Battery powered COB LED illumination drop off

[Richmrf]

PROBLEM:
24 SMD COB LED has too much illumination drop off over 3 hours. Currently powered by four 3V button batteries.

PREFERRED SOLUTION:
Get this battery powered LED to perform more consistently over a few hours.

THE STORY:
I’m working on a little project at home for a custom show car I built. I’m trying to get my interior lights to work independently with a small power source.

I have a few COB LED’s placed inside the car and can not run wiring without destroying my interior.

I thought I designed a perfect fix using four 3volt button batteries ran in series. The small batteries allow me to hide the power source without running more permanent wiring. It only needs to last a couple hours for the final judging.

Currently, they are lasting much longer then needed but the drop off is very dramatic. From over 100 LUX when first switched on, to 30 LUX after 10 minutes, 17 after 20 minutes, 15 after 30 minutes, then it starts to stabilize and remain around 13 LUX with very little drop off.

These lights do not need to be over 30 LUX. They are only decorative lights but the drop off can’t be so drastic that a judge can notice the difference over a 20 minute period.

QUESTION:
What are my options to get a more stable output with much less drop off in ilumination?

 

[Nigel Goodwin]

There's no 'problem' at all - you just need much bigger batteries. You can't get something for nothing, it's like you're asking how to drive 500 miles with only half a gallon of petrol in the tank.

You 'may' be able to extend the full brightness time somewhat, at the expense of a shorter battery life and a sudden 'end' where it stops working totally - by adding some kind of switchmode converter, that maintains a constant current through the LED's.

But basically there's not much power in button cells.

 

[Diver300]

I agree with Nigel.

LEDs have a voltage that has to be supplied before they will work at all. Below that voltage they won't work, but they won't take much current either. Typically for white LEDs the voltage is around 3 V per LED. You might have three in series for car lighting, so 9 V of LEDs. There will be a resistor to limit the current, or possibly the resistance of the LEDs and the batteries will limit the current enough.

When the batteries are new, you have 12 V or a bit more, so there is about 3 V to overcome the resistance and drive the current.

As the batteries deplete, the battery voltage drops to much nearer 9 V, so there is less voltage to overcome the resistance, so less current. From your figures, it's 13 % of the light, so I would guess 10 - 13% of the current. With that much less current flowing, the batteries are being discharged slower, so of course they last longer.

It is a common problem with LED lights run from batteries that the peak light output is good, and the life is good, but you don't get both at the same time. Only a tiny part of the life is anywhere near full brightness, and most of the life is at really low brightness.

 

[rjenkinsgb]

You can get a reasonably constant illumination by using a small LED driver PCB; they regulate the LED current rather than voltage and can work over a fairly wide input voltage range.

The LED will run as constant brightness until the batteries get below the minimum input voltage.

There are many types on ebay - a couple of examples:

6-30V 0-1.2A LED driver, Arduino PWM compatible, ready built, kit or bare PCB | eBay

The max current is set using a 1206 resistor. The board is available as a bare PCB, a kit of parts to assemble yourself and a ready-built unit ready for use.

www.ebay.co.uk

700 MA Constant current LED driver with PWM control PCB mount UK Seller | eBay

Internal power dissipation: Vin = 24v, 5LEDS maximum 700mW. Full load efficiency: 96%. LED +: access the LED anode. LED-: then the LED cathode.

www.ebay.co.uk

You need to get one that provides the appropriate current for the LED(s) in use.

When the battery voltage is well above the LED voltage, the input current is rather lower than the LED current, so you also get longer battery life than with a simple LED + resistor setup.
The batteries must still be able to supply the required current though. Too small a battery and the voltage will "sag" and possibly make them appear flat when they are not.


If I'm misreading it and the LEDs are part of lamp units, eg. 12V devices, then you need a "boost converter" board that gives 12V out, instead.
eg.
**broken link removed**

 

[Visitor]

The basic problem is the amount of energy in the batteries and using it efficiently, as the previous posts have said.

However, I think there's a much easier way to achieve what you're after. You say the LEDs are generating 100 lux now, but 30 lux might be sufficient. The attached graph shows that reducing light output by about 2/3s will cut the current drain in half. It also shows you're in a shallower part of the curve, where a change in current drain results in a smaller delta of light level.

Try adding a series resistor to reduce light level to the lowest acceptable level, which will reduce LED current. This in itself extends battery life, and the useable operating time is further extended because light fall-off vs voltage is reduced.

 

[Richmrf]

There's no 'problem' at all - you just need much bigger batteries. You can't get something for nothing, it's like you're asking how to drive 500 miles with only half a gallon of petrol in the tank.

You 'may' be able to extend the full brightness time somewhat, at the expense of a shorter battery life and a sudden 'end' where it stops working totally - by adding some kind of switchmode converter, that maintains a constant current through the LED's.

But basically there's not much power in button cells.

This may be good news.
My goal is to lower the draw on the battery at first.For the first 20 minutes, it turns out to bea little too bright. Having it cut off completely and and using all that wasted energy in the first 2-3 hours may do the trick. Can you explain how to do this?

 

[Richmrf]

The basic problem is the amount of energy in the batteries and using it efficiently, as the previous posts have said.

However, I think there's a much easier way to achieve what you're after. You say the LEDs are generating 100 lux now, but 30 lux might be sufficient. The attached graph shows that reducing light output by about 2/3s will cut the current drain in half. It also shows you're in a shallower part of the curve, where a change in current drain results in a smaller delta of light level.

Try adding a series resistor to reduce light level to the lowest acceptable level, which will reduce LED current. This in itself extends battery life, and the useable operating time is further extended because light fall-off vs voltage is reduced.

View attachment 125424

Unfortunately I can’t help by providing any information about the LED’s. I have testers and I can test a few things if that helps.
is their any chance you can tell me what size resistor to try?
I apologize. I have very little electronica experience but I have an electrical background.
I really appreciate all of your time.

 

[Richmrf]

Victor,
Adding a series resistor is a great idea for a next step. It wouldn’t take up any space to speak of and space is my biggest issue.
Can you explain the best way to try this out? Can you give me a starting point? What size resistor should I try to start with? I assume I should order a few different sizes to experiment with so if you could provide me with a list to test out, I would appreciate it.
Thank you so much for your time.

 

[Visitor]

Do you have access to a digital meter? If you could measure the current draw we'll have a starting point. If you don't, we can make a decent guess based on battery life. What size are the batteries you're using?

Do you have any specs for the COB?

 

[Richmrf]

I have to get my Fluke from my desk on Monday. I only have a voltage tester and an ohm meter until then.
I’m using 4 CR 2032 button batteries.
I forgot to mention that I bought the cheapest batteries you can find on amazon. If I get a better quality, that may help.
As for specs on the COB, I have 5 different types I purchased on amazon. They are actually sold to replace other stock interior bulbs in cars. Where I want to instal these, I don’t have the luxury to use existing power source.
I’ll look to see if any of them have specs listed but I don’t believe they do.

 

[Richmrf]

The pictures I attached just show the crude setup I have for my tests. This light is still going after 12 hours. It isn’t nearly bright enough anymore but if I could get this energy to be used in the first 3 hours, I would be in good shape.
I guess the best way to explain is that I want to use up all the power in the first few hours and maintain a more consistent amount of light in that time if that is possible.

 

[Diver300]

Your CR 2032 batteries will not be able to give out all their energy in a few hours.

https://lygte-info.dk/info/BatteriesLowCurrentDischarge UK.html and the graph for the CR2032 is https://lygte-info.dk/pic/BatteryLowCurrentDischarge/CR2032/Discharge capacity Panasonic CR2032.png

The nominal capacity of the CR 2032 is about 220 mAh, but if you discharge at 10 mA, you only get 3 - 4 hours. At 5 mA you get about 4 times the life. In effect you are overloading the batteries and not getting the best from them. It's really not possible to get the power you want from those batteries for more than a few minutes.

It's a bit like when cranking a car. You could have an 80 Ah battery in a car, and cranking could take 500 A. If the battery were 80 Ah at 500 A (which it isn't) you would be able to crank for 80/500 hours, or nearly 10 minutes. It just doesn't work like that. The battery won't crank a car for more than a minute or so. The 80 Ah is measure when you discharge the battery over 20 hours.

The COB light appears to have a 20 Ohm resistor in it. I think that the light has 3 LEDs in series, giving about 9 V, so with a 12 V supply there is about 3 V across the resistor and about 150 mA. You need batteries that can supply approx 150 mA. If you could manage with say half the light, you could go to about half the current with another 20 Ohm resistor in series, giving a total of 40 Ohms and about 70 - 80 mA.

You would do better with larger coin cells, and better still with 8 x AA batteries. Three lithium ion rechargeable cells in series would do better, or on with a voltage booster would also be fine.

 

[Richmrf]

I understand and unfortunately, anything larger then 2 or 3 AAA’s, or 2 AA’s wouldn’t fit. It would become too large for the solicitous.
I’m basically trying to find a way to get a COB LED to be no larger then 3 AA batteries when ALL the components are together. Or about the size/weight of 2 AA’s and the LED.
I assumed I could make it happen because you can buy a 2 AAA cell flashlight with these types of LED’s. I know they have other components inside them but I just don’t know much about electronics.
Couldn’t I put a resistor, Capacitor , or a step up converter together with these coin batteries or2 to 3 AAAs?

I did throw an DMM on and cane up with 35.5mA but drops dramatically as expected.

I’m going to attach a picture. Couldn’t I get this to work if I inc. the correct components to get up to 12 V DC? It would be even better and not as tight of a fit if I were able to get it to go with two AA’s.

 

[Diver300]

The problem is that most non-rechargeable batteries are just not good at high power, where they are used in an hour or two. Even 3 AAAs will struggle to run one of those lights.

An alternative is rechargeable coin cells. They are much better suited to being discharged quickly.

https://uk.farnell.com/multicomp/lir2450/coin-cell-lithium-120mah-3-6v/dp/2009025

They are 120 mAh. The discharge graphs in the data sheet show that at a discharge rate of C (meaning 120 mA for that battery) they have a capacity of over 80 mAh and they hold up the voltage well. So three of those would run your light for around 40 minutes without the voltage changing by much.

You do need to get a charger designed for that type of cell if you are going to use them.

 

[t1d]

A "Jewel Thief Circuit" might be helpful. It will allow you to get the last drop of energy out of a battery and even run the LED on battery voltages lower than the LED's rating. This is no joke. Google it.

 

[Diver300]

A "Jewel Thief Circuit" might be helpful. It will allow you to get the last drop of energy out of a battery and even run the LED on battery voltages lower than the LED's rating. This is no joke. Google it.

It's "Joule thief"

 

[t1d]

It's "Joule thief"

Oh, it absolutely is! Great catch. Thank you, for the correction. They would have been searching for hours, with the wrong spelling. Much appreciated!