# High power Led project form a single lithium cell

#### marcelobm

Joined Nov 18, 2020
12
With Lithium cells, you have to be mindful of the instantaneous current demand, not just the average.
The lithium batteries I'm planning to use have a maximum discharge current of 20A.

#### marcelobm

Joined Nov 18, 2020
12
Sure...similar, but 30 watts?

I don't think Papa's calculation accounted for the pulse, which would reduce that 10 amps to 1 average.

100ms per second...right?

I think the 5.6 needed for the 3 watt LEDs divided by 10 is reasonable, that's about 560mA average.

I have no idea how long the charge would last.
Yes it is 100ms per second, so if I take the previous comments about the accepting the fact that if I power them directly from the battery, they will dim, and knowing the battery can handle the 5.6 amp then I should be ok right?

#### Papabravo

Joined Feb 24, 2006
14,700
Again thanks for your comment, I must say that I didn't wake up one day and say I want to build a rocket ship. This seems a rather simpler project, yes it is over my current knowledge and that is why I'm posting this.
Thank you any way.
SMPS (Switched Mode Power Supplies) are among the most complicated and difficult to understand circuits out there, because there is so much going on. I got my Master's degree in electrical engineering in 1970. It has only been since I retired in 2014, that I have spent any time in the study and understanding of them. I consider myself a mere dabbler having just scratched the surface.

#### Papabravo

Joined Feb 24, 2006
14,700
The lithium batteries I'm planning to use have a maximum discharge current of 20A.
And you don't want to have them spend much time discharging at that rate or they won't last very long. It's your time and your resources so go find out the hard way. Blow things up and burn them down. You're only young once. Never mind learning things before you leap into the abyss.

#### marcelobm

Joined Nov 18, 2020
12
And you don't want to have them spend much time discharging at that rate or they won't last very long. It's your time and your resources so go find out the hard way. Blow things up and burn them down. You're only young once. Never mind learning things before you leap into the abyss.
You are missing the point, the first thing I said in the first post is, I know this isn't right.
I understand the complexity of getting the 12v setup, and that is the reason I evaluated the 3.4v setup for the 3W leds instead.
In that case isn't simpler? I understand what you are saying, you seem to have great experience and knowledge and it would be great to get useful tips/pointers etc instead of just getting a "yes go ahead and burn thing" that is not helpful

#### ElectricSpidey

Joined Dec 2, 2017
1,336
You "may" be ok, that's why I asked you for the pulse specs of the LEDs.

If you can't find this information, then just limit the current using a resistor.

Personally, I would limit each LED to 500mA. (you won't notice any real difference in brightness)

The question then becomes at what voltage do you want the 500mA at.

You could start with 700mA at the full battery voltage, but the batteries will last longer at 500.

#### marcelobm

Joined Nov 18, 2020
12
Hi GUys,

So based on the thread I did some changes,
1. I've set the leds to be the cree xlamp lm2 which have a forward voltage of 3.18V @3A each, added a resistor to each one of the leds of 0.34 ohms, that was calculated assuming full charge of the battery 4.2V.
2. The leds are running directly from the battery which can provide up to 25A constant draw.
3. Added a charging module for the battery
4. Added a 3.3V voltage regulator to power the attiny85
5. input the battery to the attiny85 for monitoring and stop powering the leds on low voltage and turning the low voltage led on.

The cicle is still a 100ms flash every second.

This one I haven't tried it since I don't have the components at hand.

Again thank you in advanced for you help.

#### ElectricSpidey

Joined Dec 2, 2017
1,336
Looks good to me.

I'm curious, are you going to buy the LEDs already mounted to a spreader such as the typical "star" board, or bare surface mount?

#### bertus

Joined Apr 5, 2008
20,888
Hello,

Be sure you are using a logic gate mosfet.
A basic mosfet will not turn on fully and heat-up.

Bertus

#### DickCappels

Joined Aug 21, 2008
6,657
Remember that the LED will only have a 10% duty cycle, so with a good capacitor on the power supply's output the average power requirements can be relaxed some.

#### Sensacell

Joined Jun 19, 2012
2,634
You would be wise to change 2 obvious issues:

1) Re-design the gate drive so that the full battery voltage is applied.
You are going to need to drive the gate with everything you have to get the MOSFET Rds-ON low enough.

2) Use a resistor divider for the battery voltage monitor input.
Applying a voltage greater than Vcc to an input is going to cause problems.
The inputs are typically clamped with internal diodes to Vcc, these will start to conduct and force the Vcc to rise over your regulator voltage- not good.

With these changes- the circuit will work- "sort of" - the LED current will diminish rapidly and the results may be disappointing.

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