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6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

KrisBlueNZ

Sadly passed away in 2015
OK, here's my suggested circuit. The changes are only in the LED circuit.

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Some features of this diagram are needed for LTSpice, the circuit simulation software, and aren't meant as changes from your original circuit.

R1~4 represent the two trimpots. They're shown as resistors in this diagram for simplicity. The actual value of the trimpots is not critical as long as they aren't so low that they waste significant current. Personally I would use 20k or 50k trimpots.

U1 and U2 are shown as separate LT1001 op-amps but that's just because LTSpice doesn't have the LM358 and it doesn't support dual op-amps properly. Use your LM358 as before.

The 2.5V reference voltage is shown as a voltage generator in the diagram, but should still be an LM336Z with a 27k resistor to VCC.

The LEDs are shown as a simple diode and an LED marked QTLP690C. That's because LTSpice doesn't have many LEDs. They should be a bi-colour (red/green) LED in a three-pin package with common cathode.

R5~10 and Q1,2 are genuine components. They're all new except R5, which replaces the two 1k5 resistors in the original design. This means that the current consumption of the LEDs is always about 10~15 mA, unlike the original design where the current consumption would be twice that much when both LEDs were ON simultaneously.

In this design, when both LEDs are ON, they share the current; the value of R6 determines how much current each LED gets, and therefore determines the shade of orange that you will get. You could replace R6 with a 500 ohm trimpot so you can fine-tune the orange colour to your preference.

How does it work? It relies on the fact that red and green LEDs have different forward voltages. When you feed 10 mA through a green LED, it will drop about 2.8V (typically), whereas 10 mA through a red LED will cause a voltage drop of about 1.8~2.0V.

When the supply voltage is greater than the highest threshold (I've used 22V here), the outputs of both op-amps are high, i.e. close to the positive rail. There will be little voltage across R7 and R9, and Q1 and Q2 will not conduct. You can ignore all of the LED circuitry apart from D1B (the green LED element) and R5.

A current of about 10~15 mA (depending on the battery voltage) will flow through D1B and R5, producing a green indication. There will be about 2.8V across D1B.

When the supply voltage drops below the higher threshold, U1's output will go low and this will turn Q1 ON. There is now a current path through Q1, R6 and D1A. Since D1A has a forward voltage of about 1.8V, there will be about 1V remaining across R6, and the value of R6 will determine how much current will flow through D1A. If you reduce R6, more current will flow through D1A and less current will remain to flow through D1B. This makes the orange indication closer to red. If you increase R6, the orange colour will become greener.

When the supply voltage drops below the lower threshold, U2's output will go low and Q2 will turn ON. This effectively connects D1A in parallel with D1B. Because D1A has a forward voltage of only 1.8V, there will not be enough voltage across D1B for D1B to draw any current, and the LED will indicate only red.

In all of these cases, as the battery voltage drops past each threshold, the current drawn by the circuit actually increases slightly. Each increase in load current will cause the battery voltage to fall slightly, and this change reinforces the change that caused it. This avoids all the situations where instability and oscillation can occur.

Those situations will only occur when a change (e.g. a drop in battery voltage) causes a change (e.g. a reduction in load current) that then reverses the original change (reduced load current will cause the battery voltage to rise). The system can get into an unstable state, with the battery voltage oscillating above and below the threshold voltage, because a change causes an effect that negates, or reverses, the change. With this design, all the effects reinforce the change; nothing negates the change. Therefore there's no need to add explicit hysteresis around either of the op-amps.

The other advantage that this design has over the original design is that its load current is fairly constant, as I said earlier. The original design draws 10~15 mA when indicating either red or green, but twice that when indicating orange. This design has a single current-setting resistor, and in the orange state, that current is split between the two elements of the LED.
 

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I just had a thought,how about making the LEDs light up ONLY when the level is low?
say, a red LED when the b.level is around 18v as this is the most critical in the whole battery operation?
 

KrisBlueNZ

Sadly passed away in 2015
Sure, you can do that. And that would save current - at least until the battery voltage gets low. You wouldn't be able to see that the circuit was working until the battery became low.

I assumed you wanted the three-way indication. If you don't, then a simple low battery indication circuit would not be difficult.
 
The more I looked at your revised circuit the more I liked it! Impressive! So,I'd like try this one rather than go back to the old design!

Sorry,I'm not too sure about the following values;

Is R2=25k, actually in the circuit? if so, can I substitute a 27k? 25k is a difficult value!
R6= 300R.....can I use a 270R 1% instead?

You have indeed a fantastic talent & ability to explain things very meticulously & in a clear way way! (even for a dummy like me!)

I've redrawn it & hope haven't made any mistakes?

Cheers!
 

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KrisBlueNZ

Sadly passed away in 2015
Thanks :)

The resistor you have shown as R2 should not be there, and the 27k resistor marked R3 should be connected to the top end of the LM336Z, not in series with the other trimpot. Both trimpots should be connected directly from VCC to 0V.

The value of 300 ohms for R6 is only approximate, and the best value depends on the characteristics of the LED and the shade of orange that you want, so it's probably best to use a 500 ohm trimpot for R6.

You might want to clarify the markings on the LED. Definitely delete the "QTLP690C" marking. You might want to draw a rectangle or circle around them to indicate that the red and green LEDs are part of the same component.

Other than that, it looks good.
 
Ahh..I see the mistakes & have corrected them. Unfortunately I don't have any 2N3906so have already ordered a few. I also need to get some 470ohm trimmers as well & should have these by next week!

Thanks again & have a pleasant weekend!
 

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KrisBlueNZ

Sadly passed away in 2015
Cool :) You too.

You can use any general purpose PNP transistor. If you're in England or Europe, BC557B, 557C, 548B or 548C are more common than the American 2N series.

There's a place for your location in your profile and it can help others if you fill it in.

Edit: suitable BC5xx transistors are 557B, 557C, 558B, 558C. Not 548 as I listed above; those are NPNs.
 
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KrisBlueNZ

Sadly passed away in 2015
Oops! I listed some wrong transistor part numbers in post #27. You can use 557B, 557C, 558B or 558C. Not 548.
 
No problem! I too found that out just now while going through a few i had. I have a few BC556B & BC559C of which the latter has a very low self noise figure & seems to
fit the bill!
I'll modify the existing circuit & will report back.

Cheers.
 
Yes,it works very nicely indeed! Thanks! All I need now are a few rundown batteries to set the 18v threshold? I'll probably have to do it with a pair of 9v PP3?

Just a thought, the "yellow state" when both LEDs are on, will draw the most current then?
 

KrisBlueNZ

Sadly passed away in 2015
Yes you can try two 9V PP3s, or build, buy, or borrow a variable power supply.

Actually that circuit will draw most current in the red state, but the difference in current drain between green, orange and red is pretty small.

BTW you won't see yellow. Green and red make orange.
 
I think I'll use a couple of 9v batteries. Yes, of course, it'll be "orange"!

Thank you ever so much for being so patient & guiding me through this. I'm so grateful.
It will indeed look very impressive on my onboard acoustic guitar preamp & not in the least keep an eye on the batteries!

Best regards
 

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The circuit works but I'm finding it difficult to set the LED threshold points.

1. R4 can be set to trigger/lightup either red or green easily at any voltage!

3.I've tried R6 with both 470R & 270R values ...not much effect though!

The biggest problem is, having set the green to light to come on at say,23v, it stays "green" even at 18v!

What am I doing wrong?
 

KrisBlueNZ

Sadly passed away in 2015
Most likely there's a mistake in your board layout.

Did you draw a diagram of the layout before you built it up? If so, can you post that?
Can you also post clear photos of both sides of the board?

Disconnect R6 temporarily and work with R4 only.
Connect the circuit to a variable power supply.
You should be able to adjust R4 so the LED changes between red and green at a specific voltage. There is very little hysteresis, so the change from green to red and the change from red to green should occur around the same voltage.

I'll have a look at your circuit board when you post the pictures.
 
Yes I did as I always draw a layout with all my projects. I do have a method to my madness!! Lol!

I tried to photograph the stripboard but couldn't get it right as the lighting wasn't correct even with the flash! I will try again in the morning & will post it. In the meantime please do have look at my layout & see if you can spot any mistakes.

Thanks.
 

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KrisBlueNZ

Sadly passed away in 2015
That layout looks good.

Yes, natural light, without direct sunlight, is the best for taking photographs of circuit boards etc.

Do you have a variable DC power supply?

Open-circuit R6 and set R4 so the LED changes from green to red at the desired low voltage threshold - 18V I think.

Then can you measure the voltage between the positive supply rail and the top end of R5 when the supply voltage is above, and then below, the threshold voltage.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Yes I did as I always draw a layout with all my projects. I do have a method to my madness!! Lol!

That's a good method you have there.

even with the flash!

There's your problem right there. Don't use flash. Use a broad indirect lighting source. Easiest option is outdoors in shade (or under moderate cloud cover if you can arrange that).
 
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