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My son and I are designing a 3D printed body for his pinewood derby car.

We want to add some colored LED lights. I found some lights that I think will work well, from a mechanical fit and looks perspective. I need a battery that will power the lights for about 3 hours.

Weight is a concern, but shouldn't be a problem. The car can only weigh 5oz maximum. Tungsten weights are typically added because you want the car as close to 5oz without going over. Ideally the weight is compact and distributed properly. So I can possibly use the battery as weight, but I'd like to keep it as light as possible because I can use tungsten strategically to distribute the weight better.

The LEDs I'm looking at have a forward voltage of 3.0-3.6v and a current of 20mA.

I need to have 6 of them. So if I'm not mistaken (and I might be) I would need 360mAh of battery (20mA x 6LED x 3hours = 360mAh). It appears I could use a coin type battery for this. I see some coin batteries at 1000mAh capacity for 3.6v.

Does this seem like a possible solution? I'm assuming I could wire the LEDs parallel to the battery and would not need any resistors at 3.6v.

I'm open to any ideas and considering a rechargeable battery. If there is an off the shelf charging PCB (I can make an inline charger unit or integrate a small PCB into the body.)

Not asking for battery weight analysis, etc. We are in preliminary design and just want to figure out what is practical/possible.

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  • \$\begingroup\$ You won’t get more than 3V thus no more than 10mA with a 5mm white or blue LED from a small coin cell \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 14 '19 at 3:59
  • \$\begingroup\$ Probably no more than 2mA with 1.7g \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 14 '19 at 4:05
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The LED's I'm looking at have a Forward Voltage of 3.0-3.6v

That would be white, green, and or blue LEDs for that Vf range.
They are probably low efficiency LEDs. Not good for a battery powered project.
The Vf is too high for a high efficacy LED. Efficient LEDs will have a 2.7-3.0 Vf.

I'd use either a Lithium or Li-ion battery. Using a good efficient LED, such as the Cree C503B series LEDs, you could use a single Lithium CR-2032 coin cell (3 grams). A better (but heavier, 45 grams, and larger) option would be a $4 18650 Li-ion and would last many (100s) hours per charge or you can run these Cree LEDs super bright.

The Cree LEDs will cost under 25¢ each and will pay for themselves in batteries compared to cheaper LEDs.

I would be leaning toward the 45 gram 18650 Li-ion. I'd use a milling machine with an 18 mm ball cutter to embed the battery in the bottom of the body. If using the CR2032 coin cell(s) I'd use a 20 mm end cutter.

See also this answer: Will this schematic work - basic LEDs + strips

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  • \$\begingroup\$ I realize now I could have better defined the initial LED selection. We want a color selection of (Orange, Red, Green, Blue, Yellow, Purple) for the "infinity stones". A large viewing angle is also desirable. I found the following type had all the colors needed and a large view angle. lighthouseleds.com/led-component-lighting/… \$\endgroup\$ – Diznaster May 15 '19 at 4:21
  • \$\begingroup\$ I could probably put a 18650 in the car, parallel and close to the rear axle and get a good weight distribution. \$\endgroup\$ – Diznaster May 15 '19 at 4:30
  • \$\begingroup\$ So you will have a wide variety of intensities and forward voltages to deal with. The 120° 10,000 mcd should be putting out 5-10 times the amount of photons as the 15° 50,000 mcd Cree LEDs. Good choice. I would use an LM317 adjustable regulator with the 18650 set at about 3V (or a 3V fixed). This way you can adjust the resistor values and intensities and they will remain consistent over the full discharge cycle. The 18650 should not be discharged below 3 V. \$\endgroup\$ – Misunderstood May 15 '19 at 4:50
  • \$\begingroup\$ Good idea with the LM317. If I'm not mistaken that should shut off the battery when voltage is below drop out (slightly above 3v). Also it won't have issues as battery voltage drops (3.6v-3.1v). I appreciate the help, this gives me enough to know what I need to learn/research. Then I can teach my son what I learned! \$\endgroup\$ – Diznaster May 16 '19 at 1:26
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I have red and white keychain LED lights. The red lights have one 2032 lithium coin battery, the white lights have two 2016 lithium coin batteries in series. They don't have any series resistors so the light output varies considerably as the batteries discharge.

I don't have an easy way to measure the in-circuit voltage or current, but I did remove a white LED and test it by itself. At 5mA the drop is 2.88V, at 15mA the drop is 3.12V. This is consistent with a typical LED I-V curve and convinces me that there is no series resistor inside the LED package.

I believe that lithium coin cells are suitable for your application. You could always divide the batteries and LEDs into 2 separate circuits to get more battery life if necessary. Run some experiments with your LEDs.

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