Years ago I bought a rechargeable torch as a present for a friend of mine who was a fisherman. For some reasons I was not able to give him the present. I put in the basement and forgot about it. I found it few months ago again and decided to use it. I tried to charge it for hours, but the only result was that the LED lights were glowing very dimly. I was curious where the problem is and I dissembled it. I found that inside the torch was installed a lead acid accumulator battery. I tried to charge the battery in different ways - but without any success. The main problem of the lead acid batteries is that if they are not used for a long time, they sulfide and it is impossible to use them more. The only way is to try to desulfide them. For that purpose a desulfator is needed. Some research in Internet lead me to this site. The main credits for this project go to Mikey Sklar. The desulfator, which I created is based on his work, but :

it is done with very cheap parts and costs less than 7-8 USD
It can be reproduced very easy and does not require any knowledges of microcontrollers, their programming...etc. - it can be done even by people without any experience in the electronics.

ATTENTION: In this project a work with high voltages, which can be dangerous for your life is required and safety requirements for work with such voltages must be strictly followed. Some of them : The devices shall be inserted in the outlet socket only in assembled state and touching any part of the device, when under high voltage shall be avoided. Shall be used isolated high voltage clips. The battery terminals shall be connected or disconnected with the device only if the device is not inserted in the outlet socket. Only when the battery is connected, you can insert the device in the outlet and only when the device is disconnected from the outlet, you can disconnect it from the battery. The other advices given in the site linked above are also valid.

Step 1: Schematic and Parts

The schematic of the device is very simple. It is shown on the first picture. When assembling such device the main problem always is where to insert it. The case must be small, electrically insulating, easy to use and nice looking. A lot of requirements :-). Wondering where to mount it I found that I have an empty case of Devolo ETH dLAN adapter. It seemed to me to be very suitable for the project. You will need also a small protoboard. A small push button is also required. The device uses three high voltage ceramic capacitors. Four diodes from the type 1n400X are required where X>=4. You can use also Gretz assembling for voltage over 300V. In this design instead to use microcontroller and LCD display to show the current battery voltage, I decided to use voltmeter LED module. It costs less ten 0.7 USD. It must have 3 wires and must have max input voltage of 100V (voltage spikes in the beginning of the recovery process can reach even 100V). To connect the PCB of the device to the case pins I used the contacts of PC MOLEX connector. The easiest way to supply the LED voltage meter is to use some separate, but very small AC/DC module. I had one like these adapters (the coloured ones, which costs less 1USD) and I cut it and extracted the ACDC module from it. having all parts the assembling can begin.

Step 2: Soldering Works

The extracted from the MOLEX connector contacts I fixed with epoxy glue to the pins of the dLAN case.

On the PCB I soldered the capacitors in this way : two of them connected fully in parallel, the thirds first terminal connected to the other caps terminals, the second terminal connects through the push button to both caps second terminals. In this way I can have two or three capacitors connected in parallel and control this with the use of the push button, I soldered the PCB with the caps to the MOLEX contacts. The small ACDC board I fixed in left of the capacitors board with epoxy glue and soldered its terminals in parallel with the AC input of the capacitors board.

Step 3: Mounting of the LED Voltmeter

For the LED voltmeter I cut a small hole in the front of the case, under the HV capacitors. I fixed the voltmeter again with epoxy glue. Its supply terminals I soldered to the ACDC 5V outputs ( the USB connector was removed before the fixing of the board in the case). The voltage sensing wire was soldered to the positive output of the capacitor board. The ground nets of both boards were shorted together.

Step 4: Fixing the Charging Cable and Closing the Case

To close the bottom of the case I used a piece of PLA plate, printed by 3D printer some time ago. the cable was inserted through a rubber bushing. I soldered also the third capacitor terminal wire to the push button from one side. The other push button terminal was soldered to the node where other two capacitor terminals are joined together. After finishing all connections I closed the case and fixed it with screws. At the ends of the charging cable I soldered two insulated clips. Everything is ready now for test.

Step 5: Battery Recovery

First I connected the battery. After that I inserted the desulfator in the outlet. In the beginning the voltmeter became crazy showing very different voltages jumping from 90,70 to 4,5 volt and back. All this were accompanied with very scaring noises, but this continued a short time. After around two hours the voltage on the led stabilised in the range of 5-6 V. You can see this on movie here. The recovery process I started with three capacitors connected. After few hours, I took the device of the outlet, pressed the push button to disconnect the third capacitor and inserted the desulfator in the outlet again.I charged the battery until the voltmeter started to show more or less stable voltage of 7.2V. The whole battery recovery procedure is described in the site quoted in the introduction.

Step 6: Torch Assembling

I inserted the charged battery inside the torch case and mount over it the control board holder. I reconnected all cables according a picture, which I took months ago, before disassembling of the torch.

For all my pleasure now all were working perfectly.

I hope that this instructable can be very useful for you and you can save a lot of money recovering you dead lead acid batteries. The device can be used also for charging other type of batteries - how to do this you can find in the FAQ of the site quoted before.