I have often seen schematics of Joule Thief chargers for use in charging Ni-MH rechargeables from used alkaline cells. The idea is to boost the voltage of the used alkaline cell up to high enough to get current the flow into the Ni-MH cell. One setback is that conventional JTs are only 50% efficient, so half the power is lost in the conversion. That’s very wasteful and there should be some other way to solve the problem.
One way is to change the JT to my Supercharged Joule Thief circuit. This increases the efficiency to 70 to 80%. That’s a substantial gain, but why do we need a JT to begin with, if the voltage of the alkaline is higher? We can connect the alkaline to the Ni-MH, positive to positive and negative to negative, and current should flow, with no external losses at all.
But eventually the alkaline will eventually be depleted and the voltage will drop to the same as the Ni-MH. The current will drop to zero, and eventually will go negative as the alkaline cell takes charge from the Ni-MH cell. This is the time when they should be disconnected. The problem is how to do this automatically, without human intervention.
One simple way is to use a diode between the two cells. The cathode or banded end will go to the Ni-MH cell. We can’t use a regular diode. A regular diode has a forward drop of .5 or more volt, which is too much, and will stop any current from flowing. A Schottky diode such as the 1N5817 will be much better, with about .25 volt drop, and will let some current flow, but will still leave charge in the alkaline cell.
A better solution, I think is to put two alkaline cells in series. First, connect the two alkalines, one at a time, until each has discharged most of its charge into the Ni-MH cell. Then connect the two alkalines in series until they are discharged.
Again the problem is how to determine when the alkaline cells have given up most of their charge. In the case of the two alkalines in series, again a Schottky diode can be used with minimal loss. A higher current Schottky diode can be used, such as the 1N5820, which is 3 amps, or even higher current if you can find a cheap one.
An ‘ideal diode’ with no voltage drop would be even better. I’ve been thinking about how to control a reed relay, with a coil of low resistance and low voltage drop, so that it will act like a diode. The major problem is that any circuit is going to waste some power and reduce the efficiency. My Supercharged Joule Thief with the diode on the output may be the best solution, given its simplicity.
I would like to hear from anyone who has an idea how to make a magnet close the reed relay, and then using the coil, keep it closed until the current gets low. I think that this is at least one way to get the job done.
Perhaps a latching relay would work and be efficient, as it’s unpowered most of the time.
Yes, a latching relay would hopefully have no voltage drop. But connecting an alkaline directly to a NiMH battery might result in more than 10 amps current and could damage the relay. I’ve read about using a power MOSFET for a nearly zero voltage drop rectifier. These can handle many dozens of amps at very low voltages. I would need a source of about 9 to 12 volts to turn the gate on or off.