The Low Power Desulfator

Help and information for builders

• This page will give assistance to builders of the desulfator circuit shown in Home Power magazine, #77. The circuit has been tested sufficiently to know that it is effective on 12 volt systems up to 1000 amp-hours, and possibly higher. It can be used in automobiles with starter batteries that have become marginal. Most small solar systems will also be benefitted. It is very simple and easy to duplicate.
• Schematics:All the following circuits work quite well. There are some parts that may be easy or hard to get, so you can pick the one that suits you best.
  • Here is the pulser circuit from Ron Ingraham. It uses an N channel FET, which is easier to locate for many people.
    
    
  • Here is another 555 alternative schematic from Don Denhardt.
    
    
  • Some say that P channel units were hard to come by, but the original circuit from the article, using the P channel FET, is here. Thanks to Glenn Brown, we have single sided PCB layouts. Here is the trace layout: traces.gif. Here is the placement grid grid.gif. And here is the nomenclature: parts.jpg. Don's hand wired versions look like this: Top and bottom.
    
    
  • Here is another N channel version from Trevor Andrews in the UK, by way of the BBS discussions. It has some EU parts designations. The 555 will not handle very high voltages, so be careful not to use on open circuit batteries. For utter simplicity, here is a blocking oscillator that Trevor also uses.
    
    
  • Alastair Evans in the UK sends his N-channel version which has some additional protection. Here is the double sided layout for etching a circuit board: TOP and BOTTOM. He also has some release notes as well. Thanks !
    
    
• It is possible to use a simple peak detector across the battery to watch the progress of desulfation. as the battery internal voltage goes down, so will the peak voltage which the pulser develops across the battery. Here is a simple circuit from Trevor A.
• There is now an FAQ for additional helpful hints.
• Don Denhardt has taken the initiative to put together some parts kits to make it easier for home builders. He has this circuit designed at 12, 24, and 36 volts. He has also begun development of a Stamp based, intelligent desulfating circuit. The Stamp module is a microcontroller chip that runs BASIC and hooks directly to the serial port of a PC. This allows for many different possibilites, and anyone wanting to experiment in this area has a great open opportunity availiable. Click here for Don Denhardt's parts kits.
• Note: The value of C2 is wrong in the original article. It should be .0022uF, not .022uF. This explains why some have had problems with frequency/ pulse width being off.
• I have used parts from Digikey.com, and also from Mouser Electronics. The only item that is remotely critical is the inductor L1. I have used a variety of inductors taken from old switching supplies and TV chassis, and they all work fine as long as they are somewhat close to the value indicated, have low enough series resistance, and that the output of the 555 timer is trimmed accordingly. If you have a larger valued inductor, you would lengthen the pulse width, if you have a smaller value, you would decrease the width. Read below for more about this. L2 is totally noncritical, as long as it doesn't get too warm at the current level you are using. It's purpose is simply to isolate the current pulse from the rest of the circuit, and to control the current going into C4. More on this below.
• The MOSFET can be any unit that has a sufficiently low "on resistance" rating. Power dissipation is low, but I use a high power unit to lower this resistance, as that is part of what limits the peak pulse current generated. The voltage rating needs to be high enough to stand the highest peak voltage, which might get to be 100 volts. If you want to try for higher peak currents, you can parallel MOSFETS together, as they share current well.
• The diode D1can be anything that is fast, has a high peak current capacity, and more than 100 volts peak inverse rating. Again, the higher current devices are not required by the circuit, but they will help to increase the peak current by virtue of low resistance.
• The output leads should be made from fairly heavy wire (#16 or less) and be as short and direct as possible.
• Here are some hints for making higher voltage versions. The same principles apply.

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. The latter looks to be the best bet however.

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	---	---	---