How can i use LM3914 to display state of charge ? -schematic attached.

Thread Starter

anishkgt

Joined Mar 21, 2017
529
hi anish,

Thats exactly the same configuration that I posted in post #12.
View attachment 175456

View attachment 175455

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Ah initially i did not understand so i just played around in LTSpice and came out with what was in my mind. So here is it the R1 and R2 that needs to be adjusted ? R1 needs to be changed to a lower voltage than the minimum voltage of 3, correct ? R2 would remain same as that is the current limiting resistor for the LEDs.

Am i correct in assuming so ?
 

ericgibbs

Joined Jan 29, 2010
10,513
hi.
What is the minimum and maximum voltage you would allow on one battery.?
Also do you want to have LED #0 lit at the lowest battery voltage and LED #9 lit on the highest voltage.??
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I suggest you use DOT Mode on the LED's, as this will minimise the LED load on the battery.
 

Thread Starter

anishkgt

Joined Mar 21, 2017
529
hi.
What is the minimum and maximum voltage you would allow on one battery.?
Also do you want to have LED #0 lit at the lowest battery voltage and LED #9 lit on the highest voltage.??
E
I suggest you use DOT Mode on the LED's, as this will minimise the LED load on the battery.
The lowest LED should be 2v and highest should 4.2v. Yes the led 0 should light up 2 and 9th one at 4.2v(approximately). Yes I had planned on to use dot mode.
 

ericgibbs

Joined Jan 29, 2010
10,513
hi,
OK, I suggest you set the lowest to say 2.2V and the upper to 4.2v, this will give you a 2V span, at 0.2V/LED.
Use the LM3914 equations from the d/s to calculate the resistor value, post what you calculate
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Attachments

Audioguru

Joined Dec 20, 2007
11,249
A Lithium Ion cobalt battery cell is 4.2V when fully charged, 3.7V when discharged halfway or is in storage and is 3.2V when its load should be disconnected. If it is discharged as low as 2V or 2.2V then its capacity is ruined and it might not take a charge from many chargers that detect that it is ruined.

My electric radio controlled model airplanes pulse the motor when the battery is at 3.15V per cell and turns off the main motor when 3.05V per cell. They say that relying on the 3.15V warning to land will shorten the life of the battery.
 

ericgibbs

Joined Jan 29, 2010
10,513
hi anish,
Why have you chosen a lower level voltage of 2V, when the video uses 6V [ for two cells] when you have only one cell which is 3v.??
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Thread Starter

anishkgt

Joined Mar 21, 2017
529
The pack has several cells in parallel as well so the 2v is just for an indication purposes, more or less like a visual warning. It would never go below 3.3v but just in case if any of the cells does go, I would know.
 

ericgibbs

Joined Jan 29, 2010
10,513
hi anish,
I guess you realise that the two resistors 13k and 10.9k are designed for a voltage range of 3V thru 4.2V.
It is the two 10K resistive battery voltage divider which makes the video's LM3914, measure a battery voltage 6V thru 8.4V

So a simple mod would be to remove the bottom 10k resistor from the divider, the top 10k would act a current protection limit resistor for the LM3914 internal OPA.

Do you follow OK.?
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ian field

Joined Oct 27, 2012
6,539
In that case, the LM3914 is the classic way to do it for a single cell or battery but I am not sure it will behave as expected in your application.
If you can get hold of a 3914, they've been discontinues a long time and NOS is getting a bit thin on the ground.

I'd go for an LM324 and make a ladder window comparator (almost a start on what's in a 3814 anyway) - the full 10 LED resolution isn't worth the trouble of working any harder for.

There might still be bar graph chips from various SE Asia manufacturers - but they're probably taper law for audio level indicators.
 

Thread Starter

anishkgt

Joined Mar 21, 2017
529
I guess you realize that the two resistors 13k and 10.9k are designed for a voltage range of 3V thru 4.2V.
It is the two 10K resistive battery voltage divider which makes the video's LM3914, measure a battery voltage 6V thru 8.4V
Yes i do understand any two same values of resistor in series will always be half the voltage across it. Hence a 4 volt through R1 and R2 with resistance of 10k, 10k respectively would be 2v between the R1 & R2 and 2.5v would be 1.25v. basically all the actual input voltage is divided by 2.

So practically when the actual voltage is 4.2v, SIG would read it as 2.1 and the corresponding LED would light up, here the top most. When the actual voltage is 2.5v for example SIG would read it as 1.25v and the lowest LED would light up. I guess i am correct here.

Now RLO is the voltage reference for the internal set of comparators and to set the RLO, we have a voltage from VR to play with and a resistor divider R1 and R2. But how is RLO set, i know a resistor divider is the answer but how can i calculate resistance with R = V/I where V is 1.25 but what is 'I' ?
 

ericgibbs

Joined Jan 29, 2010
10,513
Hi,
Do not change the 13K and 10K9 they are already set for 3V to 4.2V.
Remove the 10K voltage divider lower resistor and connect the upper 10K between the battery +V and pin #5.
The LEDs will indicate the battery voltage between 3V and 4.2V.

Do NOT let the battery voltage fall below 3.2V else they will be damaged, read @Audioguru post #28, he has already warned you that the battery could be ruined.!

E
 

Thread Starter

anishkgt

Joined Mar 21, 2017
529
Thank you. I am aware of the safety of the of Li-ion cells.

The 2v I had earlier mentioned was just a visual indicator only. Would the below mentioned schematic be correct as you have suggested ?
Capture.PNG
 

Audioguru

Joined Dec 20, 2007
11,249
You have the values for R1 and R2 much too high. They set the LED current. Your values set the LED current at about only 1.3mA which is very dim.
Also you are missing a resistor from pin 5 to pin 2 to make a voltage divider.

The biggest problem is if a battery cell voltage drops below 3.0V then the LM3914 for that cell will not work properly and the LEDs with it might not light up.
 

ericgibbs

Joined Jan 29, 2010
10,513
hi ag,
The LM3914 is set up for Rlo =3V and Rhi at 4.2V.
The TS is measuring one nominal 4V cell, he does not need the divider.
The video used a 10K/10K divider as he was measuring two cell in series, ie: 6v thru 8.4v, the divider was used to drop pin#5 to 3V thru 4.2V

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@Audioguru
 
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