### Browsed byMonth: January 2011

Make a Joule Thief Battery Charger

## Make a Joule Thief Battery Charger

Recover the last bit of energy from a “dead” alkaline battery. When your modern electronics gadget turns off because the alkaline batteries are “dead” it just means the voltage in the batteries has dropped below a usable level for that gadget, which depending on the electronics that voltage could be around 0.9 VDC to 1.2VDC per cell.

I found this nice graph on http://www.powerstream.com/AA-tests.htm that shows the discharge curve for alkaline batteries. You can see that when the alkaline battery is below 0.9VDC there is not much usable energy left, but if there is 1.2VDC left in the battery there is about 28% of the energy left in the battery.

So what can I do with this “dead” alkaline battery? I can use a Joule Thief to make a battery charger that depletes the remaining energy from the alkaline battery and recharges a NiMh battery.

To make a Joule Thief battery charger is a quick and easy project. Here is the Joule Thief battery charger schematic:(to view full size images click image then click image on following page)

I am in the process of building a battery charger this week and will put data about this project as I charge batteries.

Some notes about using the Joule Thief to charge NiMh batteries:

1) This probably is not the most efficient way to recover the energy, but hey it is quick, cheap, and easy to do. The batteries were going to the trash so I might as well try to recover the lost energy from them.

2) The LED in the schematic probably uses half of the energy that would be recovered, but it is the only good way to see if the circuit is still running. You could also modify the circuit and charge up to 4 NiMh batteries in series (of course this will reduce the charge current, since the boost voltage has to increase).

If you use a white LED the circuit can be used as a night light, but the white LED (3.5V forward voltage) will consume about 79% of the energy when you are charging one NiMh Cell. If you charge four NiMh batteries in series the white LED will consume about 41% of the charging energy, the LED will be dimmer since the current will drop.

If you use a standard red LED (1.7V forward voltage) the LED will consume about 57% of the charge energy when you charge one NiMh cell, with four series NiMh cells the red LED will consume about 25% of the charge energy.

3) This circuit, if built properly, will run the alkaline battery down to 350-400mV which will truly make it a dead battery.

4) As long as your NiMh battery has a high enough capacity you will not overcharge it with this circuit, provided you do not exceed its C/10 rating (capacity/10). I found this on http://www.powerstream.com/NiMH.htm
“The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mA/Hr battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature–at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.”

5) It can take several “dead” alkaline batteries to recharge a 1500mAH NiMh battery, before I experiment I am going to estimate that it will be somewhere between 6-15 batteries.

Update: I built the circuit to charge 4 batteries in series. The battery charger circuit was working great for several days until the charged batteries got up to around 5.4v then they started to discharge. I was really perplexed for a while as to why this was happening. I finally figured out what went wrong, the LED reverse breakdown voltage was somewhere around 5.4V and it ended up destroying the LED and discharging the NiMh batteries. I have a new circuit design that will be more efficient and will charge the batteries quicker.

I have ordered parts for the new higher power joule thief circuit. I will build some and try them out, if they work out well I will add them as a new high power joule thief kit.

Using a Joule Thief to Harvest Energy from a Candle

## Using a Joule Thief to Harvest Energy from a Candle

Today I made a compact thermal electric generator ( TEG ) using a thermal electric cooler ( TEC a.k.a Peltier device ) and a Joule Thief.

This TEC produces about 1.8VDC when heated on one side and cooled on the other (this setup uses a candle as the heat source and cooling is from ambient air). The advantage of using the Joules Thief circuit in this setup is that it will boost low voltages to higher usable voltages. The open circuit output voltage of the Joule Thief in this circuit was about 31V Peak. It takes about 14VDC to forward bias and light the four LEDs.

The down side of this circuit is that there are conversion losses, but still it costs a lot less to buy one TEC then to buy nine of them and put them in series to get to the voltage required.

From the time I light the candle it takes ~36 seconds to light the LEDs, and they continue to get brighter from there. The LEDs stay lit for ~2 minutes after I blow the candle out, as the residual heat moves from the bottom heat sink through the TEC to the top heat sink, not shown in the video.

Here is a video of the circuit in operation:

Here is a picture of the major components, from left to right: top heat sink, thermal electric cooler, Joule Thief, bottom heat sink, and candle. (to see full size images click images, then click image on following page, still have to figure out why you have to do this to get a full size image? )

Here is a picture of the assembled cooler and heat sinks, I added several pieces of 12AWG solid wire and a 3/4inch copper coupler to direct the heat.

Here is a picture of the TEC Generator connected directly to a DMM without the Joule Thief boost circuit. The DMM is reading 1.792 VDC

And here the circuit is operation, producing ~14VDC to light four white LEDs. :

Joule Thief Charge Curve for Low Voltage Solar Cell Experiment

## Joule Thief Charge Curve for Low Voltage Solar Cell Experiment

I put a 0.5v 1A monolithic silicon solar cell on the front end of the Joule Thief circuit. Added a schottky diode and 2000uf cap on the output. I wanted to see how long it would take the Joule Thief to charge the cap bank. There are two 1000uf/50V caps in parallel to get the total 2000ufs. In full sun it took 3 mins 34 sec. to charge to 21.8 V.

Took voltage measurements every 15 seconds. Here is the data:

Here is the Joule Thief circuit that shows the connections for this experiment (to see a full size image click image then click image on following page):

Cheers!

Joule Thief Kits are Availible Now

## Joule Thief Kits are Availible Now

Our Joule Thief Kits are available now. They are on sale for \$4.99 each.

If you are student and you plan on using any of the Joule Thief products for a school project please contact us at sales@madscientisthut.com for a discount.

To purchase this item click –> Buy Joule Thief Kit Today

This Joule Thief Kit is available as a thru-hole board kit. This kit allows you to substitute components and includes all parts and a quality PCB. The PCB is double sided with extra copper around each hole on both sides, all traces are redundant top and bottom of the PCB. The PCB design allows for the components to be replaced many times as long as good soldering practices are followed. Substituting components will allow the experimenter to try for different voltages or efficiency.

Kit Contains:
* 40 inches red magnet wire
* 40 inches green magnet wire
*1ea – NPN TO-92 transistor
*1ea – torroid
*1ea – 1/4watt axial resistor
*1ea – PCB
*1ea – Ultra bright white LED

For detailed product information please consult the product forums section of this website. http://www.madscientisthut.com/forum_php/JouleThief

For assembly instructions click here: Assembly Instructions

BIG educational discounts on kit quantities of 20 or more. Educator kit also includes 1 assembled and tested Joule Thief, please contact us at sales@madscientisthut.com for more information on customized kits.