Make an LED (20 LEDs) circuit battery efficient

Question

I want to wire up a miniature wargaming figurine to have 20 LEDs. As I've poked around options my concern is the battery life and how often the battery would need to be replaced. Since each gaming session lasts 4-5 hours, it'd be great if I could find a battery that would last multiple gaming sessions (though I think this is impossible...)

I'm VERY NEW to all of this so I would appreciate explanations/solutions that understand I have little knowledge of the terms or techniques (I still don't understand what current is...)

EDIT: Here are more details on what I'm trying to do

I am trying to light up this figure (Knight Castellan).

Locations for the lights would be (and their specs):

• 8 blue for right arm (3.4V, 20mA)
• 2 Red-Blue-Green for the eyes (3.4V, 20mA)
• 10 red/orange lights for left arm and engine (2.2V, 20mA)

So altogether that's 20 LEDs, all at 20mA and none above 3.4V.

The battery would go inside the torso -- I can't find an exact image but the dimensions are roughly 2" wide, 2" tall, 3" deep.

My concern is battery life, as I'd like the batteries to last at least a few game sessions (so let's say 10-15 hours). I don't mind replacing or recharging a battery, but there's no point if I have to do it halfway through a game.

I should add that I don't know how bright these LEDs are (the specs say 8-10,000 mcd for the blue ones), and that I don't know how to dim them, but would prefer them bright since they are usually in a well-lit room.

Answer 1

You don't want to run all of these at 20mA. 20 leds in a 6" cube of a model like that knight (It took a while to find the size of these things) at full brightness are not just visible in daylight, they will absolutely illuminate a room. You can power them at a lower current, and they will still be visible in daylight and quite nice in the dark.

Since you power them at a lower current, they will have a lower forward voltage so you can use a smaller battery for longer. 20 leds at 1 or 2 mA will run for days off a pair of AAA batteries. Now you could power them all individually with a resistor each, but that's a bit wasteful and take up space. They can be powered in parallel with a single resistor (Ignore those that scream that's a sin!). The only issue with that is that you want red/orange, so you would have one string for the blue and one for the red.

The last thing is the RGB you want for his eyes. You haven't said how you want those to work. A fix color (4 pin RGB led that you combine multiple resistors to get a rough color)? Or flashing (2 pin led with a built in micro controller that runs through a pattern, just add power). The flashing led would work just the same, normally expecting 20mA at 3.3 volts, so power it at less and it's good.

^{simulate this circuit – Schematic created using CircuitLab}

Example schematic. 3x AAA batteries for size. About 13mA for both the blue and red strings. 10 for the two color changers. About 22 hours with good Alkaline batteries (plus more due to the way the batteries discharging lead to dimmer leds which pull less current, but that depends on how dim you mind them going). You could use NiMH 1.2V batteries but would probably want to use 75Ω resistors instead. You could use 75Ω resistors if you want them brighter but obviously that changes how long they last by a bit.

Answer 2

The capacity of this battery is not 1000mAh, it is about 750mAh at low currents.

Note that the linked battery only provides 9V for a short period of time. The rated capacity assumes that your device still works if the voltage drops to 5.4V. You will actually get about 700mAh and the voltage will be 7V to 8V. If the LEDs are not in series and you need 400mA you should expect no more than 1.75 hours of operation.

It will be difficult to use 2 LEDs in series if the voltage drops close to 7V, and the brightness will change greatly as the battery discharges. Even if that was OK, you will get about 3.5 hours of operation in this case.

You should be looking for another approach, possibly one that uses a high-efficiency switching regulator as a constant current source.

Answer 3

At 20mA your LEDs will be incredibly bright for such a small figure. There are several solutions to the problems you have:

1. You could use the current sources I showed in the other answer and provide a small PWM to lower the brightness. This would reduce the power consumption considerably. Using a small simple MCU could give you PWM control of perhaps 3-5 strings each at a preset brightness for the individual LED strings.

2. You could use the WS2812 integrated RGB LED to get any color or intensity you want. It may even be possible to reduce the number of LEDs required by making a Fiber optic connection to a small number of RGB LEDS. Plastic Optical Fiber (POF) is dirt cheap (see here) and readily available. Use a small hole in some plastic to position multiple fibers above one of the LEDs.
   The WS2812b comes already in LED strips form from many sources, and might be a good starting point for you. This could be a very efficient means of providing your lighting and getting the runtime out to many hours. The controller for setting brightness and color would not actually need to be within your model, you could send the serial data when you turn on the model, and then remove the controller. The same controller could be used with multiple models (there are endless opportunities and variations for creating the control system).

The WS2812 works from 3.5-5.3V so a single LiPo battery without any regulation is all that is needed; this would significantly simplify your wiring.