We regenerate these type of alkaline batteries here: http://en.wikipedia.org/wiki/Alkaline_battery (not normal rechargable ones…)
The schematic is rather trivial. A voltage regulator brings down a supply voltage (which might be from a car battery) to 5V. An additional built-up voltage regulator generates an adjustable voltage of 1,95V. The reason why to use two voltage levels is to distribute the heat losses better, and to have the final voltage more stable. But if your power supply outputs a stable 4 to 6 V, you can omit the 7805.
An oscillator generates roughly 10 Hz with symmetric impulse/pause ratio. This signal is fed to the transistors below the batteries and switches them to the regeneration voltage. So this is “charging” with pulsed current. In some publications also reflex charging is recommended, but it works equally well with just having a “relax” time instead of a reflex time (inverted charge, short-time load). The impulse/pause ratio is 1:1, or the impulse/period time is 50%. This is conservative, as some publications use longer charge, less relax time.
The chemical process of regenerating is different from charging a rechargable battery. It is more an activating of hidden reserves than an exact reversal of the chemistry during discharge.
Don’t regenerate the battery if:
Regenerate the battery if:
Between these extremes it is your own compromise between saving money for new batteries or losing money due to spoiled appliances by leakage.
Sometimes I take some risk and use rather old batteries, but then I use it only in a low-level application. I put some tissue around the minus range and some silicone grease on all contacts to keep the damage in limits if the battery should leak.
How much regeneration?
It depends on the battery condition, age and your risk level. This is why we cannot so easily add charge stop criteria like as it is with NiMH.
A rule of thumb: Regenerate that the battery voltage has been 6 hours on 1.7V or 3 hours on 1.75V. The charge time is between 5 hours for a slight, and about 18 hours for a full regeneration.
After some experimenting you can use a time switch, converted to self-holding. Adjust a short time first, test, add some regeneration, write down the times and make your own lookup table by experience.
This seems to be a new idea. Measure the length before and after regenerating. If you use a metal caliper, put a piece of thin paper between, else you make a short-circuit. Increased length indicates that the minus-lid is being pressed out and the risk of leakage is rather high. Also, if the minus-lid becomes spherical, this indicates some danger. In another discussion in this forum, someone proposed to let the batteries jump back from a hard surface. I guess that this correlates with a belly shaped lid in combination with internal gas pressure, making something like a football structure. Springy cells = bad and non-springy=good.
Removing the voltage peak
The fresh regenerated battery has still 1.70 to 1.75V. As I guess that the voltage correlates somehow with the internal pressure, I take away this voltage peak by connecting a small bulb for a few minutes. When the battery is down to 1.65V it goes for quarantine.
I put the batteries on a piece of tissue for a week. If there is no leakage, they come into the refrigerator for storage, but for safety they are stored in tissue in a plastic bin.
How often can I regenerate the batteries?
About 5 times. The older the battery is, and the more it has been regenerated, the least it can take a regeneration. In case of an old battery, it is advisable to regenerate only with 1.70V and accept, that the use voltage is just 1.4V after regeneration.
Where to use regenerated batteries?
Where not to use regenerated batteries
The consequent damage of electrolyte leakage is so large that the saved money for new batteries is not justified.
Can it explode?
Unfortunately, not. You need to mix up your own gunpowder 🙂 The worst what happens in case of overcharging, is, that the minus-lid comes slowly out and the electrolyte drips out.
Is a leakage dangerous, is it poisonous?
The electrolyte is potassium hydrochloride. http://en.wikipedia.org/wiki/Potassium_hydroxide This fluid is a base (the opposite of an acid). It causes corrosion in electronics, can make stains in clothes and slightly attacks the skin. When you get it on your fingers, it is recommended to wash them soon. I tried it, the stuff is far less aggressive than lead-battery acid. Of course, don’t eat it.
Preventive actions against a possible leakage
Toilet paper or kitchen tissue around the minus range, small hole in the middle for the contact. silicone grease on all contacts. Possibly additional tissue in the battery pack. This limits the damage if a battery should develop leakage. The water of the electrolyte evaporates, leaving crystals in the tissue.
Flight pressure (!)
An airplane with pressure cabin has a pressure inside, corresponding to ca 2000m height. This reduced atmospheric pressure is already enough to make freshly generated batteries leak.
What about rechargeable alkaline batteries, RAM?
In Europe we have Accucell and I-go-green, in the USA among others Rayovac and Grandcell. The difference to a one-way cell is mostly the pressure-resistant casing, the internal chemistry is nearly the same. I made my experiments with http://www.accucell.de/technik.php They can withstand more regeneration cycles than a one-way battery but also start leaking after some years of usage or ca 5 charges.
A special effect
It can happen, that during regeneration, or some time afterwards, the voltage suddenly drops down to 0.4V and below within a few hours. In this case, crystal growth inside the cell has caused a short-circuit. It happened to me in 3 or 4 cases out of ca 100. The battery is then dead and has to be disposed.
Homepage of Rolf Zinniker, Switzerland: http://www2.ife.ee.ethz.ch/
Günther Hager, Modellflug und Elektronik http://www.aero-hg.de/
describes a very simple schematic. It needs active supervision, as there is no voltage limit (in german)
Elektor, 2002 Schaltungen, Page 4216 there is a simple reflex regenerator.
Elektor, 2002 Schaltungen, Page 4491: Alkali-Akku Lader, more sophisticated schematic.
Note: Elektor (Elektuur) is available in dutch, german, english in parallel. Comes originally from the Netherlands.
ELV 5-2000 Ladeschaltung für RAM-Zellen im seriellen Betrieb. A balancer is described which allows the use of solar cells to buffer RAM cells to make the lifetime longer.