12v Battery Meter Question

[revans]

For a project of mine I was planning to use this 12v battery level monitor here: **broken link removed** with a regular 12v SLA battery. However, while checking out prices for the components of it I found this pre-made kit here: **broken link removed** which is meant for car batteries. Now as far as I know car batteries are generally 12v SLAs, so the kit should work for me, right? It looks pretty similar to the diy one with 10 leds, what looks like a LM3914 IC and trimpot so I think it will be alright. I'm not 100% sure on this though so if someone could confirm that it'll work fine that'd be great.

 

[ericgibbs]

hi,
Car batteries are not SLA [sealed lead acid].
A SLA is sealed and has a 'gel' for the electrolyte, a car battery is not normally 'sealed' and has a liquid electrolyte.

As the battery monitor is for a 12V battery, it should be OK for both.
I cannot see the spec for the ready built unit, but it looks like a LM3914.

 

[revans]

Ok, so despite the kit being for a car battery which would probably not be an SLA, I would still be able to use the kit with an SLA anyway as it is a 12v battery - Correct? Thanks for your clarification, Eric.

Also, just a slightly non-electrical question that someone might know: would the rectangular LEDs in the Jaycar kit work well with frosted acrylic over the top of them to provide a diffused look, instead of just seeing the LEDs?

 

[jst3712]

Also, just a slightly non-electrical question that someone might know: would the rectangular LEDs in the Jaycar kit work well with frosted acrylic over the top of them to provide a diffused look, instead of just seeing the LEDs?

Hi, how thick is the acrylic? I'm guessing 3 or 5mm?

In my experience, the rectangle LED's don't give out much light at all, so you'd be struggling to see what LED's are lit through diffused acrylic - especially during broad daylight! (in my experience).

The standard 'water-clear' round type LEDs, even the 3mm, give you nicer colours (especially the green), and if they are too bright, you could always increase the resistances according to your taste

Jason

 

[revans]

So it might be a good idea to replace the stock square leds then, preferrably with water clear ones? The rectangular LEDs from Jaycar are 15mcd which I suppose isn't much, while the clear ones are 300+. Thanks

 

[Leftyretro]

Ok, so despite the kit being for a car battery which would probably not be an SLA, I would still be able to use the kit with an SLA anyway as it is a 12v battery - Correct? Thanks for your clarification, Eric.

Also, just a slightly non-electrical question that someone might know: would the rectangular LEDs in the Jaycar kit work well with frosted acrylic over the top of them to provide a diffused look, instead of just seeing the LEDs?

"Correct? Thanks for your clarification, Eric."

Correct, that monitor would work with both types of 12V lead batteries.

Lefty

 

[Willbe]

I think the chemistry for SLA and unsealed LA is identical. The monitor should work.
If you want to unseal an SLA connect it to two 12v car batteries in series (just kidding)!

 

[revans]

@ jst3712 and anyone who cares: the frosted acrylic will be 3mm

 

[revans]

Ok I'm probably going to get the kit and use some brighter leds with it but looking at some bright/clear leds on the Jaycar website I noticed they have different forward voltages. (red 4000mcd = 2v, green 3000mcd = 3.5v, yellow 4000mcd = 1.6v). Will this matter? Do I need to find leds with the same forward voltage? The rectangular leds Jaycar sell (which I suppose will be the same as the ones in the kit) have a forward voltage of 2v, so should I be aiming for around that?

 

[ericgibbs]

Ok I'm probably going to get the kit and use some brighter leds with it but looking at some bright/clear leds on the Jaycar website I noticed they have different forward voltages. (red 4000mcd = 2v, green 3000mcd = 3.5v, yellow 4000mcd = 1.6v). Will this matter? Do I need to find leds with the same forward voltage? The rectangular leds Jaycar sell (which I suppose will be the same as the ones in the kit) have a forward voltage of 2v, so should I be aiming for around that?

hi,
The LED drive output from a LM3914 is constant current, the current is set by a resistor/s on the pcb.
The different voltage of the LED's shouldnt be a problem.

You may have to increase the LM3914 current output to give the brightness you want [within the limits of the LM3914]

 

[revans]

Well, the stock rectangular leds have a forward current of 10ma with a peak forward current of 50/160ma.
The brighter leds I mentioned below have a forward current of 15/30/50ma with a peak forward current of 100/200ma.
From this, do you think that I would need to change a resistor to get the most out of the brighter leds?

 

[ericgibbs]

Well, the stock rectangular leds have a forward current of 10ma with a peak forward current of 50/160ma.
The brighter leds I mentioned below have a forward current of 15/30/50ma with a peak forward current of 100/200ma.
From this, do you think that I would need to change a resistor to get the most out of the brighter leds?

hi,
Looking at the LM3914 data, shows a 20mA max drive for the LED's.

If the kit resistor is set for 10mA, then a resistor change is required if you are going to drive the LED's at 20mA.
Thats assuming the kit designer has chosen 10mA for that kit LED.

 

[revans]

I suppose I'd need a multimeter to figure that out, or could I work it out throuugh the circuit?

 

[ericgibbs]

I suppose I'd need a multimeter to figure that out, or could I work it out throuugh the circuit?

On pins6/7 of the LM3914 is a resistor, call it R1.

Ic LED = 12.5/R1.

You may have to change another resistor, call it R2, connected to the bottom end of R1.
This is because Vref = 1.25(1+[R2/R1])

If you can post a circuit diagram of the kit it would help.

The datasheet explains it.

 

[revans]

Unfortunately I don't have a circuit diagram of the kit, but the closest thing would probably be the circuit diagram of the diy monitor I was originally planning to do (see 1st post).
Would I be right in calculating that the resistor R1 over pins 6 & 7 would need to be 625 ohms to give a current of 20ma?
Ic LED = 12.5/R1
20ma = 12.5/R1
R1 = 12.5/.02
R1 = 625 ohms

 

[ericgibbs]

Unfortunately I don't have a circuit diagram of the kit, but the closest thing would probably be the circuit diagram of the diy monitor I was originally planning to do (see 1st post).
Would I be right in calculating that the resistor R1 over pins 6 & 7 would need to be 625 ohms to give a current of 20ma?
Ic LED = 12.5/R1
20ma = 12.5/R1
R1 = 12.5/.02
R1 = 625 ohms

The sums look OK, its the R2 value that may need to be altered.
You will be able to sketch out the pins 6/7 when you get the kit.

 

[revans]

Sweet, thanks for your help Eric; will it be fine if I private message you when I get the circuit so I can sort out the resistors?

 

[revans]

I decided to go with making this circuit (**broken link removed**) instead of using the Jaycar kit as it would work out cheaper for me. So, seeing as there's a circuit diagram to go by, could someone please evaulate whether I would need to change any resistors to give the appropriate LED drive current of 20ma for the brighter leds discussed above?

 

[ericgibbs]

I decided to go with making this circuit (**broken link removed**) instead of using the Jaycar kit as it would work out cheaper for me. So, seeing as there's a circuit diagram to go by, could someone please evaulate whether I would need to change any resistors to give the appropriate LED drive current of 20ma for the brighter leds discussed above?

hi Rhys,
Answered your PM, ref this resistor change.