Joule Thief Power Bank

Hundred millions of electric batteries are thrown away each day because their voltage is not enough to be used. Although with lower voltage, these batteries still contain a lot of energy. In this way millions joules remain unused and are forever lost. The idea of the presented in this instructable device is to try to extract as much as possible energy of these batteries and to help in this way keeping our planet alive.

The principle of work of this device called further Joule thief power bank (JTPB) is based on the parallel work of multiple Joule thieves. The reason to use this approach is the following : There a lot of different step up DC/DC converter on the market, but all which I found work with voltages over 0.8V. Some of the Joule thieves (JT) I made was able to work at voltage close to 0.2V. As reference design for the Joule thief I used this site.

My primary idea was to force multiple JT to work in parallel. As beginning I created a PCB supporting 6 JT's.

Their output voltage should be applied to the input of step down 5V converter, which could supply or charge some external device. It also should supply an Arduino used to measure the battery voltage and to display it on some screen. I chose to use small OLED display because of the low consumption. During the design different weak points in the planed approach appeared and solved them in different ways. Keep reading to learn more :-)

As first step I designed a PCB containing 6 JT's. I soldered one of them and started playing with it. The main difference between this one and that shown in the reference design is that mine is not loaded by LED, but its input is connected through diode to big electrolytic capacitor - no DC load at all. I found that the output voltage increases continuously and can reach values over the breakdown voltage of the capacitor. Based on this I decided to use step down converter to 5V. This decision was not right because when loaded the whole system output voltage drops below 5V and the DC/DC converter is not able to produce exactly 5V but some lower voltage. The correct choice would be to use combined step up/step down module - it would produce 5V independently on the input voltage. Other interesting phenomena I found is that even without any connected battery the output voltage on the capacitor increases slowly with the time - that means the JT is able to harvest energy from the air.

As mentioned before, the JT board is the core of the device. It supports 6 parallel working JT's. On the pictures can be seen the parts needed for it :

6 ferrite beads

6 2N2222 BJT transistors

6 1 KOhm resistors

6 Schottky diodes - I used 1N5819.

The PCB was produced in PCBway in a short time with high quality. I had them in two weeks - well packed and in perfect state. The project files can be downloaded or PCB's can be ordered directly here.

I used 7 battery holders for different battery types. Two of them are connected in parallel - the coin cell holder and one AAA battery holder - in this way only in one of them a battery shall be inserted. We need also a power switch, a plastic case, an Arduino (I used nano), a DC/DC converter (step up and down), a 0.96" I2C OLED display module. Additionally I used a push button with embedded LED.

At first I cut a hole in the plastic box for the OLED screen. I fixed the module with epoxy glue. in the front of the OLED module I created a protection layer using UV resin.

As I wrote before my first idea was to supply the Arduino also with the produced by the DC/DC adapter 5V voltage. For that purpose I soldered a cable to its output. The nodule I fixed again with epoxy glue to the case.

Because the Arduino measures the voltage of the 6 batteries I used for that purpose 6 of its analog inputs : A0, A1, A2, A3, A6, A7, A8. The pins A4 and A5 are used for the I2C communication with the OLED. I soldered a flat cable to all of these pins. Additional cables were soldered to the 5V and GND pins (common ground and ground/ supply for the OLED module). Cables were also soldered to D8, D9 pins - used for the push button and the its LED.

All connections were done according to the schematic (only the connections of the switch to D8,D9 are not shown there). I mounted the battery holders on the top of the case using moment glue.

Here is the Arduino code used in the JTPB. You can download the needed libraries from the Adafruit site. The other missing libraries can be found in Github.

In the video can be seen the working of the JTPB.

Note that I supply the Arduino with external supply. The reason - the DC/DC converter is not able to produce 5V output voltage when the system is loaded, so the voltage reference for the Arduino ADC is not exact and the battery measurement is imprecise. Using combined DC/DC converter may solve this problem. It seems that for supplying higher output currents only 6 JT's are not enough. More of them must be connected in parallel. You have to collect your half-empty batteries and when you have enough of them, then you can switch on the JTPB. You can use also the device to check the battery level of each battery lower than 5V. When the power bank works with multiple batteries, this with the highest voltage works in the hardest load mode and loses its capacity faster than the others. In this way during the work the final voltages of all batteries becomes uniformly.

As conclusion : Despite its problems the described device can be used for charging and supplying of different electronic devices saving costs and energy resources