LED Strip battery level indicator - Zener diode or LM3914?

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
75
Short version:
Can't really understand if and how zener diode or LM3914 can be made to work for my battery level indicator for LED strip that draws about 800 mA when connected directly to a 12v SLA battery. I feel that they really would not help me here since SLA batteries below ~11v are considered empty or drained or do I need to play with voltage dividers/resistors.

Long version:
I am building a LED lighting for my pantry. Since there is no readily available outlets and to avoid having to hire electrician to wire up one I had to resort to a SLA battery to provide power to LED strip, at least for now. I've already built up a circuit that allows me to control brightness using PWM. But now I've realized that since I can vary brightness manually it becomes harder to catch a moment when battery needs to be recharged to avoid draining it completely.

So, I started looking for a circuit examples that give me at least 2-3 point level indicator. There are tonns of tutorial that use zener diodes and some using IC LM3914 which seemed to be reasonable to me at first sight. But when I started researching more about SLA voltages I found that SLA batteries below ~11V are considered dead. Which brings me to my confusion point. Is that voltage they are talking about is load voltage or no-load voltage?
Wouldn't battery voltage vary depending on the load or good and compatible battery should be able to provide 11-12v irrespective of the load? If an acceptable voltage range is only within 1-2v how can zener diodes or LM3914 be really used here or can they? Or you have to play around with voltage dividers/resistors to trigger corresponding level LED?

Anyone is able to tell me if and where I am right or wrong?
Any help is appreciated.
Thanks
 

Reloadron

Joined Jan 15, 2015
6,584
A 12 volt SLA battery when fully charged has a voltage of about 12.6 volts and considered discharged at about 11.6 volts.

A LM3914 (see the data sheet) is a popular choice as a battery voltage indicator. More on that in a bit. When a battery voltage is measured with the battery under load and the load removed yes, the battery voltage will increase under no load conditions.

The LM3914 has been around a long time being used as a battery voltage monitor. A nice feature is that two of them can be cascaded affording a greater range. See the linked data sheet. Also the monitored range can be adjusted so for example if you wanted a range of 11 volts to 13 volts it can be done. Zener diodes would be a poor choice since they have a knee in their response meaning they are not a simple On/Off device. Actually a 12 volt battery level indicator using a LM3914 inclusive of LED display can be had off the shelf relatively inexpensive. A Google of "12v battery level indicator" should bring up some off the shelf module solutions.

Ron
 

Papabravo

Joined Feb 24, 2006
18,408
I think you should look for a 2 and 1/2 digit voltmeter on a board. They are built around a small microprocessor with an A/D input. The half-digit will be a blank or a 1 and you'll know the voltage under load to the nearest tenth of a volt. Like this one:

https://www.digikey.com/en/products/detail/sparkfun-electronics/PRT-14313/7564745?utm_adgroup=Essen Deinki&utm_source=bing&utm_medium=cpc&utm_campaign=Shopping_DK+Supplier_Other&utm_term=&utm_content=Essen Deinki&utm_id=bi_cmp-384720322_adg-1301822093609990_ad-81363949567673_pla-4584963495352066_dev-c_ext-_prd-7564745&msclkid=0bb1f5bbb79914458671973ecb3c7295
 
Last edited:

Reloadron

Joined Jan 15, 2015
6,584
I think you should look for a 2 and 1/2 digit voltmeter on a board. they are built around a small microprocessor with an A/D input. The half-digit will be a blank or a 1 and you'll know the voltage under load to the nearest tenth of a volt. Like this one:

https://www.digikey.com/en/products/detail/sparkfun-electronics/PRT-14313/7564745?utm_adgroup=Essen Deinki&utm_source=bing&utm_medium=cpc&utm_campaign=Shopping_DK+Supplier_Other&utm_term=&utm_content=Essen Deinki&utm_id=bi_cmp-384720322_adg-1301822093609990_ad-81363949567673_pla-4584963495352066_dev-c_ext-_prd-7564745&msclkid=0bb1f5bbb79914458671973ecb3c7295
Actually the best logical choice. Keep forgetting that today we have all these inexpensive volt and current meter solutions. This is the way to go. :)

Ron
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
75
I think you should look for a 2 and 1/2 digit voltmeter on a board. They are built around a small microprocessor with an A/D input. The half-digit will be a blank or a 1 and you'll know the voltage under load to the nearest tenth of a volt. Like this one:

https://www.digikey.com/en/products/detail/sparkfun-electronics/PRT-14313/7564745?utm_adgroup=Essen Deinki&utm_source=bing&utm_medium=cpc&utm_campaign=Shopping_DK+Supplier_Other&utm_term=&utm_content=Essen Deinki&utm_id=bi_cmp-384720322_adg-1301822093609990_ad-81363949567673_pla-4584963495352066_dev-c_ext-_prd-7564745&msclkid=0bb1f5bbb79914458671973ecb3c7295
Thanks! This is a good option. There are few reasons why I prefer to go the "hard" way.

1. What is the power consumption of this module? Does not seem to mention in datasheet. With battery power source I can limit the current used for"display".
2. I am occasional hobbyist and have good amount of zener diodes (along with other components) from sets that I have purchased before and may never find a use. So might as well use them now.
3. This would be an opportunity to learn more about zener diodes and how they operate in practice. The knee of course is not ideal but does not bother me that much.

So, considering all of this. If I go the zener diode route my only option is to arrange resistors as voltage dividers and control leds using diodes? Or there are other approaches that use common components that I can look into?
 

Papabravo

Joined Feb 24, 2006
18,408
Thanks! This is a good option. There are few reasons why I prefer to go the "hard" way.

1. What is the power consumption of this module? Does not seem to mention in datasheet. With battery power source I can limit the current used for"display".
2. I am occasional hobbyist and have good amount of zener diodes (along with other components) from sets that I have purchased before and may never find a use. So might as well use them now.
3. This would be an opportunity to learn more about zener diodes and how they operate in practice. The knee of course is not ideal but does not bother me that much.

So, considering all of this. If I go the zener diode route my only option is to arrange resistors as voltage dividers and control leds using diodes? Or there are other approaches that use common components that I can look into?
My solution for that is to use the module as it is and have it on a manual switch or a timer, so it is not consuming continuous power unless you want it to. For the price I'd just get one and fool around with it. Then you can use it for something else or give it as a stocking stuffer.
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
75
Thanks! This is too large. I wanted to 3D print a small box that would fit small 4x6cm PCB and couple of LEDs. Probably overkill for the basic PWM brightness control project for the LED strip.

By how much is your battery indicator going to reduce your battery life?
Trying to minimize consumption as much as possible that aligns with my knowledge and experience. But I am not aiming for absolute minimum. In the worst case, if nothing else works the way I want it to, I can consider single LED to indicate low battery.
 

MisterBill2

Joined Jan 23, 2018
11,877
Years ago I built a batteru check package that used a single transistor, a red LED and a green LED that served as the voltage reference The green LED came on when it was powered and the red LED started lighting about 12.3 volts and became full brightness about 12 volts. It was a very small package and told us when the battery needed to be recharged. I just used a button for checking the battery. Unfortunately I did not save a copy of the circuit. And the variable brightness was adequate for knowing how close to recharge time we were.
So a battery monitor can be really simple and really cheap.
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
75
Zener diodes would be a poor choice since they have a knee in their response meaning they are not a simple On/Off device.
I did a sample circuit and I think I see what you mean. Seems like the knee is too large and usable battery voltage range can "fit" inside that knee. I could not find any info about that in the datasheet and/or not sure how to read that from whatever is available.
 

Reloadron

Joined Jan 15, 2015
6,584
You can toss around a circuit as MrBill suggest. I have seen it done with two transistors. Regardless of choice there will be some current drain. You can also use a small module meter capable of measuring it's own supply voltage with a button push or you can work the LM3914 route rolling your own.

A Google of zener diode knee should further what the knee is all about. :)

Ron
 

MisterBill2

Joined Jan 23, 2018
11,877
The circuit I mentioned did indeed draw a bit of current,so it did include a small normally open push-button switch to allow a convenient battery check. Most battery voltage monitoring circuits do draw some current, unless they are externally powered.
 
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