The "Reverse Joule Thief" Battery Charger

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Author:qs

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Here is a totally different take on the Joule Thief (JT) circuit commonly found in garden lights. Instead of charging a 1.2v battery directly from the solar cell and converting the power to run a 3-volt LED, we'll be using the JT to convert the output from the solar cell and charging a Lithium battery first. Then when night falls, the battery is used to drive the LED directly.

 This method has some advantages: (1) the Lithium cell that was chosen here (and avialable for $2here) has an output of 3-volts, which can drive a White LED directly; it also has a huge capacity (800mAH) and very low leakage. (2) The solar cell normally can only charge the NiCd battery in full, direct sunlight, but, with the JT circuit, it is able to deliver power to the Lithium cell even on overcast days.

Step 1The 'Reversed' layout.

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A look at the circuit will tell you this is not a run of the mill JT configuration. Most obvious will be the fact that there is only a single coil involved (the 220uH) - we are using a second transistor (Q2) and C1 to take over the timing requirements. This allows us to use a wider range of coil values, as well as operate over a larger voltage range.

Besides reversing the charge / discharge order, this circuit also reverses the location of the driver transistor and the coil, but wait, that's not all! The transistors all have reversed polarities, and even the output voltage is reversed!

No, it isn't an error! Diode D1, the LEDs and the charged battery all have their polarities  reversed! That's because this Joule Thief is configured as a voltage inverter. This arrangement was chosen due to its advantages for this kind of application.

To improve efficiency, the traditional JT relies on a fairly constant battery supply (over a millisecond or so) to give it a boost when it is delivering power. With the limited output from a Solar Cell, we have to store all its power in C2 and feed it into the Lithium in one big pulse, meaning the capacitor will be "empty" for the few critical millisecond, cancelling the 'kick' the normal JT requires to work well.

Our 'Reversed' JT circuit will work as a regular JT - without the 3v Lithium load, an input of 1.2v will light up the LEDs quite nicely. Not strictly necessary, the LEDs are there so you can SEE the system working, and also to prevent the battery overcharging.

Solar Cell. 2-volts with 100-ohm load

Q1,Q3 BC327 PNP. Can be any low-signal amp of sufficient current rating (>100mA)
Q2 BC337 NPN. Most will work but if you change Q1, Q2 or L1, you may need to adjust R1 for best performance (Try 3.3k to 15k)

D1 1N4148 or 1N914 or similar
LED1 Blue or White LED
LED2 Red LED
LED3 100mA (1/2W) White LED

C1 220pF. Can be 150-500pF
C2 50-200uF

R1 10k-ohm
R2 330-ohm (use 470-ohm for longer run time)
R3 3.3k-ohm
R4 6.8k-ohm (use these values instead of the one on the schematic)
R5 100-ohm. Go as high as 220-ohm for lower brightness.

L1 100-500uH. Many home-made ones will work.

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The second image shows the waveform measured at the top of the coil. The portion above the tag (2) is the charge stored in C2 fed into the coil. The sharp negative going pulseis the battery being charged.

So, is this safe for the battery? What if it were a 2800mah 3.7V li ion battery?

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May 30, 2010. 7:23 PMqs (author) says:

Once the battery reaches about 3.3v, the LEDs will turn on to make sure the battery doesn't overcharge. A 3.6v Li-Ion at full charge is 4.2v, so the circuit as shown will charge it to about 80%. By replacing LED1 and LED 2 with a 4.8v zener, you will charge the Li-Ion to 100%.

NiMH Battery Charging Basics

 Basics

NiCad and NiMH batteries are amongst the hardest batteries to charge. Whereas with lithium ion and lead acid batteries you can control overcharge by just setting a maximum charge voltage, the nickel based batteries don't have a "float charge" voltage. So the charging is based on forcing current through the battery. The voltage to do this is not fixed in stone like it is for the other batteries.

This makes these cells and batteries difficult to charge in parallel. This is because you can't be sure that each cell or pack is the same impedance (or resistance), and so some will take more current than others even when they are full. This means that you need to use a separate charging circuit for each string in a parallel pack, or balance the current in some other way, for example by using resistors of such a resistance that it will dominate the current control.

The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

The minus delta V bump that is indicative of end-of-charge is much less pronounced in NiMH than NiCad, and it is very temperature dependent. To make matters worse, new NiMH batteries can exhibit bumps in the curve early in the cycle, particularly when cold. Also, NiMH are sensitive to damage on overcharge when the charge rate is over C/10. Since the delta V bump is not always easy to see, slight overcharge is probable. For this reason PowerStream does not recommend using minus delta V as a termination criterion for nickel metal hydride batteries.

As the battery reaches end-of-charge oxygen starts to form at the electrodes, and be recombined at the catalyst. This new chemical reaction creates heat, which can be easily measured with a thermistor.. This is the safest way to detect end-of-charge during a fast charge.

Overnight Charging

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 mAH 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. Examples of this kind of charger are shown athttp://www.powerstream.com/NiMHWM.htm . This charger uses a microprocessor to report the state of charge via an LED as well as performing the timing function.

Faster Charging

Using a timer it is possible to charge at C/3.33 for 5 hours. This is a little risky, since the battery should be fully discharged before charging. If the battery still has 90% of its capacity when the timer starts you would have a good chance of venting the battery. One way to ensure this doesn't happen is to have the charger automatically discharge the battery to 1 volt per cell, then turn the charger on for 5 hours. The advantage of this method is to eliminate any chance of battery memory. PowerStream does not currently have such a charger, but the microprocessor board used in the C/10 charger http://www.powerstream.com/NiMHWM.htm could easily be modified to do the discharge. A power dissipating package would be needed in order to dissipate the energy from a partially charged battery in a reasonable amount of time. Another example of a 3 hour charger is http://wwww.powerstream.com/9vnmh.htm . This is a very inexpensive microprocessor based 9 volt "transistor radio" battery charger that that drops to low current when the battery voltage indicates a full charge.

Fastest Charging

If a temperature monitor is used NiMH batteries can be charged at rates up to 1C (in other words 100% of the battery capacity in amp-hours for 1.5 hours). The PowerStream battery charge controller shown inhttp://www.powerstream.com/product3.htm does this, as does the battery management board shown inhttp://www.powerstream.com/product5.htm.

This board also has the ability to sense voltage and current for more sophisticated algorithms.

When terminating on temperature rise the dT/dt value should be set at 1 to 2 degrees C per minute.

Trickle Charging

In a standby mode you might want to keep a nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and .05 C. The voltage required for this is dependent on temperature, so be sure to regulate the current in the charger.

dT/dt versus -dV/dt

These two termination methods work well for NiCads, and are both applied to NiMH as well. dT/dt measures the temperature rise at the end of charge. After the battery is fully charged it starts new chemical reactions in order to absorb the unneeded current. In nickel hydroxide style batteries this consists in generating and recombining oxygen. This process heats the battery. The sudden increase in temperature rise can be used to terminate the charge.

Another effect of the oxygen generation/recombination cycle is to depress the voltage of the battery slightly. If you can detect this voltage depression you can use this signal to terminate the charge. Of course, -dV/dt is the easiest because it doesn't require a temperature sensor. The best method for NiMH is the dT/dt method. There are two main reasons. With the NiMH battery the voltage depression is smaller, and harder to detect than with the NiCad battery. This almost always ensures an overcharge, which will limit the total number of charge/discharge cycles before battery failure. Second, a new NiMH battery has false peaks early in the charge cycle, and so the charger will terminate too soon.

There are new algorithms that use microprocessor control to use the -dV signal to detect the end of charge. These can work very well and several of our chargers use this technique, which involves pulsing the charger on and off to do the voltage measurements. This technique seems to be sensitive to imbalance in the capacity of the cells. The dT/dt is still more reliable, especially for large packs, but in cases when only two wires are available solutions are now available.

The Ultimate Charger

Sometimes the most important issue is the lifetime of the batteries or the total lifetime cost of the system. In this case PowerStream is in a good position to offer the ultimate charger because of our wide experience in microprocessor controlled battery chargers and power supplies. Specs for the ultimate charger are:

1. Soft start. If the temperature is above 40 degrees C or below zero degrees C start with a C/10 charge. If the discharged battery voltage is less than 1.0 Volts/cell start with a C/10 charge. If the discharged battery voltage is above 1.29 V/cell start with a C/10 charge.

2. Option: if the discharged battery voltage is above 1.0 Volts/cell, discharge the battery to 1.0 V/cell then proceed to rapid charge.

3. Rapid charge at 1 C until the temperature reaches 45 degrees C, or the dT/dt indicates full charge.

4. After terminating the fast charge, slow charge at C/10 for 4 hours to ensure a full charge.

5. If the voltage climbs to 1.78 V/cell without otherwise terminating, terminate.

6. If the time on fast charge exceeds 1.5 hours without otherwise terminating, terminate the fast charge.

7. If the battery never reaches a condition where the fast charge starts time out the slow charge after 15 hours.

8. Fuel gauge, communication to the device being powered, LED indicators all possible.

Custom design and manufacture of state-of-the-art battery chargers, UPS, and power supplies for OEMs in a hurry!

NiMH & NiCd rechargeable batteries charging time – mAh – mA – hours :
Battery type : 2000 mAh NiMH size AA rechargeable battery Require  : 6.0 hour(s) to be fully recharged with 400 mA charger
Step 1: Enter a mAh capacity of your Rechargeable Battery * Enter a mAhr capacity value as shown on one battery only (to charge 1 to 4 batteries at once.) * Precision in resuslts is how many numbers after decimal point (0 – 9)
Enter a mAh number of your battery (ei: 2400 ) : mAh   –   Precision :
Step 2: Select a rechargeable battery size/type to calculate from	Step 3: Select a battery charger mA current output rate for time results
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