D.C. (Direct Current) Storage Systems

Direct Current (D.C.) is a smooth continuous flow of electrons that is polarized ( + positive and - negative). D.C. current can be stored inside a lead acid battery. These batteries using lead acid have been around for many years and are found in most all mobile equipment and solar system (PV) charge panels.


Below are several pictures of the lead acid batteries written on in this page text.

The one cell lead acid battery far left has one red cap for one cell to add electrolyte (acid) and made up of lead (Pb) plates (lead acid battery).



In the middle picture is a 3 cell battery with three white caps for three individual cells to add in electrolyte.



On the far right the battery pictured is a 6 cell battery with six yellow caps for 6 cells to add in electrolyte.



In this illustration you can see one large cell, three smaller cells and 6 very small cells and compared to the one cell battery pictured here on the far left. Important is to understand that these three different size lead acid batteries all have something in common, each cell no matter what the size has the potential of 2.2 Volts DC (2.2 Vdc). A one cell battery the size of a piece of toast produces the same exact voltage as a cell as big as a large freezer. Each cell when fully charged will measure 2.2 to 2.4 Volts per cell (100% optimum capacity). Capacity is the major difference between all of these and electrons are the electrical volume or capacity, current is the flow of those electrons. For example one gallon of water can be also said to have droplets and these droplets are exactly the same as the electron or in a specific quantity measureable they are called watts. When someone pays an electric bill it is calculated in watts and when a specific amount of droplets of water add up to occupy a one gallon container that specific volume can also be compared to the watt in the sense it is the common smallest measured amount when paying for them in a bill.


A specific amount of electrons result in one ampere or 1 Amp. The Amp is multiplied by the battery pressure or volts (EMF electromotive force) to calculate watts. The one cell at 2.2 Vdc can power something electrical that can be operated on 2.2 Vdc. When current is drawn from the cell @ 1 Amp the watts would be 2.2 X 1 or 2.2 watts. The 3 cell battery would calculate 3 X 2.2 = 6.6 volts @ 1 Amp = 6.6 watts. The 6 cell battery would calculate 6 X 2.2 = 13.2 volts or 13.2 watts @ 1 Amp current draw. Watts are used by each second of time so to calculate total watts used the seconds are the multiple.



In all cases of these batteries the optimum 100% full charge rating are those calculations given in above text. After the 13.2 Vdc lead acid battery is used or in service when checked a good battery that has been setting around for a week or so is most likely to read 12.7 Vdc  to 12.58 Vdc with absolutely nothing connected to it (no load). So in the calculations that would end up to be a 12.7 / 13.2 or reduce to approximately 3.8% loss or 96.2% of battery left. So in the electrical calculations of efficiency the state of the battery results in two factors electrolyte specific gravity and percentage of charge available. A direct relation of Voltage and current exist whereas the lack of volume will result in a reduction of force potential or electrical voltage.



In an air system the tank holds the air volume and pressure results and builds as more air is pumped into the tank. After the pump reaches the desired pressure someone has set into the controls it stops. Example is the air system where they range from 100 pounds per square inch pressure to 200 pounds air pressure measured at the air tank. The volume of air is directly proportional to the dimensional size of the air tank. The volume keeps things running and the pressure is required to begin moving the load or work effort. The battery is like the air system in the way of pressure (voltage) and capacity (Amps). The cell size in a battery or dimensional volume is exactly like the air tank sizes, larger volume bigger capacity. More capacity in an electrical storage system means that the pressure required to operate an electrical device remains at the specified value and can run the device longer than a smaller cell in dimensional volume. In the days of older technology and to modern times of todays newer batteries it is fact that the type of batteries used are of a certain weight in pounds and for example a lead acid cell that weighs 100 pounds will have 100 times the capacity of the same type of cell weighing 1 pound. So the weight is actually a type of guideline for electrical capacity in the lead acid systems.



A Caterpillar equipment battery that is 12Vdc and weighs 50 pounds has more storage potential in volume than a Caterpillar equipment 12 Vdc battery that weighs 25 pounds, the concept is comparing apples to apples. After understanding that; each electrical device has it's own operating voltages dictated by the manufacturer, that voltage need to be maintained as long as the product is expected to work properly. 



Most all DC low voltage systems (2.2 Vdc to 52.8 Vdc) found in solar systems as well mobile equipment rate the nominal voltages of the device powered at a commonly used 6 volt 12 volt 24 volt or 48 volt DC lower limit. All electrical devices will operate until the voltage in the system drops to a point that the device stops working properly and develops excessive heat eventually burning out.



NOTE: In all systems electrical , pneumatic, or hydraulic as the pressure drops at one point only heat is the result ending up as the product that quit operating. No matter how much volume is contained at the point of low pressure limit no work can be accomplished in any of these systems. A DC device like a motor will run fine at it's rated voltage and as the voltage drops so does the speed of the motor shaft.   


Charging a battery cell relies on the voltage potential of the charging system, either a PV panel or a generator system needs to develop more voltage than the optimum voltage rating for the system or bank of cells. In the 2.2Vdc cell (one) there is no bank of cells, there is a bank of lead plates but in mechanical terms more than one cell is called a battery of cells. It's simply a generic term used to describe a DC voltage storage tank or container.



In this description we will use the one cell to gain a foundation on charging cells properly. A 2.2 Vdc cell can be charged when the voltage drops to the point a load stops working due to low voltage. For the one cell you would want to charge it when it was at no load measuring 1.74, note the differences in decimal because they are very important to specifically calculate charge potential. If the setting on the battery charger or system voltage was 2.2 Vdc the rate of current (volume) would continue until the charging cell reached 2.2 Vdc. No more current can flow into the cell when the voltages are equal to one another.



That is not sufficient to keep the system at it's optimum so setting the charge voltage higher makes more electrons to flow into the cell. The rate at which they flow from the charger causes heat in the cell and this is dependent on the applied voltage so when setting up a charge pattern a measurement of temperature should be taken with either an infrared temp gun or a thermometer you can dip into the cell acid. For example when the cell is first checked @65f and you start charging it @ 2.2 volts and after 10 minutes the temperature rises to 70f you can safely turn the charger voltage up. In the same cell after the voltage is turned up and the cell goes to 115f the charge voltage is to high within that 10 minute period. As long as the lead acid battery does not reach a temperature that is uncomfortable to touch by heat dissipated you are ok with the charge rate.



This holds true for a battery of cells also and for a simple every day example we take the conventionally known 12 volt car battery or the solar battery. If either one has signs of acid on the top of the battery you are severely overcharging the system, that means the charge rate is causing the acid to boil. Heat is one simple aspect that kills cells in batteries. On and every day battery charger (12 volt) you will find an Amp charge rate which in simple terms what is changed is the output voltage. A slow rate of charge would measure 13 volts, a high rate of charge would be 19 volts and when the charge is shut off the battery should normalize @13.2 Vdc. The important fact is voltages and the current flow are proportional, as the voltage goes higher so does the current flow. Time of charge is a factor and has to equal the time of usage to keep the battery of cells operating efficiency and extending the life of the cells.



Think of the electron as a tiny soap bubble and like in the air systems a compressible bubble. The more electrons you can squeeze into the cells of a bank the better the batteries will perform for a longer period of time. The colder the battery the more it naturally discharges, if a battery is not charged and sets on a cold floor it discharges at a small percentage per day and then if the battery is not charged it will go dead. That is normal for a lead acid battery of cells.



Rate of voltage drop in time with no loads applied to the system of cells is used to find the capacity of the cells. After charging a 12 volt battery to 13.2 Vdc take a meter reading and set the clock, after 10 minutes take another reading and the system should be slightly lower in voltage but not below 12.8 Vdc. Keep checking every 10 minutes without applying a load to the cells. If the battery cell bank goes down to 12 volts in an hour then you could be sure the battery cells are almost used up. A good 12 volt battery with no load will not drop under 12.4 Vdc in several days and it depends on battery temperature. Setting a battery on a cement floor that is cooler than the outside temperature or drops below 60 degrees f causes a faster natural discharge of the cells. Cold conditions are really hard on battery cells as is heat above 115f.



Calculating your storage capacity in a solar PV system is critical to the life of the battery bank. Capacity is what costs the money not the voltage, the more the capacity the more the initial cost. Under sizing your storage system will cause you problems in any application of DC batteries. Over heating your batteries damages the cell plates. Letting a battery bank system set without charging will cause the specific gravity to change in the acid water mixture causing the cell to go bad. Cells go bad and loose capacity to produce voltages and store DC electrical current by absorbing the water from the mixture of acid and water. It is important to keep the specific gravity balanced all of the battery cell life.