Batteries Basics From Greenwried the battery experts Batteries are used in many applications for storing energy. Think of your cell phone, flashlight, or home energy system. The key thing to remember is that batteries can’t produce energy; they can only store the energy that we have created. Think of a full battery as a full tank of water. As you use electricity you are slowly draining that tank, the more you use the less water (energy) is in the tank. This is known as a state of charge (SOC), a full tank has a 100% SOC a half full tank has a 50% SOC. The Converse of this is called a Depth of Discharge or DOD. So when you have a full tank and a 100% SOC you have a 0% DOD. When you take 20% of the power out of your battery (or tank) you decrease the SOC and increase the DOD so an 80% SOC is a 20% DOD. How does this effect battery life? Battery life is rated in number of cycles, or a discharge and then a charge. For large storage batteries such as L‐16s, golf cart, or sealed/ AGM batteries, there a direct correspondence between number of cycles and the Depth of Discharge DOD. A typical graph looks something like this: As you can see in this graph the deeper you cycle your battery the fewer number of cycles you have. If you drain your battery bank down to 20% DOD every day of the year you are right around 3000 Cycles or roughly 8 and ¼ years. Not a bad life span for a set of batteries. Now if you take the battery down to 50% DOD every day you are looking at somewhere around 1000 cycles or 2 and ¾ years. Not that great for an expensive battery bank. How do we know the DOD on our battery bank? With proper metering this can be as easy as a meter in your home giving you a digital read out of your battery state of charge (SOC). (Check out the Trimetric Battery Monitor.) Another way to keep track of your SOC is using a volt meter. The trick with this is that when charging or discharging the battery voltage can fluctuate rapidly, meaning that to use voltage as an actual measurement of the SOC you need to let your batteries sit for at least 2 Hrs (preferably 6 Hrs) with no charge or discharge before you can accurately predict the SOC using voltage. Below is a chart showing (for 12 V DC) the correlation between SOC and battery voltage for flooded lead acid batteries (industrial, l16’s, and golf cart batteries): For a 24v system multiply the 12V battery numbers by 2 For a 48v system multiply the 12V battery numbers by 4 Physical (flooded Lead Acid) Battery Maintenance: (all persons working near batteries should wear proper protective equipment. Safety Glasses, gloves, and acid proof clothing should be worn at all times) Aside from maintaining the battery state of charge there are a few physical maintenance issues that need to be kept up on. Battery Water: Flooded lead acid batteries must be kept wet. The Acid in your batteries must be maintained by adding distilled water to each cell. During normal charging cycles batteries off gas where water will evaporate out of the cells. To prevent them from going dry we need to fill batteries with distilled water. It is recommended to check battery water levels once a month adding water when needed. It is also important to add water when the batteries are charged. If you fill batteries when discharged and then charge them they will likely overflow spilling the electrolyte (acid) on to the top of the cells. Water consumption is a key indicator to battery state of health. Aside from a complicated load test recording the amount of water used by a battery can be one of the best ways to know the state of health of a battery bank. In short when a battery can no longer store the energy put into them they use that energy to boil off the electrolyte. This means that as batteries get older they will use more water. If your monthly water checking is requiring large amounts of water added it is time to start thinking about a new set of batteries. Battery watering options: Hydro Caps (reducing water consumption) Battery watering systems (providing an easy way to fill batteries) Connections and corrosion: (most common cause of battery Failure) Batteries connections over time will both loosen and corrode due to heat and the acidic gasses present in the battery box. Connections should be checked and tightened twice a year to prevent loose or pour connections from decreasing battery charging and life. This inspection should also include removing any corrosion that had built up on the terminal connections. Tools to help: Battery corrosion compound (prevents corrosion by applying to all metallic surfaces at installation or after a cleaning) Insulated Wrenches (allow for tightening battery terminals reducing the risk of shorting) Battery Charging: Flooded lead acid batteries like to receive three stages of charging. Most inverter charges and solar charge controllers are programmed to give the batteries three stages of charge but some need adjustments to charge and voltage times. Many simple battery chargers (car battery chargers and trickle chargers) are not designed to adjust charge volts and amps to charge flooded lead acid batteries through their three stages. Below is a list of the charge types and what they mean: Bulk Charging: First stage of battery charging where current (amps) is at its maximum rate that the batteries will accept until the battery reaches ~80% SOC. Absorption Charging: The second stage of battery charging. During absorption the voltage remains constant and the current (amps) gradually tapers off as batteries resistance increases. This is the highest voltage in the normal charging cycle. Float Charging: The third stage of battery charging. After a battery has reached a full state of charge, charging voltage is reduced to a lower voltage allowing only a small amount of current (amps) into the batteries. This is to reduce off gassing and water consumption. Often called “trickle charge”, this charge setting is designed to keep a battery fully charged. Equalization: Once a month it is recommended to give the batteries an equalize charge. This is a “controlled over charge” bringing battery voltage to a high voltage and keeping it there for a set period of time to allow for the voltage in all cells in the battery bank to come to an equal level. This can be done by setting the charging system preferably solar to manually or automatically equalize this will only be achieved if they receive first a full charge then an equalize charge. Equalization should only be performed on flooded lead acid batteries. Battery charging voltage Set Points Absorption/Bulk
Voltage
Per 2 V Cell
12 Volt
24 Volt
48 Volt
2.37-2.45
14.2-14.7
28.4-29.4
56.9-58.8
Per 2 V Cell
12 Volt
24 Volt
48 Volt
2.20-2.23
13.2-13.4
26.4-26.8
52.8-53.5
Per 2 V Cell
12 Volt
24 Volt
48 Volt
2.58 -2.67
15.5-16.0
31.0-32.0
61.9-64.1
Float
Equalization
Now you have the knowledge to use and maintain your batteries for a long and productive life.