Thursday, June 18, 2009

Homemade DIY Battery Desulfator / Charger

I had posted a video on a few weeks ago showing my first very crude and simple battery charger/desulfator. Someone commented on how well it worked for repairing nicads. I'd like to explain that a bit. The nicads don't get sulfated like lead acid, but they do degrade. Little microscopic filaments that look like hair build up and eventually short out the plates. When you hook up a regular charger and it is trying to charge at half an amp, it does that by putting out a fixed voltage. The battery plates are so "fuzzy" that it never takes a charge.

But the desulfator / charger is a constant current device. If you set it for half an amp by using a 12 microfarad "Run" capacitor, then it will push half an amp at whatever voltage it needs. It can go as high as 170 volts DC. And then it pulses it 120 times a second. Believe me, that fries those little hairs pretty quickly. Then the voltage drops down to normal and you can charge for a few hours after that. I normally put a timer on it so it doesn't overcharge.

If you have a nicad pack that is charged at half an amp, don't set up the charger to push 3 amps into it. (or if you do, don't do it for too long) That is bad for the battery and could be dangerous. You change the amps by putting in a different capacitor. A 24 MFD capacitor will charge at 1 amp. A 50 MFD would be just over 2 amps.

You can see the post here that talks about making one.

There are so many old nicad battery packs out there just waiting to be fixed.



  1. Just arrived and new here, but... I find all those things simply like what I was caring and looking for for a while!! Thanks!!

  2. Hi, sounds great. Is it possible to use this on a system with 6 2Volt Batteries connected in line with each 1080 Amps?
    How can I build one myself?
    Please contact me using

  3. Hi, sure it can charge any battery or cell up to about 150 volts or so. It puts out 170 volt or so DC pulses. Check here to see how to make it.
    I have charged nicads, nimh, lead acid, and gel cells. Don't do lithium ion though. Just change your capacitor to the appropriate size for your battery.

  4. Richard, I have a question about desulfating batteries for you. I have 4 6V batteries, and while I know that the IDEAL way would be to desulfate battery 1 first, then battery 2 and so on, I'd like to cut down on the length of time it takes by hooking up all 4 batteries at once. Do you think that hooking them up in series or parallel would be better? How about hooking them up in two 12V banks and paralleling the banks? I know that it'll handle large banks, but I'm wondering which configuration is better.
    I'm new to this desulfating stuff, but I've built myself a 25/175 desulfator according to your design and am eager to try it out on these "dead" batteries.

  5. My power supply is 230/40v so would the capacitor etc need to be different?

  6. Hi Richard, please excuse my ignorance here. In New Zealand mains power 230v plus or minus 6% and 50 Hz. Need to know what adaptations to make if any. From your last answer I assume a 16uF cap for low power would give 1 amp and then two more 50uF in parallel would give approx 7.25amps. what wattage isolating trannie required? Would a 35amp rectifier be OK? Are any changes needed for the shunt? Would it be simpler to keep your specs and step my voltage down to 120v?

  7. Brian, well it might be easier to just step down to 120 volts. But the 50 hertz might still make the numbers slightly off. But close enough and that could be your isolation transformer as well.

    You are right on your numbers above at 7.25 amps for your voltage. At that amperage, you would want 150 watt transformer per 12 volts that you are charging. So, for example, use 300 watt transformer if you are charging a 24 volt bank. And the 35 amp rectifier is more than enough and the shunt would stay the same.

    But if you step down the voltage and use the same capacitors, then that would be 4.6 amps instead of 7.2. In that case you would need at least about 100 watt transformer per 12 volt battery.


  8. Hi again Richard,
    Thanks so much for your help but I have one more question. Do the capacitors have to be run capacitors and do they need to be rated for my mains voltage of 230v or could voltage rating be higher than that? I purchased your "Poor Man's Guide to Welding with Batteries" a few days ago and am trying to get this desulfator going for that. Cheers

  9. Brian,

    They have to be Run capacitors. If you use the start capacitors, they will smoke after about 30 seconds and eventually flame up. The rating should be higher than the mains. Just multiply the mains by 1.4. So, 230 volt mains needs a 330 MFD or better. If you don't have big enough capacitors, then you can put two in series to raise the voltage rating. Make sure they have the same MFD rating though.

    Thanks for reading my welding book. What did you think?

    You should've seen what happened when a friend and I used a 24 volt forklift battery to weld. The rod melted right through a 1/2 inch thick piece of plate steel. I think it was about 30,000 watts of raw power.


  10. Richard,
    Thanks for that. I am impressed with the welding book and believe it is just what I require for the small amount of welding I do. All I have to do first is get the desulfator going. I can get the required run capacitors at 440v so hope they will work OK.

    Currently looking at Kevin Dixons battery reconditioning book which I guess you are affiliate to so no real use asking for an unbiased opinion about it. Trying to decide between it and another. Seems to be some validity to both.

    Many thanks for your time and generous help.

  11. Hi Richard,

    In India, the mains power is 230v/60Hz. What will be the adaptations in this case.


  12. Hi Richard
    Can you elaborate a bit more about the desulfating side? how long does it take to desulfate a severely sulfated battery (say 50 AH lead acid battery - assuming that plates are not corroded and no shorts inside the cells).
    I'm asking because from your explanation you mentioned that it gives 120 cycle / sec. , while normal desulfators uses 1 - 5 kHz and it needs about one month to repair such batteries.
    My second question if I may, can I replace the stainless steel rod (shunt) with a resistor(s) and what values do you recommend?


  13. Ranma,

    Yep, just hook in series and hook your desulfator to the two open terminals. Just make sure the polarity is correct. In other words, hook up positive to positive, etc.


  14. sorry wrong account oh well.

  15. thanks for the reply i was thinking that but just wanted to make sure. Also keep up the good work on the blog.

  16. very nice work , keep it up.

    How much it may take to desulfate a working but low gravity battery say 1200 all 6 cells