may want to put in another regulator/DC-DC converter prior to the arduino.  

Edit:
Just noticed your hysteresis requirement (12v cutout, 13.4v startup). You can lookup LM393 circuits with "hysteresis" in the search. Substitute this for the TC54 although it pulls 0.4 ma which is much more than the TC54.


Not sure if those are absolute voltage values there, alas, I'm a poor estimator, but I want something that would drop the circuit out at a bare minimum battery voltage needed to start my truck, and reenable it only when the battery is at a safe charging voltage, (ie, the truck is running and the alternator is doing its job.) I'm not 100% sure what a bare minimum starting voltage would be, nor am I entirely sure what the value of the battery is when the vehicle is running,

Hope that doesn't ruin your day!
Take 'er easy cause there's two miles of ditch for every mile of road.

skelectrician

#12
Jun 10, 2014, 03:43 am Last Edit: Jun 10, 2014, 03:44 am by skelectrician Reason: 1

Explain how the relay shield , Fob, Arduino, and cellular shield are meant to interact and what you are trying to achieve.  Do they only activate when the ignition is on or do you need them always active (listening)?


The system is to act as a text message unlock system. I send the "launch code," and the relay shield closes the contacts where the fob's pushbutton used to be for a second or two. I need the shield sandwich to be on at all times for the system to work as intended. the relays on the shield are not energized until given the launch code.
Take 'er easy cause there's two miles of ditch for every mile of road.

lemming

#13
Jun 10, 2014, 03:45 am Last Edit: Jun 10, 2014, 03:47 am by lemming Reason: 1
Not that clear. Is it that a text can unlock the car OR you can use the fob to unlock the car? (i.e. one or the other can unlock it)

skelectrician

Sorry for the misunderstanding. The setup is a little rube goldberg because I dont want to mess with the security system on the truck. The fob is currently powered by a small 12v battery, but i'd like to remove the battery and power it off the 12v icvr. (cause I'm not sure if it would survive running at a higher voltage while the vehicle is running.) I'd desolder the pushbutton and replace it with leads for the NO and COM terminals on one of my relays. when a text is received, an output would go HIGH for a second or two, simulating me pushing that button with my thumb. The other fob on my keychain will work as normal like it always has.

Does that clear things up a bit?
Take 'er easy cause there's two miles of ditch for every mile of road.

Go Up