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# 3v High Capacity Rechargeable Battery?

#### PFAB

##### New Member
Hi There,
I’m trying to create a very simple circuit with one LED bulb which is battery-operated. (I know, this all sounds like LED 101)
However, I am looking for a bit of a different solution to my problem.

I had purchased some LEDs on Amazon in 2018 to create a proof of concept for a project. The one I am using is a 5mm 6v Pre-Wired Amber/Orange LED - Ultra Bright (3v, 4v, 5v, 6v) amber/orange LED, that I would like to power for very long periods (10 days at least). The LED I have is rated at 3v-6v, so my first attempt was to run this with a 5v battery pack, similar to the ones used to recharge cellphones. This works fine, but 99% of these charges have an auto-switch-off feature that turns off the charge if it doesn’t sense enough draw, which is the case with a 20mA bulb. The chargers turn off after about 30 secs on average (depending on the make).

Since this was not working, I then researched other options and came across some slightly different phone chargers, made by a company called VOLTAIC, which sells chargers of different capacities, all with a common feature of having the charger always on, no matter what the draw is. This solved my problem partially. The problem is that if I leave the LED on continuously, I will at most get about 2 and half days before the battery is completely drained. This surprised me given that the total battery capacity is rated at 6.4Ah*3.7V = 23.68Wh

From the manufacturer:
"The total capacity of the battery is actually the 6.4Ah*3.7V = 23.68Wh because 3.7V is the lithium ion battery cell voltage. The 5V USB output is the regulated output voltage and therefore is not the correct voltage to calculate capacity. Additionally, this is the nominal capacity assuming no losses. In practice we see that it is closer to 18-20 because of variation in the cells actual capacity, losses in the regulated output, and a low voltage buffer set at around 5% remaining capacity."
Still, one would think that that tiny bulb could run for longer. So, I am not sure why this might be happening.

I’ve now recently been experimenting with small votive candles that run on a CR2032 battery and seem to be able to stay on for as long or longer actually. How can a coin-sized battery provide longer power than a big brick?? This is a total mystery to me. The LEDs are 3V on these.

So here’s my question, if anyone would be so kind as to help me answer, and so that I can keep the little amount of hair that I haven’t yet pulled out from the top of my head:

Is there a way to:

-Power a single Amber colored LED from a rechargeable battery.
-Be able to have that LED stay on for long periods. (10 to 15 days)
-If only used for a few hours initially, be able to come back to a battery that has not drained after a few weeks of not being used (which happens with the current battery).

-Optional cherry on top: If the bulb could have a fading/breathing pattern, that would be tops.

Thank you for reading this far,
Any insight or help would be super appreciated. Any suggestions on a different approach would also be very welcome.

Cheers,
Fabrice

Here’s a picture of my latest set-up:

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Depending on which CR2032 the math said 20mA for 12 hours. CR2032 That battery was designed for very light loads and it will not last that long and the voltage will be down to 2V by then. You can get more life by going to 10mA.

LED amber. It needs 2V or more. LED So, using a resistor to limit the current, which you are doing, the brightness will drop as the battery gets run down. At 2V the LED is done.

There are very small ICs that will extend the life of the battery. They will keep the light output constant as the battery runs down.

The 240mAh battery lasts 12 hours. (simple math) So a 2.4Ah battery should last 5 days. (20mA)

Does the light need to go in all directions? If the light can be focused in one directions the light looks much brighter, and the current could be reduced.

Sorry I don't have picture right now. I have some small printed circuit boards from a job years ago. The boards are the same size as the CR2032 battery. Have a LED or more and a LED drives IC and a small microcomputer that controls the brightness, timers, push button on/off, blink rate. This project has been done before.

Battery: battery Here is a battery 3V, 2.5aH

I have used this IC to take any voltage from 2.4 to 6V and make a constant current into a LED.

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The CR2032 is a primary Li cell which stays near 3V for most of its capacity and by the time it has dropped to 2.85, the LED is hardly drawing any current but still dim as this is the typical white threshold voltage above which it quickly approaches about 12 ohm bulk resistance for a Vf rise of about 240 mV above 2.85 = ~3.1V which a new CR2032 might start at. So basically the LED never draws more than 20 mA and that drops over time,

This means if you had a rechargeable Li 3.7V cell your current depends the series resistance you add and that may be much more than 20 mA. Some cheap white LEDs might in fact be closer to 18 ohms due to fabrication methods and tolerances such that 20mA occurs at 3.7V or it might be 50 mA somewhere in between if you did not use a resistor. But you didn't say much about brightness or actual current, so I'll assume this to be your case. Then by the time the Li Ion battery is down to 3V where the CR2032 started under load, the cell has lost about 90% of its capacity or more if old.

You are right to assume the battery voltage must match the LED voltage, but they just don't regulate current well. So usually a little LC buck-boost chip that senses current is used in torches, which is your best bet with one that uses a rechargeable 18650 Li Ion cell. Why sweat over it.

BTW Amber LEDs are 2V and the dominant wavelength affects the shade of amber to yellow below 598 nm. I have thousands of ultra bright 5mm amber LEDs so if you want any just ask .

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This is an example of a boost regulator, that I would not expect you to build but would be 96% efficient with the LC components with very low ESR in the mohms and you would run two Amber LEDs in series to 4V boosting to 4V approx with say any 2.5 to 3.9V battery.

if you really want to make something, you'll need to tell us what limitations in construction you specify.

Since this was not working, I then researched other options and came across some slightly different phone chargers, made by a company called VOLTAIC, which sells chargers of different capacities, all with a common feature of having the charger always on, no matter what the draw is.
Just to clarify, what you are calling a "charger" is what's commonly known as a "power bank".

They use a lithium cell, plus a voltage converter to step that up to 5V. The converter part will be drawing power and draining the battery all the time it is active - that's why most types shut off when there is no load.

I'd suggest you start with a lithium cell such as an 18650 type, which are commonly between 2AH and 2.5AH (2000 to 2500mAH).

Add a a single cell USB charge controller module, or use a stand-alone charger and put the cell in it to recharge If you use a cell with a clip-in holder for your project, you could have more than one cell and exchange them.

Then add add a LED driver to run the LED at a constant current, such as one of the circuits that Ron & Tony have suggested.

With nothing running from the cell when the LED driver is not switched on, it should hold its charge for months while idle.

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