Joule Thief Charger

Let your dead battery give life to another! An open circuit Joule Thief can put out 50 or more volts. Enough to charge a AA or AAA Nicad or NiMH rechargeable battery.

Step 1 Make the Charger

Use this schematic to build a standard Joule Thief circuit with the added diode.

My joule thief uses twisted network wire passed through a small ferrite core. I use 6 turns of wire. You can find a core from a burned out compact fluorescent bulb. You can see how others have wound the coil and built the Joule Thief, since so many have done so. Just add a diode and LED in series with the charging battery. The LED is useful as a charge indicator.

A high-speed schottky diode would be the most efficient. The 1N4005 was handy at the time and works.

Comments

[blinkyblinky]

How many turns go BEFORE the 6 turns?

[botronics]

You wind the turns through the core as a pair. So both are 6 turns.

[blinkyblinky]

So technically If I wond 30 turns would it make any difference?

[botronics]

The turns is not critical for it to work. The frequency will usually go down with more turns. Experimentation is in order.

[blinkyblinky]

Thanks. On my digital meter, when I measured and checked for voltage I got 0.1 volts less than the battery had. What happened?

[namzer0]

how much is the current draw? when not charging/open circuit and when charging?

[mitchiko]

You cannot measure the current draw in the open circuit only the voltage.

[freenergyfuture]

I am very interested in Joule Thief's and i am curious how to place a variable ohm resistor into the circuit? please reply asap.

[agatornz]

i like the idea of this for hiking - using a combination of nimh rechargables and alkalines for my electronics - it would be great to be able to charge the former - is there some way you can include a simple "charge level indicator" in the circuit so one would know how well charge the rechargeable is?

[botronics]

Your better off using a solar cell for charging. You can attach one to your backpack. A picaxe mcu could be used as part of a circuit to measure amp-hours when charging.

[twighahn]

can u put this in layman's terms?

[ab3t]

What if the batteries have a voltage of 3.7 volts used? What should be changed?

[botronics]

Nothing, the battery voltage will adjust itself. Just feed it electrons.

[mikedoth]

Stupid question but why doesn't the LED blow from the high voltage alone?

[SwingNoob]

because your using a 1k resistor as well

[mikedoth]

I'm still learning but will a 1k resistor drop 52.6 volts down to 1.5 to charge the battery? How about a voltage regulator?

[botronics]

No regulator is needed if you are charging a battery.  The voltage is high because it is an open circuit, but the current is low.  When the battery is connected, the voltage will drop to the voltage of the battery. Same thing with a LED connected to the output.  The voltage drop of the led will keep the voltage down. Think of the LED as a zener diode.

[leodahsan]

more like a constant current source? ;)
btw.. what are better options than the 2n2222? I need something that generates 40v @ 20uA (the more the better ;P) from anything around 1 to 3.4v. Anything in this range, but I don't know how to start..

[knektek]

is this like a campain for charging up batteries. 'All dead batteries please save a ni-cd's life today and donate 500 milli amps each month'.

[dasimpson1981]

so hwne the battery is charge dose the led go out ?

[dasimpson1981]

i have tried this and well the charge is false the voltage is right but there is never any ampage to run anything i have tried charging then running down over and over and dose not seem to improve or work

[CyborgGold]

you should have done this logging with the rechargeable battery starting out dead... this one just hovers at full charge... actually it appears to drop slightly in charge.

[botronics]

I discharged the battery into a load till its voltage was .9 volts. That is the recommended lowest you can go without damage to the cell. When a battery just sits there open circuit, the voltage will rapidly rise to a higher level, which it did. During charge the voltage will only rise slowly. The battery was no way fully charged. It was only a test to see if the circuit would raise the voltage and allow current to flow in a charging direction. Given enough time and dead batteries, the battery will charge.

[flying pie]

circuit is too small and can u add a solar panel to the "charge donor " so u can charge it without using up a battery to charge a diffrent battery

[A good name]

Are you serious? The circuit is too small? Adding a solar panel would defeat the purpose. You're using a next-to-dead battery to charge another battery.

[botronics]

Next-to-dead batteries still have a lot of joules left. I have nite lites that run for months using next-to-dead batteries. We are just transferring whats left to something more useful.

[tinstructable]

You know you don't need that diode... the led is acting as a diode. Light Emitting Diode. Just a tip.

[botronics]

There might be a negative going spike as the circuit oscillates that can harm the led. I could just use a diode and forget the led, but the led acts as a indicator.

[electrofreak]

What is the frequency range for these types of circuits?I'm curious because if the frequency is high enough a fast type diode could be a better choice. Also it will help in determining the size of the capacitor if one decided to filter the output.

[botronics]

The frequency is in hundreds of kilocycles. Watson's e blog has a lot of information on joule thief circuits.

[lolzertank]

Wouldn't the led eat up a lot of power? Assuming that its a red led (2v), then the total load is 0.7v (diode) + 2v (led) + ~1.3v (battery) = 4v * (your charge current). Without it, it would be 2v * (your charge current) total, increasing the efficiency dramatically. Of course, since you're using dead batteries, this might not be important at all.

[webmasterpdx]

 Shottky diode shouldn't be .7V. Probably could use a germanium too. Either gives you capability of working with lower input voltage. personally, I'd get rid of the LED too (maybe have a pushbutton to see if it's working. You could also pass it through a capacitor to remove the DC component and the try a full wave rectifier. Might get better results....I'm not sure.

[yourdiyguy]

I am not able to get this kind of voltage out of my JT what could i be doing wrong? My meter always shows the around the same voltage of the battery I am using. I am able to get the LED to light up. Could it be the type of meter im using?

[botronics]

High voltage is only present with an open load and a high impedance meter such as a DVM.

[yourdiyguy]

thanks for the help. i have been pulling my hair out trying to figure out what was wrong. im new to electronics and projects like this get me learning alot really fast, sometimes i get lost : ) so thank you. i now have a reason to buy something new, ha.

[imakethings]

can i charge my nokia 3.6v Li-ion battery with this?

[ironsmiter]

See the reply's to derbert, below.

Short answer, NO!!!!!!!

Long answer... you can use a joule thief to supply power to a Li-ion charging circuit, but not to the battery directly. Initially, it's just wildly reckless, but once cell voltage rises to over 4 volt, it becomes Russian roulette, with a fully loaded revolver. the battery WILL be destroyed.

[derbert]

can i charge a 7.2v 200mAh LiPo with this?

[Re-design]

No.

[lilpepsikraker]

You need a 7.2 donor battery with a charge. LiPo batteries need a smart charger, you don't want your house to burn down. A $50 charger saves your house.