Thursday, February 21, 2013

Lead Acid Battery Desulfator

My parts have arrived, I've got the schematic sorted out (I think). I think I'm going to do a start to finish post on my current PCB fab procedure and make a new board, even though the other one seems to work. I've got the correct parts and the other pcb is together and working. I'd like to see if they work differently.

Desulfator-RevB - KiCAD 3D Render

The Theory:
When a lead acid battery sets discharged sulfate crystals form on the  plates, reducing the surface area and thus potential capacity of the battery. A desulfator pulses a high voltage / current spike into the battery that causes vibrations that knock the sulfate crystals off the plates.
The usual frequency is around 1khz, though the vibrations are usually in the mhz range. It's described as ringing a bell. The desulfator rings it a thousand times a second and the harmonic vibrations actually do all the work. Therefore the actual frequency doesn't seem to be particularly important.
It would probably be best to remove the battery from the vehicle so we don't pulse high voltage through computers, regulators, etc. It may take a week to several months to restore a battery anyway, so doing it out of vehicle is preferable anyway.

Desulfator Rev. B - Top
The Specs:
  • Unit is controlled by the venerable 555 timer, although an attiny would work and allow more control / monitoring of progress.  (Possible future design)
  • Should work on many voltages
  • May work on nicad's too.
  • This is basically a reference design. I want to see if (and how well) it works before spending a lot of time designing my own system (if I even need too, let's not go reinventing the wheel right?)
  • I'm trying for a compact design, altoids tin or smaller. preferably 50mm x 50mm, as if it works well I may get a batch made and try to sell them.

Desulfator Rev B - Bottom
  • As the battery size increases the amperage needs to increase. Two resistors can be replaced with pots to adjust the frequency and duty cycle of the 555, but this results in increased heat on the inductors. (we'll try the default settings to start)
  • Increased amperage requires bigger, more expensive inductors.
  • It's not self-powered, so a trickle charge is needed for operation.
  • No reverse - polarity protection, hook it up backwards and you will probably destroy it. (I'm going to look into this, doesn't seem it would be too hard to prevent this, definitely looking into this before selling any)
I'm basing this on Ron Ingraham's design, I don't think he followed the component numbering of the original, so if they say you can substitute something for R1, it may not be R1 in the schematic.

Update 1:
My parts haven't arrived and I've run out of patience. So I scrounged some inductors (about 600uh and 180uh) substituted some other parts and it seems to work.

Only found two problems:
1. I  missed a trace (somehow missed it in the schematic) and that prevented the duty cycle from dropping below 25% and caused the frequency to change with it. it was really frustrating. I eventually found it and added a bodge wire. Now RV2 adjusts frequency and RV4 adjusts duty cycle. I also added a 180K resistor to RV2, but that's just because I only had 100K pots (should have been at least 300K+. I highly recommend using pots for these. Then you can just dial it right in. (with a power supply and scope or by ear and watching that nothing gets hot)
2. Stupid (me) used non-polarised caps (C) instead of polarised caps (CP) in the schematic, so 3 caps were backwards.

I've corrected all these things in the schematic. I need to clean up the schematic and get it uploaded. I'm not sure if I'm going to roll all this into Rev B or go to Rev C. I have made a number of minor changes, but it's not a redesign by any means.

Rev. B Changes:
1. Replaced R2 & R4 with Pot's for adjustment. Resistors should fit instead, so it's optional
2. Added C41, C43 for more options on the low esr cap.
3. Noted in schematic different options for assembly.
4. Has component locations for several variations on the circuit. Board shouldn't be completely populated.
5. Might have placed the power connector on backwards. Oops.
6. PCB layout is Copyright for now. Free for Personal Use. I may release it to public domain eventually. I just don't want anyone selling these untested boards yet.

KiCAD + PDF Files

Make sure you add the cache library at the very top of the KiCAD libraries for the schematic. It's a PITA it doesn't do this automatically. The PDF's may or may not work, they gave me a fit. Make sure you print at 100%, no scaling.
Pictures of latest board in my PCB Fab post

Desulfator Information (mine is based on the low power - Ron Ingraham design)