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 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.

AA Alkaline Battery Discharge Curve @ 100mA
AA Alkaline Battery Discharge Curve @ 100mA

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)

Joule Thief Battery Charger Schematic
Joule Thief Battery Charger Schematic

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
“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.

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