Information taken from http://www.shaka.com/~kalepa/desulf.htm .
The links on this page will take you to the appropriate webpage off my site, however the basic information is here in case the original author's website doesn't work.
Lead Acid Battery Desulfation Pulse Generator
Some help and information for builders
(Last update Sept. 2002)
http://www.shaka.com/~kalepa/desulfparts2.htm New "peak reading" kit with digital display so you can see the desulfator making progress on rehabilitating your battery.
This page is intended to provide builders of the battery desulfator circuit, as shown in Home Power Magazine, issue number 77, with additional information. (Here is the original article, PDF format.) The volume of email has made it clear that a few more details are needed. The use of inductors and high frequency pulses makes this circuit require more than a passing DC electrical knowledge to get working properly.
To start with, take a
look at this
short note on lead acid battery
chemistry and the sulfation process.
Begin by making sure the 555 is putting out the proper drive waveform to the MOSFET. This is best done with a scope. Using trim pots in place of R1 and R2 will allow for a wide range of conditions and battery types to be accommodated. If you are unfamiliar with 555 operation take a look at this tutorial: http://www.uoguelph.ca/~antoon/gadgets/555.htm, the astable mode equations are about half way down the page. The illustration below shows what our drive signal should look like:
The frequency of the pulse is close to 1000 Hz. The width of the narrow, negative going part controls how long the MOSFET is turned on. The longer it is turned on, the higher is the peak amperage delivered to the battery, up to a point. At present, it isn't known whether it is better to pulse frequently with a small amperage pulse, or whether a slower, higher peak pulse is better. I am leaning towards the latter at the moment, as a result of the content of this patent.
NOTE:If you are having trouble with things getting too hot, it is likely that the 50usec pulse width is too long, resulting in L1 saturating. Also, C4 should not get warm, but will if it is a marginal unit with too much ESR (effective series resistance).
Here is the current pulse created by the circuit. This was taken using a .1 ohm resistor in series with the negative lead. The circuit is showing, given the scope setting, a peak current of over 5 amps. There is quite a bit of high frequency ringing on the leading edge of the pulse. This peak current can be reduced or increased by changing the width of the 555 drive pulse to the MOSFET. For small batteries, it might be wise to reduce the pulse width. For larger units, using heavier duty inductors, a longer pulse would give more current. As the 555 goes low for ~50usec, the MOSFET is turned on. Current flows into L1 from the stored charge in C4. The magnetic field around L1 builds up until Q1 turns off. The field now collapses, and as a result the inductive kick back forces a large current spike which goes from the -12 volt terminal, through D1, through L1, through C4, and into the +12 volt terminal. This is all over in less than 100 usec. The rest of the cycle allows C4 to slowly recharge, at a current of around 50 mA, through L2 until the next firing of Q1. Check the DC current drain of the circuit. It should be less than 100mA, preferably less than 50mA. If you increase the peak current output, the efficiency will go down, and so the DC current drain will go up.
The tireless Don Denhardt has supplied the following data which shows, for the Delevan chokes sold through Digikey, the relation of pulse width to peak current pulse. It shows the diminishing returns due to saturation of the inductors:
Tables showing peak amps at various pulse widths. (Pulses in microseconds.) 6 Volt battery IRF=IRFZ44N IRL=IRLZ44 L1=(See table) L2=1000uH Pulse IRF IRL IRF IRL Width 220uH 220uH 120uH 120uH 10 .75 .75 0.8 0.8 20 1.2 1.0 1.2 1.2 30 1.6 1.3 1.8 1.6 40 2.0 1.8 2.0 1.8 50 2.2 1.8 2.2 2.0 60 2.4 2.0 2.6 2.2 70 2.8 2.1 2.8 2.5 80 2.9 2.2 3.0 2.8 90 3.0 2.5 3.6 3.1 100 3.2 2.8 4.0 3.8 110 3.6 2.9 4.0 4.0 120 4.0 3.3 4.0 3.9 130 4.6 3.8 --- 3.8 140 5.0 4.2 --- --- 150 5.1 4.5 --- --- 160 5.0 4.6 --- --- 170 5.0 4.5 --- --- 180 4.9 4.1 --- --- 12 Volt battery IRF=IRFZ44N IRL=IRLZ44 Pulse IRF IRL IRF IRL IRF Width 220uH 220uH 120uH 120uH 330uH 10 1.2 1.0 1.5 1.2 0.8 20 1.9 1.6 2.2 1.8 1.2 30 2.4 2.0 3.0 2.4 1.6 40 3.0 2.5 3.5 3.5 2.0 50 3.5 3.0 3.5 4.0 2.2 60 4.0 4.2 3.2 4.0 2.5 70 4.0 4.8 --- --- 3.2 80 4.0 4.8 --- --- 3.5 90 4.0 5.0 --- --- 3.5 100 4.0 5.0 --- --- ---
A few words about the "several magehertz resonance." One of the most frequent questions is about the fact that the drive frequency is at 1 khz, but the resulting vibrations in the battery are at several MHz. Go back over the part in the article about the "plucked string". This is a very common situation in all systems with a resonant frequency. A disturbance of any sort, will tend to create vibrations at the resonance. The megahertz range vibrations are over very quickly, ie they are a damped oscillation. The pulser circuit does not drive these frequencies, any more than a finger nail drives a guitar string at its pitch.
One can see them readily using an oscilliscope, but some care is required to get the triggering just right. A better way to see the fact that batteries have a high frequency resonance is to use a sweep generator, as in the diagram:
In the original article, I put forth an idea of what might be happening in the battery to cause this resonant frequency, and guessed that it was occuring in the electrolyte itself. A recent email (Mar. '01) from battery expert Heinz Wenzl, in Germany, said this:
The next question in my quest to understand this is the following: Given the same battery type there are small deviations of resonant frequency which are no measurement artefact. Now under normal conditions (i.e. the battery not being deep-deep discharged with an acid density close to one), there are always a lot of hydrogen and sulfate ions around which can create a plasma type charge distribution. A change of the frequency could be linked to the electrolyte density which is related to the state-of-charge (I have found no real correlation here), the viscosity (this I would imagine would be linked to the IR spectrum of molecular vibrations), the current exchange density (linked to state-of-charge and surface area and catalytic properties, etc.) and to .... some others. But gut feeling would tell me, that all these effects should be small compared to the plasma properties themselves. In which case, all lead acid batteries with flooded electrolyte should have the same resonance and NiCd a different one.
So it is likely that my original idea about what may be going on needs changing. Nevertheless, the resonance still creates conditions of high peak voltage (ringing) that are favorable to the process of desulfation.
Using a resisitive load to measure battery condition is a standard method. For each battery type a standard load is defined, and if the voltage under load drops below a certain level, the battery is bad or in need of recharging. For small batteries this load can be typical, like a 5 Ohm load for a AA Alkaline drops the voltage at end-of-life to 1.0 Volts. Using a load across the battery (for a few milliseconds) is used by laptop computers to assess the charge status of the battery.
desulfator bulletin board.
Some relevant links:
details on why pulse charging is good.
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Copyright 2001-2003 Peter Ferlow