Distinct Steps and Infinite Sequential Monitoring (ISM™)
There are a number of distinct, definable charging modes, or methods, or stages, or phases, or steps within a battery charging algorithm. Not all of these steps are essential in every battery application for every type of battery. Also, given the increasing complexity of optimum charging requirements for many batteries in the 21st century marketplace, Deltran Battery Tender® battery chargers have come to depend more upon the Infinite Sequential Monitoring (ISM™) approach in the executive microcontroller code that governs the behavior of the battery chargers. In other words, although any number of specific charging steps may be available for execution, usually sequentially, in the definition of any given charging algorithm, the ISM™ executive control overlay performs the essential task of optimizing the battery charger performance over a wide range of operating conditions.
Let’s consider what we can define as meaningful steps to be included in a charge algorithm. Let’s also consider what often happens in the marketplace. The desire for a manufacturer to differentiate a product from its competitors can sometimes result in the creation of technical jargon that may not be the most beneficial in terms of helping end users really understand how the technology actually works.
So let’s talk about the details of the steps and try to avoid unnecessary technical jargon. The numbering of the steps and the order of their presentation simply indicates a typical sequence in which they would appear in any given charge algorithm. Again, not all steps are available nor are they necessary in all charger algorithms.
First Step: Initialization or Qualification.
This step has been around and incorporated in battery chargers from day one. Although it may not have been clearly defined or even considered to be a step. But truthfully, it may be the most important step in terms of safety. Virtually all battery chargers measure the condition of the electrical connection between the battery and the battery charger output. The specific limits of the parameters may differ, but the behavior of the voltage and current measured at the battery charger output give a fairly clear indication as to whether things in the battery charging world are normal or not.
As an example, if the charger output voltage is positive and the output current is zero, then that is a good indication that there is either no connection or an extremely poor connection between the charger and the battery. In technical terms, that is an open circuit, or very high impedance at the output. This is a common circumstance caused when a fuse has opened between the charger and the battery. That is a condition where it is wise to shut down the charger output and give the charger operator an indication that something is wrong, like flashing a specific color light or flashing more than one color light in a specific timed sequence.
Another common example is when the output voltage is positive and the output current is negative. That would normally indicate that the battery terminals are connected backwards to the battery charger output. You would think that the voltage would also be negative, but because of the laws of physics and electrical circuits, it is still possible for the charger to read a positive voltage. One more thing, all Deltran Battery Tender® battery chargers are designed to prevent negative current, which if unchecked, would drain the battery.
Second Step: Recovery.
This step is necessary to deal with severe over discharge situations. Both lead acid and lithium batteries may be subject to this problem. If you forget to turn off your lights on a power sport vehicle, then you can completely drain your battery in a short time. The philosophy behind recovery is to use low amplitude current to gradually build up the charge stored in the battery and support a voltage sufficient for the battery to accept a normal recharge regime. Even with a small current, there should be a minimum amount of voltage available. For 12 volt lead acid and lithium batteries, that value is about 4 volts. Anything less than 4 volts, and the recovery mode will not be employed. In the family of lead acid battery chargers, the recovery step is more of a background, on-demand type of function. In the family of lithium ion battery chargers, the recovery function is more distinct and clearly defined because the lithium ion batteries are more susceptible damage if the recovery parameters are not tightly controlled.
Third Step: Bulk Charge.
This step has the honor of occupying the unique position as being the only truly essential step in a charge algorithm, at least for lead acid batteries. Here you allow the battery to draw as much current as the charger will allow (called the current limit) until the battery voltage rises to a predetermined maximum level. When the voltage reaches that maximum level, the charger may be turned off. Before the voltage reaches its predetermined maximum level, the current will stay at close to its maximum value, or the current limit. Most charger manufacturers refer to this step as a ‘constant current charging mode’. In most cases, after bulk charge is complete, the battery will be about 80% charged. That is good enough to use it again without doing anything else.
Fourth Step: Absorption Charge.
During this step, the behavior of the voltage and the current is reversed from that observed during the bulk charge step. The voltage is held constant and the current is allowed to decrease naturally. If you look at the graphs, during bulk charge the voltage starts to increase in a straight, linear fashion. Then as the voltage approaches its predetermined maximum level, the curve follows more of an exponential curve. During absorption, the current decays following a straight, linear path, then curves and tapers into a very low level, where it stays until the value of the charger output voltage changes.
The importance of the absorption charge step directly relates to completing the full replenishment of the individual battery cells. There are very complex mathematical equations that could explain the chemistry of this phenomenon, but the truth is that most of the useful knowledge available for charge algorithm applications has come over decades of trial and error. You will be hard pressed to find an explanation that justifies the effectiveness of the absorption charge step that does not include a very strong dependence upon empirical data. That is particularly true when you consider that the absorption charge step is only completely effective if it is allowed to continue long enough so that there is a minimum of several hours, probably at least 4 hours, when the battery is drawing virtually no current, but the applied voltage is kept high, at the absorption level. At first glance, that seems to make no sense. But it is absolutely true.
Fifth Step: Equalization Charge.
For lead acid batteries, this step is important mostly for a number of batteries being charged by a single voltage output charger while the batteries are connected in a series string. It takes a number of batteries to clearly observe the effect. Four batteries are usually enough. The mechanics of the equalization step appear graphically similar to a combination of the bulk charge and absorption charge steps. The difference is that the current starts out a very low level, approximately at 2 to 5% of the current limit of the battery charger, or at simply a very low fixed level, like 0.5 or 1.0 amp.
Depending upon how the actual equalization charging current value compares to the numerical value of the battery capacity in amp hours, and depending upon the equalization charging voltage limit, the charging current will only remain constant for a very short time. Then, for the balance of the time remaining in the equalization step, the voltage and current will behave as they do in during the absorption step. However, both the voltage and the current amplitudes are different.
What is the observable effect on batteries connected in series? The fundamental definition of a series connection is that one current flows through all of the elements that are connected. If a single charge current is applied to 4 or more 12 volt batteries connected in series, then without the equalization step it is likely that the individual voltages on the 12 volt batteries may vary by as much as 0.2 volts. For example, after recharge, the voltages on the 4 batteries in a 48 volt string might be 12.85, 12.8, 13.05, and 12.9 volts. If you add these voltages together, the sum is 51.6 volts which is that same as 4 batteries with each voltage = 12.9 volts. That is the theoretical 100% SOC value for a lead acid battery.
We’ll discuss why those individual differences might exist later. For now consider that 1.5 volts represents the full capacity range on a single 12 volt battery. Therefore 0.2 volts represents about 13% of that range on a single battery. What happens to those individual voltages when we employ the equalization step? The readings change to 12.89, 12.9, 12.91, and 12.9 volts. The range of variation is now only 0.02 volts, or 1.3% of a single battery’s full capacity range. This shows that all 4 batteries are charged equally, based simply on the observation of the terminal voltage.
Why the initial difference? Remember that each 12 volt lead-acid battery is made up of 6 individual 2 volt cells. The fully charged voltage of each cell is 2.15 volts. What if the cells do not perform identically and their voltages vary to the point where their combined value varies between 12.85 and 13.05 volts. That is exactly what happened. The remedy, which is to apply equalization level charge current, does in fact “equalize” the voltages. But the explanation remains in the realm of empirical observation. Not as satisfying as solving some mathematical equation, but effective none the less.
Sixth Step: Float / Maintenance Charge.
This step is very important in terms of the fundamental Battery Tender® defining concept. The whole purpose of float / maintenance is to maintain a fully charged battery in that 100% State of Charge (SOC) condition. For nearly all batteries, that means applying a voltage to a fully charged battery that is 1 or 2 tenths of a volt above the voltage that the battery would support to indicate that its SOC = 100%. Also, the battery must be at rest, not being charged or discharged.
In most cases, a 12 volt lead-acid battery, at 100% SOC, will have a rest voltage between 12.8 and 13.1 volts. That means an effective float voltage need only be as high as 12.9 to 13.2 volts. However, most Battery Tender® battery chargers have float voltages between 13.3 and 13.5 volts. The important thing is that the float voltage should be higher than the fully charged rest state battery voltage and it should be lower than the gassing voltage which is about 13.8 volts. See the discussion about float charging on the Battery Tender® website. It is definitely worth your time to read that document.
The float voltage requirements for the 12 volt lithium ion battery, specifically the lithium iron phosphate battery are a little higher because the combined voltage of 4 lithium ion cells at 13.3 volts is higher than 6 lead acid cells at 2.15 volts.
In the following figure, the text boxes above the voltage and current graphs contain the details for the charging steps. The time scale is not proportional to any real time. It set to match up with the text boxes. That is for display information only.
Looking at the graph, the first real charging step is Step 2, Bulk Charge. After successful qualification, depending upon the charger being employed there are varying current limited, timed voltage generation tests that are not specifically shown. Given the complexity of those tests, they could certainly be considered to be a recovery mode, or at a minimum, an extended qualification mode. Suffice it to say that other factors are being considered to ensure safety and the validity of the decision to proceed into the basic charging steps.