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
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.