Introduction: Desulfator for 12V Car Batteries, in an Altoids Tin

About: Was a maker before makers became a thing. I have always enjoyed solving problems with my hands. I also enjoy writing.

Hello Everybody!

After a year or so of reading and drooling over other people's wonderful projects in these pages I decided to finally make one of my own. Here is my first instructable, a version of the ever popular Battery Desulfator, which I built in an Altoids tin.

First, some background:

My urge to build this project came when my wife's car refused to turn over after a three day weekend away. Here in Tokyo, during winter, the temperature can drop to the low 20's (F) at night and since we have no garage, her car just has to endure the cold as best it can. Many people don't realize that you don't have put up with repeated jump-starts or run to the nearest garage and plunk down 7,500 yen ($85) for a new battery every time this happens. Your old battery may just have built up a layer of lead sulphate crystals on its plates and that is preventing the acid from contacting them over their full surface area. This is caused by subjecting the battery to long periods of insufficient charge, as in the cases of unplugged golf carts over the winter, infrequently used automobiles, and PV systems that don't get enough sunlight to charge their batteries. The result is a great reduction in the battery's ability to produce electricity.

With a desulfator circuit you can reverse this process and rejuvenate the battery to like new condition. You can also save money and prevent water and ground pollution at the same time by keeping your old battery out of the local landfill. As long as nothing is seriously wrong with the battery it can last many times the two or three years that people typically use them. You can even get free batteries from garages that routinely throw them away, desulfate them, and never buy another battery again. Save money and help the environment - now there's a green ecology scheme I can get into!

Most DIY desulfator circuits in use today can trace their roots back to an article in issue # 77 of Home Power magazine written by Alistair Couper in June/July of 2000. Many versions were spawned by his design but they all accomplish the same thing, that is, they use various pulsing circuits to force the lead sulphate crystals back into the electrolyte thus rejuvenating the battery and restoring its lost capacity. The version I chose uses an NE555P timer chip for the multivibrator front end and two coils, a low ESR cap, a fast diode, and an N-channel MOSFET (hereafter referred to as a FET) to generate the high voltage (50V) spikes in the output. Credit goes to Ron Ingraham for changing the design to use an N-channel FET instead of the harder to find and more expensive P-channel types in the earlier versions. Along the way I couldn't resist adding a few tricks of my own to make the design more convenient. See this link for a description of the theory and other information on desulfators.

This circuit can be used three ways - as a standalone device powered by the battery under test; as a standalone device but used in parallel with a battery charger; or built into a charger so that the two work together as one. I chose the third option for my circuit but added a switch so I can use either device independently. Mounting the device onto my charger also allowed me to use the charger's output cables for both functions and avoid the tangle of wires that inevitably results at the battery.

Once properly adjusted, the desulfator can be left on permanently whenever the charger is charging. Just be aware that no matter what configuration you choose, the desulfator is powered by the battery under test so if you use it without a charger care must be taken to avoid deep discharging the battery.

High power versions of these circuits can be built for off-grid solar-cell systems as well where many batteries are typically arranged in series/parallel banks and attached to inverters to produce 120V AC. These battery banks can be desulfated en-masse while being charged by their solar arrays for a truly self-maintaining system minus the periodic checks for electrolyte level, as long as the desulfator circuit is scaled up in size sufficiently.

The Altoids can is the perfect box for this project as the circuit neatly fits inside it and the metal construction can shield much of the RFI that may be emitted by the output stage. You can't beat the price of these tins, and they even come with free mints, or do the mints come with a free tin, I forget... ?

So with the background out of the way, let's get to work!

Step 1: Circuit Schematic and Parts List

Here is the schematic and parts list, along with some of my pencil notes.

The list is complete except for some parts (two pots, two resistors, two switches, a LED, a FET and some grommets and pop-rivets) that I salvaged out of my junk box. Feel free to do the same, just keep to the values on the schematic as much as possible. Please note that C4, a 100uf 25V electrolytic capacitor, must be a "low ESR" type (Equivalent Series Resistance) to limit its tendancy in this application to get hot. If you choose to use trim pots instead of resistors for R2 and R4, as I did, be careful with the adjustments as C4, D2, L1 and L2 can get very hot if the 555 chip is made to send too wide a pulse into the output stage. The resistor values in the schematic should program the 555 chip to output pulses of the proper width and limit any excess heat buildup, however. We'll discuss this further in the Smoke Test Steps.

The LED can be any standard type and will only turn on when pulses are present in the output. S1 should have at least a 3A rating, and if you use a DPDT type use both sets of contacts in parallel to reduce the contact resistance as much as possible. S2, at the output of the 555, isolates the 555 from the output stage allowing you to make adjustments to the front end without risking overheating Q1, D2, C4 or the inductors.

The inductors I chose are listed on the schematic at the bottom of the "Possible Inductors from Digikey" list. They fit the can nicely but will need to have one lead extended slightly to reach the bottom of the circuit board. In retrospect, an inductor with a slightly higher current rating for L2 might be better as the one I chose gets noticably hotter than L1 even though it has the same current rating of 2.4A. Digikey part number M8875-ND should fit the can, barely, and has a 3.6A rating, but the 2.4A coil that I'm using now really only gets hot if I get too aggressive with the pulse width adjustments.

D2 is a FRED (Fast Reacting Epitaxial Diode) and should not be substituted with any old diode in your junk box as the latter will probably not work well in this circuit. If it gets too hot you can use two in parallel to double the current capacity, but again, if you keep the pulse width on the conservative side it will only get slightly warm.

The FET listed works very well and is inexpensive. I mounted mine directly on the perf-board with a piece of stick-on copper foil (available from Digikey) under it to act as a heatsink. In this configuration it doesn't get warm at all so the copper foil may not actually be needed. Be aware that the metal tab on the FET is also attached to pin 2 (drain) so if you attach the FET to a heatsink you'll have to electrically isolate it from the rest of the circuit. I also used a TO-220 transistor socket to allow easy replacements but you can wire the FET in directly if you prefer. Just avoid touching pin 1 (gate) while handling it as it is very ESD (static) sensitive.

Also, I opted to use the "Turn-off Enhancement Circuit", shown in the schematic as Q2, D3, and R5, as it helps the FET to turn off more precisely. If you use these parts do not use C2 and R3.

Step 2: Build the Box and Start Your Layout.

First, you should cut a piece of perf-board (aka breadboard) the same size as the inside bottom of the can, minus a wee bit for wiggle-room, with a coping saw or jig-saw. Use a disk sander, if you have one, or a sanding block if you don't, to clean up the edges. You'll find that the fiberglass board cuts and sands easily. If sized correctly, the circuit will sit nice and snug inside the can with no mounting screws or other hardware needed to hold it in place, yet be removeable for fit tests or repairs if needed.

Next, place the parts loosely in the Altoids can to get an idea of where you'd like to mount them. My layout roughly follows the schematic and limits the number of jumper wires needed to make connections. I'm sure there are better layouts but what you see worked well enough for me so feel free to copy it.

Early on I'd planned on bolting the FET to the lid so the lid could act as a heatsink but this turned out to be unnecessary. There's just enough room on the perf-board for it and a socket, and since it doesn't get hot at all, no additional heatsinking is required.

You'll need to insulate the metal can from the circuitry by cutting some thin cardboard to cover the bottom, lid, and sides. The "wiggle-room" mentioned above is to allow space for the cardboard sides. Later on, you'll mount the cardboard in with double-stick tape but for now leave the cardboard out while you drill the holes in the can.

The output wires will exit a 5/16" hole in the left side, and in this hole you'll fit a 1/4" rubber grommet. Start small and go gradually up in size with the drill bits as the metal is thin and soft and bends quite easily. Remove the flash with a countersink bit, if you have one, or a larger drill bit twisted with your fingers. Use something round and hard, like the shaft end of a large drill bit or the ball end of a small ball-peen hammer, to flatten the holes after drilling. Don't forget to allow room for the lid which overhangs the sides a 1/4" or so.

The vent holes around the right side are 1/8" diam and spaced about 1/2" apart. A center punch helps a lot here but a nail works as well to dimple the metal a bit to better aim the drill. I also drilled a hole in the lid for the LED so I can see it when the lid is closed. You can do the same but you'll have to measure carefully where it will go after you decide where to put the LED in your layout. Mine fitted nicely inside L1. You'll have to also punch a hole in the lid's cardboard liner for the LED to shine through.

Place the cardboard side strip in the can and tack-tape it in place, then use a pen to mark the holes from outside the can. Use a hole punch to punch holes in the cardboard exactly over the marks you made.

If you decide to mount the can to the back of your charger you'll need to drill four more holes in the bottom for whatever fastening hardware you want to use (I used pop-rivets). You can also make the circuit separate from your charger but you'll have to add lead wires and some clips to attach the circuit to your battery. The parts list shows some clip parts that I used but you may prefer larger ones. The lead wires should be made of at least 16 ga flexible wire, thicker if you can get it, and as short as comfortably possible to avoid losses at the battery. Even if you plan to wire your circuit into your charger it's a good idea to make temporary leads with clips so you can debug the circuit before permanently mounting it.

Once the holes are punched you can double-stick tape the cardboard strip and lid pieces in place and fit the grommet. For now, don't tape the bottom piece in, just use it as an insulator as you build and troubleshoot your circuit. This will allow you to pop-rivet the can onto the charger when the time comes and you can then tape the cardboard in permanently over the pop-rivets. If you don't plan to mount your can onto your charger then it's OK to go ahead and tape in the bottom piece.

Step 3: Build Your Circuit.

On small one-off circuits like these I don't bother with designing printed circuit boards. I just wire them up on perf-board using the cut off leads of the various components to solder them together in a kind of "connect the dots" fashion. Keeping the layout in roughly the same order as the schematic helps to visualize the top and bottom of the board as you assemble it. For the longer runs use 24 ga hook-up wire or some cuttings from a telephone cable if you can find one.

It's important to use a good quality soldering iron with a thin tip and good 60/40 solder as it gets a bit cramped, especially around the 555 chip socket. Definitely use a socket for the chip as you can easily overheat the chip during assembly and troubleshooting. Small needle-nosed pliers will help with manipulating the leads and in holding them in place for soldering.

I used SMT parts for the electrolytic caps because they were the smallest low ESR caps I could find. If you use the same ones solder your own leads onto the pads and wire them up as if they were normal discrete components paying attention to the polarity (see schematic).

Once you know exactly where to put it, glue the FET socket to the perf-board with CA glue. I used a nylon bolt to bolt the FET down but as long as the tab is isolated from the rest of the circuit any small bolt will do.

I also used a strip of stick-on copper foil, cut from a 6" wide sheet, along the bottom edge for a ground bus. Digikey sells the sheets by the foot and it's marvelous stuff as it can be used for making ground planes, RFI shields, heatsinks, and many other uses. My wife enjoys making stained glass items and the rolls of copper foil she uses are also perfect for this task. You can pretty much make your own "printed" circuits with these rolls, sans the etching steps, if you like, but it's not necessary with this circuit.

Step 4: Smoke Test 1 - Pots Instead of Fixed Resistors for R2 and R4

It's time to test your handiwork!

For those of you who used fixed resistors for R2 and R4, skip this step and go on to the next step, Smoke Test 2.

For those of you who used pots instead of fixed resistors for R2 and R4:

First, turn off S2, put a 555 chip in its socket and a 2A fuse in the fuse holder. Set the pots to their mid-range and attach the plus lead clip of your circuit to the plus terminal of a 12V battery. Attach the ground lead clip of your circuit to the minus probe of a multimeter, and set the multimeter to the 10A AC scale. Briefly touch the plus probe of the meter to the minus terminal of the battery. Check for smoke. No smoke? Good! Try it for 5 seconds, then 10 seconds. Still no smoke? Great!

Check the 555. Hang a scope probe (if you have one) on pin three of the chip and check for pulses. Adjust R4 for peak output at around 1000 Hz (the exact level isn't critical).

Now check the output stage. Turn on S2 and briefly touch the plus probe of the meter to the minus battery terminal. You should see a brief spark and hear a faint 1000 Hz tone come from the coils. The LED will turn on in the presence of output pulses. If it doesn't, but you hear the tone, then the LED may be mounted backwards. If you don't hear the tone, or see smoke, then something is wrong and you'll need to check your output stage wiring.

If the fuse blows try adjusting R2 down a bit (the direction of turn depends on how you have it wired). Smile when you get the meter reading below 0.8A -- you're almost there!

If all is good then adjust R2 so the meter shows no more than 0.7A on the AC scale. This should yield a good output into the battery without overheating the output stage. Finger test the coils, C4, FRED, and the FET. If all are no more than slightly warm after 30 minutes then you're in the clear. You can SLIGHTLY increase the pulse width and the current into the meter a little at a time until the circuit reaches about 1.0A but at this level my charger won't flip into trickle charge mode because the combined currents of the circuit and charger are beyond its trickle threshold. I therefore keep it at around 0.7A. Anything beyond 1.0A gets a bit too toasty after a night's use anyway. Also note that the circuit will tend to consume 0.2A to 0.3A more current and get hotter when the charger is on high charge rate. It's therefore best to stay at or below 0.7A to prevent the current from getting too high as the charger adjusts its charge rate from high to low. Be conservative, especially with an undercharged battery because as the lead sulphate crystals dissolve into the electrolyte the battery voltage climbs and this increases the current and the heat dissipated by the output components.

Step 5: Smoke Test 2 - Fixed Resistors.

For those of you who used the resistor values in the schematic:

First, turn off S2, put a 555 chip in its socket and a 2A fuse in the fuse holder. Attach the plus circuit lead clip to the plus terminal of a 12V battery. Attach the ground lead clip of your circuit to the minus probe of a multimeter, and set the multimeter to the 10A AC scale. Briefly touch the plus probe of the meter to the minus terminal of the battery. Check for smoke. No smoke? Good! Try it for 5 seconds, then 10 seconds. Still no smoke? Great!

Check the 555. Hang a scope probe (if you have one) on pin three of the chip and check for pulses. If you don't see them then check your 555 wiring.

Next check the output stage. With the meter and circuit wired as above, turn on S2 and briefly touch the plus probe of the meter to the minus battery terminal. You should see a brief spark and hear a faint 1000 Hz tone come from the coils. The LED will turn on in the presence of output pulses. If it doesn't, but you hear the tone, the LED may be mounted backwards. If you don't hear the tone, or see smoke, then something is wrong and you'll need to check your output stage wiring.

If you heard the tone then leave the battery connected a little longer and finger test your output components to make sure they don't get too warm. If they're still only warm after 30 minutes then you're in the clear and your circuit is functioning fine. If you have a scope you can check the pulses at the chip and the output but this isn't really necessary. Your meter should be indicating something under 1.0A. If it shows more than that then you'll have to adjust the value of R2 to get the output current down.

Step 6: Hacking the Charger.

Any charger will do, mine just happens to be an automatic model from CellStar made for the Japanese domestic market. If you decide to keep your desulfator separate from your charger you can skip the Hacking the Charger step, but you'll need to attach clips and output leads to your circuit so you can attach it to your battery.

You'll need to drill six holes, one 5/16" diam for the leads to enter the charger, one 1/2 " diam for the switch (if you use the same style toggle switch that I used), and four 1/8" diam holes for the pop-rivets. Drill one pop-rivet hole, put in a pop-rivet and mount the box to the charger, then drill and pop-rivet the three remaining holes in succession. Be careful with the metal shards and thoroughly blow out your charger with compressed air after you're done drilling and shaping the holes. Also watch out that the drill bits don't accidentally damage anything inside. One wire got caught in a bit when I did this and had to be repaired later.

Mount your switch and put a 1/4" rubber grommet in the lead hole.

Wire up the switch per the schematic using both sets of contacts in parallel if you use a DPST or DPDT type, and keep the leads as short as possible. This switch also becomes a good place to hang a multimeter for checking the current drain later on.

The final wiring step is to solder the leads to your charger's output wires. I prefer splicing into the wires themselves rather than tacking on the circuit board or internal components to avoid damage to those components. The output leads should be quite thick so care must be taken when splicing into them. Partial dissassembly of the charger may be required as well. Be thorough with the solder but don't overheat the insulation. Cover the splices with 1/2" diam heat-shrink tubing and shrink them down with a heat gun.

FYI: Sometimes a hair blower will suffice for a heat gun if you use or make a nozzle with a 1/2" x 2" slit opening. The over-temp sensor that all hair blowers have may shut down the blower after a few minutes but you don't need to keep it on very long. Use the blower's high heat and low fan settings if you can. Failing that, I often use a monokote heat gun normally used for building model airplanes as it's cheap ($20), reliable, and comes with the right sized nozzle. You can buy them online or at any hobby store.

Step 7: How Well Does It Work?

At this writing my circuit has only been in operation three days on a 95AH sealed car battery that a friend gave me almost two years ago. Its fully charged no-load voltage has climbed several tenths of a volt in those three days, which I consider a good sign. When it is ready I plan to put it in my wife's car and remove her battery so I can test the circuit on it in my relatively protected but unheated hobby shack. Therein lies a problem. Lead-acid batteries (and desulfators and chargers, for that matter) work best when the battery is warm. A cold day can sap 50% or more of the charge out of your battery. Because I don't have a warm garage I may just have to wait until warm weather returns before I can fully do the circuit justice in testing it.

My Internet sources tell me that batteries may take a month or more to reverse the effects of heavy sulfation. However, they also say that heavily sulfated batteries are fully restorable and that patience will be rewarded with a battery that can be put back into service instead of on the scrap heap. This site offers some tips on the general use of desulfators. Use these tips at your own risk!

This page has a wealth of info on similar designs and a link for a peak detector circuit that can help you plot your battery's improvement over the course of treatment. I've not tried this circuit so can't comment on how well it works. The page also has a link for a FAQ that can help you answer some basic questions about desulfator circuits in general.

Please be aware that I present this instructable to you to use with an Attribution Non-commercial Share Alike license. Use it at your own risk! While the circuit is not particularly dangerous, you will be using it around lead-acid batteries and relatively high voltages and currents. Deeply discharged batteries have been known to explode in the presence of sparks due to high hydrogen outgassing. Similarly, a battery accidentally or deliberately shorted can be extremely dangerous! I take no responsibility for your use, misuse, or accidents resulting from or involving any attempt to use this information.

Good luck with your desulfator! I invite your comments. If you build one, send me an email. I'd love to hear from you!

Step 8: Update:

It's been over a month now and I'm happy to report that my desulfator circuit is working well! My battery now charges to over 13.4 volts after a full charge. Before desulfator treatment it would not rise beyond 12.7 volts. This is a very good sign meaning that the plates are now much cleaner, the electrolyte is now contacting their full surface area, and that full electricity production has been restored. I kind of wish I could verify this visually but I can't due to the battery being a sealed type. For now, I'll have to be satisfied with just reading the improvement in my voltmeter.

Some notes:

1. During testing I found that my charger has no real trickle rate mode and instead stops the charge entirely when it decides that enough charge has entered the battery. Call it an idle state instead of a trickle rate. If left like this, after a full charge the battery voltage will slowly drop to about 12.2 volts within a week or so (further if I let it), which I assume is a reflection of the battery's natural decay rate plus the amount of charge being consumed by the circuit itself. I therefore, every few days, top off the battery by turning the charger's power switch off, then on again to restart the high charge rate.  A few hours later I make sure that the red LED has turned off and the green LED has turned on meaning that the charger has finished the charge and gone back to its idle state. The desulfator is then free to do its thing without interference from the charger.

2. There is a marked drop in pulse peak voltage from about 50 volts, measured at fuse F1, to about 36 volts measured at the battery. This is due to losses in the cabling going to the battery. You can limit these losses by keeping the cables as thick and as short as possible. 12 or even 10 ga wire is not too thick as long as you can solder it and it is flexible enough to not make the circuit unwieldy. If you use thinner wire just know that the circuit will still work but that the reduced voltage at the battery will take longer to recondition it. My thanks to DRZCYY for bringing this to my attention.

3. Use of wire loops to hold down the two coils can simulate a shorted winding in the coils and result in slightly reduced output. It's best to use plastic or nylon tie wraps for this purpose. My thanks to EDTEK for this tip.

I'm working on some improvements to the design and hope to offer a printed circuit board in the near future. Check back here for further updates.