12V Leisure Battery status using 2x LM3914s

[garynobles]

Hi,

I am trying to monitor my leisure battery in my VW T2 Campervan.
This is as far as I have got with the circuit design.

Firstly I think I have wired it correctly could someone confirm this?


Secondly (assuming the first is correct) I'm sure I need to tweak the resistor values.
The battery is a 110ah leisure battery which ranges from 12V to 12.66V.
(currently reading 12.55V)

For the two ICs the RLows and RHighs are the following:

3941a
Pin 4 (RLow) = 12V
Pin 6 (RHigh) = 12.24V

3914b
Pin 4 (RLow) = 12.24V
Pin 6 (RHigh) = 12.66V

I don't mind having 2 Pots (might be handy) or two fixed value resistors.

Any help is always appreciated

Thanks

Gary

 

[chemelec]

You might build this one.

http://chemelec.com/Projects/Volt-Meter-1/V-Meter.htm

 

[ronv]

Usually 1 is enough since the battery voltage is only 12.72 when fully charged and 11.9 when totally dead. If you look around you can find 3 color bar graph LEDs with red yellow and green. Makes kind of a nice effect since you don't really want to have it totally dead as it ruins the life.

 

[garynobles]

Hi, thanks Ronv, are you certain a leisure battery of 110ah uses the range 12.72 - 11.9?
In that case the Vlow, VHighs should be:
3941a
Pin 4 (RLow) = 11.9V
Pin 6 (RHigh) = 50%V

3914b
Pin 4 (RLow) = 50%V
Pin 6 (RHigh) = 12.72V

I want to use 20 LEDs, I know I can do this with three.

Chemelec, thank you for the link, it is nearly what I need, but the scalling is wrong, from 10.25 to 15 volts.
Why so many capacitors? I only have 1 in my design.

Oh also 12V supply should come from the top right and GND bottom right either side of the capacitor in my circuit diagram.
Is my wiring correct? Is it only my resistors and capacitor to determine the values?

Thanks

Gary

 

[chemelec]

Is this not a Lead Acid Battery?

The LM3914 IC's Like a Bypass Cap Directly between V+ and Ground Pins of the IC, to Avoid Oscillations.

 

[garynobles]

I've only got one ground, so only 1 capacitor? Is that right.

Yes this is a lead acid battery, but not a normal car starting battery, a leisure battery - so if ronv is right voltage between 12.72 - 11.9
These are for running things like fridges etc...

So back to my circuit, I got the LED to IC bit including pin 9 to pin 1 from the datasheet, I have assumed pins 2-8 are the same as the standard set up.

But now I must get IC1 to turn on Leds from 11.9V to 50% capacity, then IC2 from 50% till 12.72V.
This is where I get confused with resistor values and which ones I need?

Thanks again

Gary

 

[garynobles]

Here is the link for the circuit
https://www.circuitstoday.com/12v-battery-level-indicator-circuit-led-bargraph
I would rather just buy the correct resistors rather than tuning it, also I don't have an adjustable voltage supply, unless I fully charge my battery and wait till it goes flat!

 

[MikeMl]

Supposed you used a 4 1/2 digit DVM instead of a low-resolution bargraph. Is that going to tell you the State Of Charge of a Lead-Acid battery? No, the terminal voltage of the battery does not correlate well with its SOC.

Measurement of voltage at the terminals of the battery in order to determine SOC is not useful unless you disconnect all loads, wait ~ 12 hours, then make the voltage measurement with a high impedance meter, then you correct the voltage reading for ambient temperature, and then finally look up the SOC in a table. Because any battery has a finite internal resistance, measuring the voltage with varying loads is useless. You might sort-of get an idea of SOC if the battery discharge current is always the same as you make the voltage measurement.

If all you want to do is know when to disconnect the load because you have reached the voltage below which the battery shouldn't be discharged, then a single go-nogo LED is all you need. Battery fuel gauges based solely on voltage measurements are not very accurate. A voltmeter used during charging isn't very useful, either..

 

[ronv]

Mike's right of course. It will be of no use if the batteries are loaded. But I used one on my golf cart and it worked pretty good with just a minute or so for the batteries to "bounce back". But having said that it only told me when I was getting in trouble. I knew if it was at 30% I better head for home instead of 9 more holes. That's why I suggested one instead of two. It will give you 10% steps. I think Eric has posted a calculator for this on this site. We can look around. It's a pain do it by hand. You will also want to use a resistor and a small pot to adjust the Vref voltages so that the pot is only fine tuning the setting.

 

[alec_t]

These are for running things like fridges etc...

With such variable loads the terminal voltage of the battery will vary widely. As the others have said, the LEDs will not then give a high-resolution measure of the battery state.

 

[garynobles]

Ok, maybe I confused things with the fridge, the internal electrics I have are only a few internal LED lights and a 12V car computer.
I have Eriks calculator, the LEDs I want to use are 20mA, first I will test it with single LEDs then if it all works get this circular bar graph
https://www.sparkfun.com/products/11492

Ah which reminds me, I dont need all 20 positions, only 16, so 8 on the first and 8 on the second, that might complicate things a little?

So 3914a LEDs 1-8 should be 11.9V to 50%, LEDs 9 and 10 should be two steps greater than 50%.
3914b LEDs 11-18 should be 50% to 12.72V with LEDs 19 and 20 two steps higher.


The battery status is only for testing before I head off or if I have been in a field for a while to see if I should turn my internal lights on or light the candles, the battery will not have a high load, no fridge!

Can anyone tell me if my is circuit correct? and how to figure out the requirement for the resistor values?

Thanks

Gary

 

[alec_t]

so 8 on the first and 8 on the second

No. If you're determined to go this route (when a 3 LED display would be adequate IMHO ) then 10 on the first and 6 on the second would be simplest.

 

[garynobles]

Hi Alec, true use the first 10 with 6 on the other, I know 3 LEDs would do the job but the 16 LEDs is part of a bigger design, the parts for which are being laser cut, so tailored to the circular LED ring, I will try to remember to post the end result when it all comes together.

Can anyone tell me how to figure out the resistor values in the diagram of my first post? Then I can send off for the components

Thanks

Gary

 

[chemelec]

It is unlikely you will find the exact values required in fixed resistors.
(Maybe in .1% or .01% tollerance resistors.)
Best to use trimpots and adjust it with an accurate meter for checking it.

Consider also, these LM3914's are not 100% accurate to the Formulas.
The Internal resistors are typically 1% Values.
Lots of room for Errors, considering the NARROW Range of Voltage measurement your trying for.

An LCD Voltmeter would be far more Practical and consumes Much less power.

 

[alec_t]

Can anyone tell me how to figure out the resistor values in the diagram of my first post?

Can't you use Eric's calculator?

 

[MikeMl]

...

An LCD Voltmeter would be far more Practical and consumes Much less power.

Never could figure out why folks screw around with LM3914s **broken link removed** so cheap...

 

[alec_t]

Ah, but you don't get the pretty light show with one of those

 

[chemelec]

Can anyone tell me how to figure out the resistor values in the diagram of my first post? Then I can send off for the components

Thanks

Gary

Ah, but you don't get the pretty light show with one of those

First you need a circuit that can work.
That schematic Can't work as shown.

 

[WTP Pepper]

Look on ebay for DVMs with the 7.5-19V range. £3 with no P&P from HK. I have just bought 5 of them and they are fine for connecting across a 12v battery permantly ~3.5mA (once you disable the backlight to save power). Far less than messing around with a 3914 on veroboard.

 

[garynobles]

Thank you chemelec, no one responded to the main question: will this diagram work?

I pieced it together from the datasheet, but I wasnt sure about it, what have I done wrong?

thanks

Gary