# Battery low voltage indicator

Discussion in 'The Projects Forum' started by JimmieD, Feb 27, 2011.

1. ### JimmieD Thread Starter New Member

Oct 24, 2010
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0
Hello all, I know this has been hashed and rehashed many times, but I still am having trouble with my question, so I am resurrecting a very old thread to hopefully get some better answers since I am very new at understanding circuits and such.

I have searched through the forum and the internet, and have found various circuits and explanations of the LM339 TL431 and so on:
http://forum.allaboutcircuits.com/attachment.php?attachmentid=26570&d=1295139211, http://forum.allaboutcircuits.com/attachment.php?attachmentid=26655&d=1295412736 and http://forum.allaboutcircuits.com/archive/index.php/t-25238.html

Now all of these are great, I understand the diagrams and such, but here is where I run into problems, all of these are for lower voltages than what I am requiring.

I have a battery pack that is 26.4V, I would like to have a high intensity LED light-up once the voltage hits 20V or some other predetermined voltage. Here is another twist, I really do not want to use a pot!

After going through these forums and reading about these circuits, it would be great for someone to just say, here is the circuit, but what I am asking for are the equations (remember I am just a DIY novice on all of this so be gentle with the equations!) on how to determine resistor and zener sizes when dealing with different voltages with a diagram such as this http://www.eleccircuit.com/low-voltage-battery-monitor-by-lm339/ and if there is another way to get around using a pot by determining the actual resistor number through an equation.

Have enjoyed reading and learning through other responses, so if someone would be so kind as to help me out with this that would be great!

Thanks,

Jim

2. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
Remember, the OP wanted an adjustable version, hence the pot. Yes, you are correct that fixed resisitors for the circuit can be calculated (by using ohm's law). However, due to various tolerances it may be helpful to use the pot anyways in a one-off design. You don't have to use a large pot, a small pcb mount trimpot is all that is necessary. Once you set the trimpot to where the circuit works for you, then leave it alone.

Using different voltage levels creates other issues, such as finding a comparator that will work from that voltage, or having to use a regulator to drop the operating voltage (which then creates other issues). These are just a couple of examples of design issues that need consideration and a certain amount of knowledge to recognize and resolve -- which is something that people who are new or relatively unfamiliar with electronics don't understand the time and effort it takes to analyze a design for a particular application and configure one that is robust.

Engineering is a discipline of compromise, and a big part of being capable is the ability to accurately weigh the compromises and their effects. This is why it is difficult to provide pat answers to what an OP thinks is an "easy" question (questions are easy, not the answers). See some of the long posts where Sgt. Wookie or others try to take someone through an entire design cycle on something relatively "easy" for those with experience.

Last edited: Feb 27, 2011
3. ### JimmieD Thread Starter New Member

Oct 24, 2010
4
0
Thanks for the reply nomurphy,
Great, glad it is "easy" for you as someone with experience! Difference between you and others such as my fellow Marine Sgt. Wookie is, is that they have a teachers spirit and are enthusiastic about what they are teaching and want everyone to know what they know because they find it so fascinating. I wasn't expecting to have a quick "fix all" answer, just hopefully a little help to gain a little knowledge since that is what this portion of the forum is for, some help, is it not? I am actually quite offended that you would speak down to me as a child without even knowing my knowledge or background, I was a structural engineer before I retired, all I asked for was a little help in a new area that I am exploring in my life and to be pointed in the right direction not be belittled by assuming I know nothing of what it takes to learn something new!

You answered one question that I had, and I do appreciate it, use Ohm's law, now was that so hard? I had other questions in my post which I prefaced by only slightly referencing the OP due to the similarity of the question, but if making an LED turn on when a battery hits a certain predetermined low voltage level is just too difficult for someone to explain because it is too "easy" to not "just know", then I am sorry for even thinking I could come to a community forum to ask and to learn.

Good Day.

4. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Hello brother JimmyD,
Nomurphy is really a pretty good guy, and he helps a lot of people on the forums.

Please keep in mind that it's difficult to stay in the mindset that most folks are novices in this game, and it's hard to remember that at one point in time, nobody knew anything about electricity/electronics. On the other hand, there are only so many hours in the day to explain everything; I'm in a time crunch myself.

I have to wonder what your battery pack is? If it's lead-acid batteries, they should be at some multiple of ~12.8v (nominal, 77°F, fully charged). To maximize a lead-acid battery life, it should not be discharged below ~75%. Discharging it to 50% capacity will reduce its' service life by about 2/3.

Since you didn't give any specifics about your battery/batteries, I really don't know where to go with this. I frankly don't know of a single 26.4v battery, but that's OK, I don't claim to know everything - there might actually be a battery rated for that somewhere.

It would help a great deal if you had the manufacturer and part number of the battery/batteries that you are currently using, as the datasheets for them could be looked up. Following the manufacturer's recommendations for charging/maintenance is always a good bet for maximum performance and life span.

It would also help a great deal if you can produce a diagram of how the battery/batteries are connected into the circuit.

I posted a spreadsheet and image in the "Tips & Tricks" thread that's "stickied" near the top of the General Electronics Discussion forum. If you have Microsoft Excel 2003 or later, you can download the attachment and plug in your numbers to get an idea of where you should set your levels.

In the meantime, the voltage of the batteries is a concern as how to configure the circuit. Obviously you want a very low drain current, but at the same time you want to be alerted when the batteries are too low.

If the voltage source consists of more than one battery in series, you really need individual battery monitoring, as failure of a single cell will cause overcharging of the other battery/batteries in series, causing them to fail as well. This happens more frequently than you realize, particularly on military vehicles. We have a retired USMC 5-ton truck that had one battery with a shorted cell, that soon caused the other three batteries to fail. Tried to tell the guys that the bad battery had to be left disconnected, but they didn't listen. It cost \$600 to replace the four batteries.

I was a radar systems/missile fire control technician on McDonnell/Douglas F-4J and F-4S Phantom II's back in the 70's. Went on to newer and better things after I hit EAS in '80.

If I were thrown into your prior MOS with no training, I'd be doing the proverbial "fish flop" - no clue, man. I knew my MOS, how to shoot, salute, and stand inspection... that's about it.

S/F

5. ### JimmieD Thread Starter New Member

Oct 24, 2010
4
0
Thank-you Sgt Wookie for the well thought out explanation.

To nomurphy, I do apologize for wearing my "feelings" on my sleeves, just have been through quite a bit with other forums and being somewhat new to electronics and electrical types of things and being kinda pushed aside when asking questions, nevertheless, it was unfair to automatically perceive you in the same light.

I apologize, for not giving specifics about the cell chemistry or the application on which I am planning on using this circuit I am attempting to create (recreate). I was unsure if that really mattered all that much, but it helps with understanding the overall purpose and application rather than just spouting off numbers!

Chemistry is LiFePO4, 3.65± .05V (max of 3.95± .025V) charging per cell, nominal 3.2V-3.4V (my battery is 8 cells, hence the 26.4V battery using 3.3V per cell average) LVC for LiFePO4 should be 2V per cell, I am using an 8 cell protection circuit module which detects individual cells at 2.2V ±.1V before cutoff (Ideally 17.6V for a battery whose cells have perfectly matched discharge curves) One battery I have has the protection cut-out at 22.4V, while the newest one that I have built (after learning discharge curves and matching cells prior to building the battery) has the protection cut-out at 18.4V

The battery is used for an electric bicycle motor. I was looking to build a circuit which would light a high intensity LED (so I can see it and it attract my attention while riding), depending on the batteries independent overall performance, which would light once 90% of the battery voltage is used.

I can appreciate and understand different variables, such as after the load is removed, that the voltage can, and depending on the amperes of the load, will move back up, different batteries discharging a little different and the likes, but what I am very interested in, is finding if there are basic equations that can be used (or tweaked depending on variables) in determining the various resistors and diodes to be used with an IC such as the LM339. Or, is it that no matter what electronic circuit there is, new equations need to be introduced depending on what circuit is to be desired, or is at simple as applying Ohm's and/or Kirchhoff's laws and going from there. As a structural engineer I could simply reference various equations to determine a base to begin with depending on stress, load displacement and so on. Since mathematics is 'suppose' to be constant, my question revolved around that assumption in applying systematic ways in electronics as I would in structural engineering.

Here is another way to go forward with the particular question that I have. Would it be best to begin determining what resistors, diodes and other components I would need based on the particular IC comparator that I choose which would depend on the maximum rated supply voltage per the data sheets? (Yet maybe another way: Are there basic steps to follow to find the answers that I am looking for?) I know that there are devices on the market that can not only perform this, but also other information concerning battery energy consumption, I am just a DIY guy at heart! I don't mind the whole trial and error thing, that is how we learn, just would like a point in the right direction.

BTW, Semper Fi, was a load planner in combat logistics determining weight and balance for air and ship cargo. Favorite part of MOS, helicopter support team, determining weight, balance, wind, altitude, lifting capabilities and other variables for external loads. I'm a geeky former Marine that just enjoys mathematics!

6. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
An LM339 has rather limited maximum sink current; 6mA is about all you can count on. An LM311 is a single comparator that has considerably more capability.

Zener diodes are commonly used as a voltage reference in such circuits; however they have a tolerance, and their breakdown voltage can/will change over temperature somewhat, and will also vary due to Ir (reverse current). You'll want a fairly low-current Zener, as otherwise you'll consume a fair amount of power just to maintain a voltage reference.

There are very accurate voltage references available, but they tend to be rather expensive.

I'm really short on time this morning; have to run.

7. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
There are a number of people that post questions such as "I need a widget, anyone have some good designs for one?" And yet they don't provide one bit of design criteria or how it is to be used, but want someone else to do it for them.

Although something may be "simple" it still takes time. For instance, if you want to use 26V, that is a higher than typical voltage and time must be spent finding datasheet(s) and going through the comparator specs (possibly looking at multiple comparators). If, for some reason, it wouldn't work with 26V, then what is an alternate design solution? Well, you can run the comparator from +5V and use a resistor divider to measure the 26V, but what values and settings are best for doing this per your application? If a regulator is needed, then what is the regulator design going to entail to get from 26V to 5V, PS source, regulator heat dissipation, etc? Engaging in and going through all this consideration is time consuming.

Please understand that engineers work from design specifications (what is the function, what are the trip points, trigger points, voltages, frequencies, operational logic). When they don't exist, you sit down and write them out so that you know what it is you are designing to. And then, write down tests and expected results for verifying that your circuit actually behaves as intended. It should only perform as intended; not in other "unexpected" ways. The process is to test, compare results, modify as necessary, then test again; repeat until verification is complete and the circuit is fully functional.

Although typical hobbyist circuits don't fall into this category, one may consider including temperature testing, vibration testing, humidity testing, etc. As an example: think about a hobby-type circuit that's going into your car's engine compartment, wouldn't it be wise even for a hobbyist to perform some of these "extra" tests in as practical way as monetarily feasible before possibly getting stuck out on the highway somewhere?

Nevertheless -- attached is a typical reference circuit for a comparator (with the math), I suggest using an LM4040_x of a "specific value" for the reference voltage (where x is the zener voltage). Work on it ,come up with something, post it, and ask for a critique of your design. You will receive a lot of help.

BTW: I find it handy to plug such equations into an Excel spreadsheet, so that I can make quick modifications to the values and see the results. MathCad is also quite useful, but most hobbyists are not going to spend the money. Also remember that a 1K resistor, for example, is not necessarily 1K; it has a tolerance and may be 990 or 1010 ohms (@ 1%) which may very well affect the way your comparator or other circuitry works.

• ###### Comparator.pdf
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Last edited: Feb 28, 2011

Apr 20, 2004
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9. ### JimmieD Thread Starter New Member

Oct 24, 2010
4
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Thank-you Sgt Wookie and nomurphy. I appreciate the point in the right direction and for the attachment along with a few options to begin my search and testing.

Once I have something, I will be sure to post it for some critique and some helpful reviews on improving or getting a thumbs up (maybe after a few, well quite a few, tries I'll get the thumbs up!)

beenthere, I apologize if I hijacked the other thread, didn't want to start a new thread with a topic very similar to another.

10. ### wayneh Expert

Sep 9, 2010
11,910
2,854
This isn't a problem. All you need is a voltage divider - a pair of resistors in series to ground - to bring your battery voltage down to near a reference, say 5v, established with a zener or a 7805 voltage regulator, for instance. Use a comparator such as the LM339 to compare your measured voltage (on the non-inverting input) to the reference on the inverting input. Use a transistor on the comparator output so that the comparator can drive an LED. You can consider eliminating the transistor (and reversing the inputs) if you want, but the comparator can sink limited current.