Desulfator build and Lead Acid Revisit
Posted by aonomus on July 23, 2009
So my previous post regarding reconditioning dried out sealed lead acid batteries mentioned the use of a desulfator. I based my design off of a common desulfator circuit which uses 2 inductors to push pulses of 50V into the battery at 1kHz, with the only major modification being replacing the circuit between the 555 timer and the mosfet (originally a diode for fast turnon, and a resistor for bleeding charge from gate) with a 2N3904/2N3906 pair to drive the gate (and the other modifications being part substitutions.
The schematic of my version is here. The main substitutions are the fast diode (MUR860 is even faster and the TO220 package has better heat dissipation), and different inductor types (same inductor). I made my prototype using perfboard, but at some point I may make a ‘real’ version with higher voltage spikes and better current. Not shown is the 3A fuse I used.
One observation I made while making my ‘junk box’ version of a desulfator was that the inductor type is important, because the frequency is around 1kHz, ferrite cores are too lossy and saturate easily. Using larger iron powder torroids is better than rod type ferrite inductors (larger inductors didn’t saturate, and iron powder is less lossy at this frequency).
I am absolutely sure I will revisit this one, with some more revisions and upgrades. Some that I had in mind were:
- Proper circuit board
- Higher power inductors and mosfets
- ‘In-line’ design with switch to enable or disable desulfation while charging
- Adjustable pulse frequency, length
As for the lead acid revivals, unfortunately most of them eventually did fail. What I have discovered is that while you can revive the lead acid batteries, there may be underlying conditions which caused them to fail in the first place, or damage that occured while it was in the dried out, discharged state. The main reasons that I couldn’t revive batteries were:
- Cell reversal: when at a discharged state, the other cells in the circuit charge the cell in reverse
- Shorted plates: for some reason internally, the anode and cathode of a few cells I discovered had no voltage or resistance, indicating a short.
- Plate damage: some plates exhibited signs of physical damage, and most plates with conditions (1) or (2) also must have spalled off some of the active (lead oxide) material, reducing capacity.
All in all, it was a good attempt at trying to save the batteries, I did get one battery good enough to provide a range of 10-12V (not the usual 12-14V) with 1 internal shorted cell, and current capacity to handle a car headlamp. Other batteries may have different conditions, and it would seem smaller batteries die quicker under these conditions. Caring for batteries is a labour of love, but its worth it for the capacity when you want it.
And on another note if you don’t like caring for batteries, get a NiMH or a LiPO/LiFePO4 pack and a good charger.