In this article we investigate 2 simple yet powerful battery desulfator circuits, which can be used to effectively to remove and prevent desulfation in lead acid batteries. The first method uses PWM pulses, while the second method implements a ordinary bridge rectifier for the same.
Sulphation in lead acid batteries is quite common and a big problem because the process completely hampers the efficiency of the battery. Charging a lead acid battery through PWM method is said to initiate desulfation, helping recover battery efficiency to some levels.
What is Sulphation in Lead Acid Batteries
Sulphation is a process where the sulfuric acid present inside lead acid batteries react with the plates overtime to form layers of white powder like substance over the plates.
This layer deposit seriously deteriorates the chemical actions inside the battery while charging or discharging making the battery inefficient with its power delivering capabilities.
Normally this happens when the battery is not being used for long periods and the charging, discharging processes are not done very frequently.
Unfortunately there's no effective way of tackling this problem, however it has been researched that the jammed sulphur deposits over an effected battery may be broken down to some extent by subjecting the battery to high current bursts while charging it.
These high current charging pulses should be well optimized through some control circuit and should be diagnosed carefully while implementing the process.
1) Using PWM
Implementing the method through PWM controlled circuit is probably the best way of doing it.
Here's an excerpt from wikipedia, which says,
" Desulfation is achieved by high current pulses produced between the terminals of the battery. This technique, also called pulse conditioning, breaks down the sulfate crystals that are formed on the battery plates. Short high current pulses tend to work best. Electronic circuits are used to regulate the pulses of different widths and frequency of high current pulses. These can also be used to automate the process since it takes a long period of time to desulfate a battery fully."
https://en.wikipedia.org/wiki/Talk%3ABattery_regenerator
The circuit of a PWM battery charger discussed here can be considered as the best design for carrying out the above desulfation process.
How the Circuit Functions
The IC 555 is configured and used in its standard PWM control mode.
The output from the IC is appropriately amplified through a couple transistors so that it is able to deliver the said high current pulses to the battery which needs to be desulfated.
The PWM control may be set at low "mark" ratio for implementing a desulfation process.
Conversely if the circuit is intended to be used for charging normal batteries, the PWM control may be adjusted for generating pulses with equal mark/space ratios or as per the desired specs.
The controlling of the PWM will solely depend on an individuals personal preference, so should be done correctly as per the battery manufacturers instructions.
Failing to follow the correct procedures may lead to fatal accidents with the battery, due to a possible explosion of the battery.
An input current level equal to the battery AH level may be chosen initially, and reduced gradually if a positive response is detected from the battery.
2) Desulfating with a Transformer and Bridge Rectifier Circuit
To make this simplest yet effective battery desulfator with charger circuit you would just require a suitably rated transformer, and a bridge rectifier. The design not only desulfates a battery, it keeps the new batteries from developing this issue and simultaneously charges them to the desired levels.
At the beginning of this post we learned how to desulfate using PWM concept, however a deeper research shows that the process of desulfating a battery may not necessarily require a precision PWM circuit, the supply just needs to be oscillating at some given rate, and that's enough to initiate the desulfating process (in most cases)... provided the battery is still within the curing range and is not beyond the reviving state.
So what would you need to make this super simple battery desulfator circuit which will also charge the given battery, and additionally possess the ability to keep the new batteries from developing the sulfation issue?
A suitably rated transformer, a bridge rectifier and an ammeter are all that's needed for the purpose.
The transformer voltage must be rated approximately 25% more than the battery voltage rating, that is for a 12V battery a 15 to 16V supply may be used across the battery terminals.
The current can be approximately equal to the Ah rating of the battery for those which need to be revived and are badly sulfated, for the good batteries the charging current could be around 1/10th or 2/10th of their Ah rating. The bridge rectifier must be rated according to the specified or calculated charging levels.
Desulfator Schematic using Bridge Rectifier
How Bridge Rectifier Operates as a Desulfator
The diagram above shows the bare minimum requirement for the proposed battery desulfator with charger circuit.
We can see the most standard or rather crude AC to DC power supply set up, where the transformer steps down the mains voltage to 15V AC for the specified 12V battery.
Before it can reach the battery terminals, the 15V AC goes through the rectification process through the attached bridge rectifier module and gets converted into a full-wave 15V DC.
With a 220V mains input, the frequency before the bridge would be 50Hz (standard grid spec), and after rectification this is supposed to become double that is at 100Hz. For a 110V AC input this would be around 120Hz.
This happens because the bridge network inverts the lower half cycles of the stepped down AC and combines it with the upper half cycles, to finally produce a 100Hz or 120 Hz pulsating DC.
It is this pulsating DC which becomes responsible for shaking-up or knocking down the sulfate deposits on the internal plates of the particular battery.
For a good battery this 100 Hz pulsed charging supply ensures that the sulfation ceases to occur on the first place and thus helps to keep the plates relatively free from this issue.
You can also see an ammeter connected in series with the supply input, it provides a direct indication of he current consumption by the battery and provides a "LIVE update" of the charging procedure, and whether or not anything positive might be happening.
For good batteries this will provide the start to finish info regarding the charging process, that is initially the needle of the meter will indicate the specified charging rate by the battery and may be gradually expected to drop down to the zero mark, and that's when the charging supply needs to be disconnected.
A more sophisticated approach can be employed for enabling an automatic cut-off once the battery is fully charge by employing an opamp based automatic battery full charge cut off circuit (the second diagram).
Using High Voltage Pulse
The configuration detailed below provides the most up-to-date methods of desulfating lead-acid batteries. It is a circuit which routinely supplies quick yet intense pulses to the battery, while discharging the battery marginally between the pulses.
This technique, as much as recognized right now, is the best way to knock of undesirable build up of sulphate crystals and to bring back the battery plates into a good condition.
Because the voltage necessary for the high voltage pulses comes from the battery itself (this might appear a little bizarre initially, however the discharge of the battery is likewise a part of this technique), it is advised to hook up a charger in parallel with the battery and desulfator once the battery has not much capacity remaining.
Pulse generator
It could be noticed in the circuit diagram that the parts needed for the desulphator tend to be extremely humble. The circuit consists of a couple of stages: a high voltage generator constructed using IC1, IC2d and T1, that generates the charging pulses, and an indicator circuit which involves not more than 3 op amps (IC2a, b, c) and three LEDs, that indicate exactly what condition the battery is in.
Let’s go through the pulse generator initially. Just like the other parts of the circuit, its supply voltage is obtained from the battery itself through K1. Although we’re discussing the supply voltage, this must have a pretty consistent voltage and should be devoid of any spikes (except those produced by the circuit itself).
Inductor L1 works like a suppressor, and is included in order to eliminate undesirable voltage spikes, along with C2 and C3 which work like smoothing capacitors.
LED D1 illuminates as soon as the supply voltage is switched ON. To proceed with the pulse generator, IC1 (a 4047) produces a square wave having a frequency of 1 kHz and a duty cycle which typically is 50 PERCENT. When the Q output of IC1 turns high, FET T1 switches ON. This results in a (discharge) current to move through the battery by means of L2, that boosts linearly until the voltage across R4 is approximately 0.35 V; the current can now be around 1 A.
At this instant comparator IC2d changes state, triggering IC1 to be reset and T1 to become switched off. The stashed magnetic energy inside L2 now gets converted to a voltage spike, which is inflicted to the battery through D3. How big the spike is can be determined by the condition of the battery.
If the battery is in a decent condition and its internal resistance is rather small, in that case the spike voltage level may also be reduced (under 15 V). In case the battery has a high internal resistance then the peak level of the spike could be as huge as 50 V. Its highest magnitude will be restricted and equal to the value of the two series connected zener diodes, D4 and D5.
LED Indicators
Considering that the health of the battery could be dependent on how big the charging pulses are, we have included a straightforward LED circuit which indicates the optimum value of the pulses. The 3 comparators IC2a-c evaluate the peak value inside C4 and changeover at voltages of 15, 20 and 30 V correspondingly.
Therefore in case the battery is in a reasonably good shape, the green LED (D8) illuminates, with a under-performing battery, the yellow LED (D9) lights up, and with an incredibly bad battery the red LED (D10) glows.
We have an information that needs to be pointed out regarding the indicator circuit: in order to prevent all three LEDs from illuminating simultaneously in response to a high peak voltage, they are attached in parallel to a single common series resistor (R9).
Since the red LED carries a smaller voltage drop compared to yellow LED, they may in no way illuminate together. But the yellow and green LEDs have got a identical voltage drop, so a similar technique will not do the job here, which explains why the green LED comes with an normal diode (D7), hooked up in series with it.
You will find three alternative methods through which the desulfator may be used. The first is to apply it within an existing system (inside a car as an example) to avoid sulphation from taking place inside a battery having minimum sulphation.
The advanced desulfator circuit is built-in with the system by hooking it up straight to the battery using shortest possible cabling. Because the circuit could be kept connected forever, absolutely nothing more needs to be done.
The current consumption is approximately 20 mA, therefore the battery might discharge in case it is not charged up from time to time. Recovery of batteries which have previously sulphated can be carried out in a couple of techniques. The first method would be to charge the battery, eliminate the charger and after that hook up the desulfator circuit.
Since the power for the charging pulses is derived directly from the battery itself, it is going to gradually discharge. This technique needs to be observed carefully because a completely discharged battery must be recharged quickly.
Most likely in real life many charge/discharge cycles will probably be necessary before a terribly sulphated battery could be restored to life. Since the approach described above needs a great deal of attention and has a danger that the battery could be left in a discharged condition unnecessarily (which can be extremely harmful to a lead acid battery!), another method can be perhaps much better.
The battery is coupled to the desulfator circuit, using a trickle charger hooked up in parallel. This implies, no chargers must be integrated that supply a current of 7 A or higher, yet one that provides a optimum of 1 or 2 A. This could be left coupled to the battery endlessly with no issues.
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