LED battery question

The battery contains 4 D cell batteries. A D cell has the capacit of 12,000mAh. There are 20 LED warm white lights with a draw of 20mA. If my math is correct, under stable environment those batteries should last 120 hours. With a timer set for 6 hours on, 18 hours off the lights should last 20 days. The question I have is whether the resistor will impact this at all, and if I have a resistor, do I still need a driver. At a 20mA draw, how many lumens am I going to get for each light? Any responses will be welcome.

Are the 20 LEDs each drawing 20ma? Or is it 20ma total draw from the circuit?

What is the voltage of the battery pack? (NiMh or Alkaline)
What is the forward voltage of each LED?

Without knowing the nominal voltage of your battery pack, or the forward voltage of the LEDs, its not possible to calculate how much impact the resistor will have. If you plan on using resistors with any kind of efficiency, you need to make sure that the battery voltage is as close as possible to the forward voltage of the LED string. Also, as the battery voltage drops, the current to the LEDs is also going to drop. This is going to make the actual runtime much more difficult to calculate.

Thanks for your reply. I believe that each LED is drawing 20mA, so a total of 400mA. The battery pack most likey will be Alkaline (although I am pushing for NiMh) and I am not sure the forward voltage, is this in regards to the chip or to each bulb? how would i find this out?

mAh is a pretty useless number, unfortunately. Capacity of a battery or cell is more useful when described as Watt hours. If we accept that there are 12,000mAh in a D cell, that equates to about 14.4 Watt Hours per cell. Multiplied by 4 cells, you have about 57.6 watt hours of capacity. You'll never get every last watt out of a battery or cell, so lets say there are about 45 usable watt-hours of capacity there.

The forward voltage of the LEDs is a very important number in this equation. If they are white LED's, we can assume a Vf of about 3.2V. If each LED is pushed to 20mA, than each LED consumes (.020 * 3.2) .064 Watts. 20 LEDs will consume 1.28 Watts. If you had a 100% efficient method of driving the LEDs, than those 4 D cells could power those LEDs for 35 Hours. Since there is no known circuit that can regulate power with 100% efficiency, we must assume that there will be some loss. A pretty decent SMPS will give you about 90% efficiency, so it's really more like 31.9 Hours. With 6 Hours On, 18 Hours off, You'll get about 5.3 Days.

You mention a resistor, so I'm assuming you're planning on not using an SMPS, and instead direct driving the LEDs using a current limiting resistor. This is a horrible idea for many reasons.
1) yes, the resistor burns energy, and it's burn rate must be considered and will subtract from the number of hours that the LEDs can be lit for
2) Alkaline and NiMh cells change voltage over their charge cycle, and the LEDs will be much brighter when first powered up than they will be after the cells burn down
3) there is no way to get even 80% of the Watt Hours out of the cells by direct driving the LEDs. A SMPS can discharge the batteries much further because it can continue to power the LEDs as the voltage sags.
4) If you are going to drive the LEDs in parallel, each LED will be a different brightness and will get a different amount of current depending on it's exact Vf. This can be quite significant.

hi bbawkon,

When you mention an SMPS (switched mode power supply?), is this available in an IC format....or are you referring to a power supply circuit?

bbawkon,

What is the price difference from using a resistor to using a SMPS? I see what your saying but I have to justify it if the cost is much higher.

There are plenty of ICs that are smps controllers. most (all?) require a few external components (inductor, diode, cap). Personally, I would use a boost converter and run all of the LEDs in one series string.

An IC like this would be useful
http://www.micrel.com/page.do?page=/product-info/products/mic3223.jsp

You need constant current boost LED driver (like one build using LTC3783). I don't know how many you need but if you thinking about production you need to design good driver (of find one ready to buy).

As said before resistor is not a good idea. Besides efficiency you need input voltage higher than LED to add resistor. It means that your 20 LED's must be connected in parallel each one with its own resistor. With resistor price about few cents cost probably will still be low but wiring will be a mess.

If you need few and can't find driver than easiest way may be to buy Christmas lights. Use them as is or extract electronics (I hope they don't have 20 resistors inside ).

So I just got an email and they claim that with 20 LED warm lights with a power sourc of 3XAA batteries the life cycle of the batteries would be 36 hours and the CCT would be 2800K and a resistor size of 10Ohms would be added to the battery box. Based on what I've read from you guys this is ludicris and will construct an inferior product. Should I go back to the drawing board? How can I save expenses and create a good product (SMPS or driver)?

I just did some quick math and giving the AA a generous 2300mAh I figured the Wah is about 31.05 at 100% efficency. They are claiming 36 hours, but I dont know how that is feasible, and with a resisor of 10ohms wont it waste a ton of wattage and possibly overheat.

gerrardsreds said:

I just did some quick math and giving the AA a generous 2300mAh I figured the Wah is about 31.05 at 100% efficency. They are claiming 36 hours, but I dont know how that is feasible, and with a resisor of 10ohms wont it waste a ton of wattage and possibly overheat.

Click to expand...

Possabilities:
They're lying about run time. Yes, I know, shocking.
They're running at < 20 mA
The loss in brightness curve adds to the battery life
a combination of the above, especially the first

The one benefit to running without a driver is that the battery use becomes somewhat self sustaining. As the cells discharge, their voltage drops. As their voltage drops, the LEDs see a lower Vf which in turn means they draw less current. The interesting thing is that since an LED can generate light down to INCREDIBLY low current draw, it becomes an equation that gets closer and closer to zero but never is. I've actually seen direct driven LEDs connected to a coin cell (tiny - 100mAh or less) stay lit for months. For your purpose, this is pointless though, since after a few hours - even though it's lit, its not producing any amount of usable light.
a 10 Ohm Resistor might get hot or it might not, depends on how things are wired and how much voltage drop and current it's seeing. If a 1/4 watt resistor is dropping 1/4+ watts, yes it will get very hot.

If you want cheap, go direct drive and accept the limitations
If you want quality, design a proper SMPS

Wow, I am learning so much from you. Thank you for you wisdom and incredible insight. I understand alot of what your saying, and would like to know if I could PM you if I meet more issues?