Updated April 2005!Introduction - Battery Desulfation
This is a brief survey of Battery Desulfator projects accessable on the web.
Firstly, you may ask, "What is sulfation?" (Battery Plate Sulfation For Us Dummies/Aviation Battery Plate Sulfation and Resultant Degradation Explained)
Sulfation occurs when a battery discharges and the lead in the plates combines with sulfuric acid to form lead sulfate. When you recharge the battery, some of the the newly formed Lead Sulphate will not 'convert' back into lead and sulfuric acid, but rather some of the sulfate begins to crystalize; these crystals act like insulators, the more you discharge a battery, the less capacity it has as the crystals begin to cover the plates and deprive plate area contact with the electrolyte. After awhile, the sulfate crystals may also become somewhat 'permanent', and are hard to remove (or 'force' back into the electrolyte).
So now, what is desulfation (aka "Pulse Conditioning")? It's the 'bringing back' of a lead acid battery by putting the sulfation back into the electrolyte. Those who have been at the forefront of this technology are those who need the utmost of performance from their battery banks, e.g. anybody running 'off the grid' (from photo-voltaics alone) for instance.
Battery Desulfator Survey
This is a brief synopsis of two different Lead Acid Battery Desulfator projects accessable via the web.
Noteworthy about these two designs are: a) their history (the 1st design dates back to 2000) and development b) the technical details of the circuitry c) the technical details of their operation in use d) the testimony of the positive (no pun intended) results from e) a large, self-supporting user base (in the case of the two-terminal Desulfator by Alastair Couper) via an ezBoard discussion group ("Lead Acid Battery Desulfation") and last but not least f) their simplicity.
The Desulfators.
- Two-terminal Lead Acid Battery Desulfation Pulse Generator
www.shaka.com/~kalepa/desulf.htm
This web page describes 'the original' desulfator which is suitable for most solar systems, vehicle starter maintenance, and gradual battery reclaimation. There are several flavors of this circuit described and documented on the above web page.
Noteworthy about this design is the history and experience documented here by one Alastair Couper back in 2000 in this Rosetta Stone-class article titled: Lead-Acid Battery Desulfator which appeared in Home Power magazine. A large amount of amateur research and testing on desulfators that has followed on the heels of this paper.
This is a two terminal device that draws power it's power from the battery as it performs desulfation. This may confuse some people, as it would seem counter intuitive to the purpose at hand. It is recommended that a charger be connected in parallel with this device for extended desulfating of a battery. The history for this design exists in the form of a .pdf file here. This design uses the v= L * di/dt (inductive kick) trick to generate it's pulses.
Update March 2005!
Documenting the operation of a two-terminal Alastair Couper Design - My experience. I have built three units so far, the first about 2 weeks ago before building two more, so I have built three in toto.
While testing the second two units I had a chance to take a few more scope photographs to allow more 'documenting' of the operation of these units - here is the result of that effort:
- AC-line Powered Pulsing Battery Desulfator
www.rst-engr.com/kitplanes/KP0204/KP0204.htm
This is a 120 VAC AC line powered 'pulsing' de-sulfating charger originally designed for use on 35 Ampere-Hour class light general-aviation aircraft batteries.
This desulfator uses a common doorbell transformer in the power supply portion of the circuit to charge electrolytic capacitors through a voltage doubler; the output pulse is delivered using a 'brute force' technique wherein a power FET (IRF HexFet) is switched on for the duration of the desired 'pulse' at the desired low rep rate (repetition rate).
I took the liberty of elaborating and annotating the o-scope waveforms appearing on the above website here:
http://www.dallas.net/~jvpoll/Battery/aaPictures.html
I annotated these waveforms to more clearly convey what information can be gleaned when using an oscilloscope to observe the waveform seen across the battery during a 'pulse'; this info indicates the battery's health by interpreting the voltage waveform during a pulse as it relates to dynamic battery impedance (or resistance).
References:
- DOE Battery Handbook - PRIMER ON LEAD-ACID STORAGE BATTERIES
www.eh.doe.gov/techstds/standard/hdbk1084/hdbk1084.pdf
- Capacity Loss in PV (Photo-Voltaic) Batteries and Recovery Procedures
www.sandia.gov/pv/docs/PDF/caploss.pdf
Abstract excerpt:
To date, laboratory and system testing consistently identified the incomplete recharge of PV batteries as the predominant cause of premature capacity loss resulting in a lower than rated cycle-life.Incomplete battery recharge introduces battery degradation mechanisms such as electrolyte stratification, gas bubble entrapment, excessive sulfation, and degradation of the positive active mass.
Recovery of the PV battery after extended periods in a deficit charge condition may or may not be possible depending on the extent of battery degradation and the resources available for recovery.
- The History, the chemistry of Lead in Batteries
www.du.edu/~jcalvert/phys/lead.htm
Excerpt:
The present lead-acid cell consists, in a state of full charge, of a negative plate, or cathode, of spongy lead in a grid of hard lead, a positive plate, or anode, of PbO2 paste in a grid of hard lead, and an electrolyte of dilute sulphuric acid of specific gravity 1.28. This [electrolyte] is a 37% solution, with 472.5 g/l of H2SO4.At full discharge, the electrolyte is of specific gravity 1.05, an 8% solution containing 84.18 g/l of acid. Both plates are coated with PbSO4. Approximately 4 moles of acid are used per litre, which corresponds to 213 A-h of charge (an ampere-hour is a current of one ampere flowing for one hour, or 3600 coulomb).
Assuming that 4 moles of Pb are reacted at the cathode, and 4 moles of PbO2 at the anode, the total weight of active materials is about 3 kg. This gives a weight-to-capacity ratio of 14 g/A-h. Of course, this is much lower than is required for a practical battery, with case, electrode grids and other necessities. However, a limit of perhaps 25 g/A-h represents the maximum that can be expected of a lead-acid battery, and a limit of about 200 A-h per litre of electrolyte volume.
- Charging Algorithms for Increasing Lead Acid Battery Cycle Life for Electric Vehicles
White paper and Power Point presentations on:
"Charging Algorithms for Increasing Lead Acid Battery Cycle Life for Electric Vehicles"Paper:
www.nrel.gov/vehiclesandfuels/energystorage/pdfs/evs_17paper.pdfPPT Presentation:
www.nrel.gov/vehiclesandfuels/energystorage/pdfs/evs17pres.pdfAbstract excerpt:
The purpose of the project was to develop charge algorithms specifically aimed at improving the cycle life of VRLA batteries to 1000 deep discharges for electric vehicle applications. The motivation for the project was based on the hypothesis that VRLA batteries reach end-of-life prematurely with the "normal" constant voltage charge because of insufficient recharge at the negative plate and the "oxygen cycle" or recombination reactions interfering with recharge of the negative plate.During Phase 1 of this project, we developed zero delta voltage and current interrupt charging algorithms and strategies that improved the cycle life of VRLA modules from 150–200 deep discharge cycles to 300–350 deep discharge cycles.
During Phase 2, we implemented a current interrupt charge algorithm on a 24-module battery pack that resulted in 700 deep discharge cycles. We found no correlation between operating temperature and failure when batteries stayed below the manufacturer’s recommended temperature limit of 60oC. However, warmer modules appear to have longer lives.
- A new pulse charging methodology for lead acid batteries
www.ipenz.org.nz/ipenz/publications/transactions/Transactions98/emch/2wilkinson.PDF
Lead acid battery cells have low energy density and relatively low life-cycle, yet because of their cost effectiveness they are still considered the preferred choice by many electric vehicle (EV) developers and are likely to continue to be so for the next 5-10 years.Excerpt:One method of improving the performance of a battery powered EV is to improve the battery charging methodology since EV performance and range is largely determined by the capacity, weight and charge/discharge characteristics of the on-board batteries.
This paper describes a method for fast charging lead acid batteries using current pulses of controllable magnitude and duty called ‘pulse charging’. It is used together with constant voltage/current profiles to increase charge acceptance, improve the charging time, and to potentially increase the life cycle of lead acids cells.
2.2 Invariant pulse chargingResearch overseas has shown that the hydrogen and oxygen gas development in a battery is not immediate but has a time constant relating to the state of charge of the battery [5]. Therefore if an applied current pulse is short enough, most of the current will be consumed by the charge reaction rather than producing hydrogen gas.
This is the principle of pulse charging - applying relatively large currents into a battery at periodic intervals with a defined pulse width to reduce or avoid gassing and thus increase charge acceptance and efficiency. An additional advantage is that this principle can even be applied to almost fully charged batteries.
- A white paper which adresses mathematical modeling of internal battery processes under pulsed conditions
Mathematical Modeling of Current-Interrupt and PulseAbstract:
Operation of Valve-Regulated Lead Acid Cells
Venkat Srinivasan, G. Q. Wang, and C. Y. WangElectric and hybrid electric vehicles use valve-regulated lead acid (VRLA) cells that are subjected to dynamic operation with charge, rest, and discharge periods in the order of seconds. Such operation requires more sophisticated models that incorporate the electrochemical double layer. While this effect has been incorporated in a handful of electrochemical systems, the lead-acid cell, with its sluggish reaction kinetics, is one of the few where it is significant. This significance is demonstrated with use of the current-interrupt technique, where the model is used to provide guidelines for the estimation of various resistances. The usefulness of the modeling approach is exemplified by its ability to explore the effect of changing electrochemical area and concentration with state of charge, and the role of parasitic side reactions in the voltage response of the cell.Simulations of pulse charging and dynamic stress test of VRLA cells, where considerable differences are shown when including the double layer, illustrate the need for modifying the presently used modeling approach. In addition, simulations are compared to current-interrupt experiments on commercial cells in order to evaluate the applicability of the model and to identify the differences.
- Battery Desulfator Report 2003/5/14
www.kwantlen.bc.ca/electech/FinalProjects/p2003/Battery_Desulfator_Lin_Jao.pdf
- Lead Acid Battery Desulfation Pulse Generator
www.shaka.com/~kalepa/desulf.htm
This web page describes 'the original', low power desulfator version which is suitable for most solar systems, vehicle starter maintenance, and gradual battery reclaimation. There are several flavors of this circuit described and documented.
This is a two terminal device - it draws power from the battery as it performs desulfation. This may confuse some people, as it would seem counter intuitive to the purpose at hand. It is recommended that a charger be connected in parallel with this device for extended desulfating of a battery. Extensive history for this design exists in the form of a .pdf file in the next reference.
- Lead-Acid Battery Desulfator by Alastair Couper
www.homepower.com/files/desulfator.pdf
This seems to be the 'Rosetta Stone' for a large amount of amateur research and testing on the subject of desulfators that has followed on the heels of this paper.
Opening excerpt:
Lead-Acid Battery DesulfatorAlastair Couper ©2000 Alastair Couper
It was twenty years ago that I left my on-grid home, and my job as an electronics engineer, to begin life on an alternative energy oriented organic farm. In the intervening years, I have installed, maintained, and experimented with numerous RE systems in my area.
What I have come to understand from this experience is that off-grid life tends to become very much focused on the battery bank and its fate.
All power sources and loads breathe through this crucial pathway. Batteries are heavy, toxic, inefficient, and—to the amazement of many—electrically very fragile. Weak or failing batteries are a very likely cause of breakdown, especially in smaller solar-electric systems.
Most newcomers to renewable energy are quite familiar with using water tanks or gas tanks, and naturally use this familiarity in trying to understand their battery banks. Everyone knows that a bigger water tank is better than a small one. Unfortunately, batteries are not like tanks, and the result is trouble.
It is definitely not true that a big battery bank is necessarily better than a small one. An oversized battery bank can be almost impossible to charge properly. Without a minimum daily exercise regimen, it can become the equivalent of a couch potato. The main culprit is sulfation, which is a gradual crystallization of the battery’s plate material, rendering it electrically inactive.
Some Theory
Past issues of Home Power (see Access) have gone into the details of keeping lead-acid batteries healthy, so I will only touch on the main points here. The usual practice in maintaining a battery in good condition is to apply a periodic equalization charge over and above what would be a normal full charge. Unfortunately, this is an energy-wasting tactic. It ultimately results in clean battery plates, but at a steep price, especially if the energy must come from a generator.
I initially went to the Internet to find any available information on the problem of sulfation. The search engines turned up several commercial sites that give useful details on the fine points of battery charging and equalization. A second resource is the IBM patent server (www.patents.ibm.com). I found relevant patents there, using keywords like "desulfate" and "rejuvenate."
What this wealth of data shows is that there are numerous strategies for charging and electrically desulfating batteries. Most of them were designed or developed in the last twenty years or so. Considering that lead-acid batteries have been around for more than a century, this is a relatively new innovation. Virtually all of the devices and patents I found have in common the use of some form of pulsing charge current. This is in contrast to the constant or slowly varying currents generated by sources like solar-electric panels.
I distilled and simplified these various techniques, and came up with a basic circuit that will keep small to medium sized batteries in desulfated condition. It can even be used to bring old, sulfated units back into service. Use of the circuit has dramatically reduced the need for equalization charges in my own home system.
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Healthy Batteries
I have used this circuit in my main system for over a year, and have not seen the need to equalize in that time (I do not own a generator). All of the cells’ electrolyte levels remain in step with each other, and there has been no problem with starting big loads—a sure sign of battery health.
Patience is required in reclaiming weak and tired batteries, and no amount of desulfating will help a battery with a shorted cell, or one that has lost plate material through excessive use.
The device is especially useful for automotive batteries that sit for long periods. If you use a generator for equalization, this technique is a must. When you live off-grid, silence is golden.
MORE - see link above.
- www.rst-engr.com/kitplanes/KP0204/KPtext.pdf
Excerpt:
... battery sulfation has been with us for a very long time. The problem is that in a typical wet (sulfuric acid) battery, the lead plates want to be exercised. That is, they want to be charged and discharged on a regular basis. If they just sit there, the acid slowly, slowly builds up a film of sulfide that eventually causes the battery to "go weak".This "weak" has everything to do with the fact that lead sulfide is a fairly good insulator, and as the sulfide layer builds and builds over weeks and months of disuse, the internal resistance of the battery goes up and up.
Finally it gets to the point where most of the voltage of the battery is dropped in the internal resistance of the battery and darned little gets to the point of intended use…like the starter motor.
...
4. There are going to be some batteries that are so far gone that leaving the desulfator on charge for a month will only get you four weeks and change. In my experience with these circuits, if you get the battery right when you notice that it is laboring to turn the starter, you have half a chance to make the desulfation process work. If it is so far gone that it won't even pull in the master switch relay, the odds of being able to save it are slim to none at all. See photo #6 for an example of a battery that will probably never be able to be brought back to life.
5. The sulfation process took weeks or months to develop. The desulfation process will take the same order of magnitude of time. Don't expect to put the battery on desulfate today and back in the airplane tomorrow. I've left batteries on this system for a month before I was happy with the end result.
- A several-year account of managing a 'battery plant' aboard an electric vehicle
This is a diary centered on the experiences and ownership of an electric vehicle utilizing 13 Optima deep discharge batteries.
If one does a search on the term "desulfator" on this page once can see where this individual reports his experiences from using a two-terminal Alastair Couper type pulsing desulfator.
www.los-gatos.ca.us/davidbu/sparrow/s_diary.html Excerpts:
Sun Jan 6 18:18:34 PST 2002 - 4996 Miles
I am excited to be reaching the 5000 mile milestone. The desulfator has been cycled through the batteries under the seat and is starting to make a pass through the batteries under the hood. It appears to be making a difference. As I drive up the main street of Los Gatos (a slight uphill grade, 20 MPH speed limit) the voltage has been climbing a volt or so every day.Thu Jan 10 20:50:39 PST 2002 - 5018 Miles
The desulfator has made a round trip through the pack. I am going to start switching the device from one battery to the next every morning. The pack seems "happy." Resting voltage at the bus stop (about 3 amp-hours down the road) is 168 volts. That is excellent.
...
Optima claims "thousands" of cycles lifetime when their batteries are lightly discharged, as mine are with this trip. Theoretically my battery pack could last for many more years with this kind of use. We will see!Thu Jan 17 16:12:13 PST 2002 - 5062 Miles
...
The desulfator seems to be making progress. This morning I checked the batteries under the seat, and found that the two batteries which had most recently been desulfated were accepting more charge than the others. That is a good sign! They probably are recovering capacity. The cycling of the desulfator through the pack continues. I am encouraged enough to consider building a second one. They are not too expensive ($25 for parts) but it takes time and care to assemble the parts. It's a good weekend project.Thu May 2 20:18:14 PDT 2002 - 5458 Miles
...
I also plan to build a second desulfator, as the one I have seems to make a difference, but 12 days between each battery with no desulfator seems to allow the sulfation to return. I hope two or three of them will stay ahead of it. I am adding switches to the voltage clamp circuits I built as well. When the desulfator is on a battery, the corresponding LED's glow just a bit. I suspect some of the desulfator pulse voltage is being clamped by the clamper circuits. The switches will enable me to turn the voltage clamper off for the battery being desulfated.
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