Bitteroot Solar:

Battery Capacity and
State-of-Charge for Deep Cycle, Lead Acid Batteries
by Steve Bell

Often questions arise about battery capacity and determining a battery's State-of-Charge (SOC). Battery capacity describes how much available power, normally in amp-hours (AH), the battery can provide to the load under a given set of circumstances. Battery SOC describes how much of the battery capacity is available at a given moment. Both capacity and SOC vary depending on a collection of factors. A 12 volt-210 AH will be used as an example throughout this article.

The available capacity of a battery varies with the rate of discharge, the battery temperature, age, and cycle history. Battery capacity is normally shown as 210AH @ 20 hr. rate at 25 degrees C (77 degrees F). What this is saying is, the battery at 25 degrees C can supply 210 AH of en if it is discharged at a steady 10.5 amps per hour for 20 hours (10.5 amps x 20 hours = 210 amp-hours, AH). At the end of 20 hours the battery will be fully (100%) discharged (to 1.75 volts per cell). The 20-hour rate is the typical industry standard for comparison although 6 or 8 hours are sometimes used for UPS applications.

Often, the battery is discharged over a much longer time period, 48 hours or 100 hours. The slower a battery is discharged, the more energy (AH) it can provide. This same 210 AH battery can supply 240 AH if discharged over 48 hours or 265 AH if discharged over 100 hours. Similarly, if the battery is discharged over only 8 hours, it can only provide 180 AH.

Battery temperature also affects available battery capacity. As a battery gets colder, the chemical reactions slow down. This causes a reduction in available capacity. When the battery warms up the capacity returns. For example, our battery was rated at 210AH @ 25 degrees C (77 degrees F). If the battery temperature were 10 degrees C (50 degrees F) the battery would only have 88% of its rated capacity, 185 AH. If the temperature were -10 degrees C (14 degrees F), it would be about 68% or 143 AH. Conversely, if the temperature was 40 degrees C (104 degrees F) the capacity would be 108%, 227 AH. Note, it is not recommended operating a battery at elevated temperatures, it will significantly shorten battery life.

Battery age and cycle history both effect how the battery capacity will change with usage. Over time, as a battery is used, subtle changes occur in the battery, it internal resistance slowly increases. This internal resistance acts like a miniature load on the battery and is described as the Self-Discharge Rate. With new batteries, this rate can be from 1% per week to 1% per month (of the rated capacity) depending on the exact configuration of the battery. As the battery ages, this rate slowly increases to 3 to 5 times that amount.

How the battery is cycled, charged-discharged-charged, effects both battery capacity and battery life. The more completely or deeply a battery is discharged, the shorter its functional life. The percentage of total battery capacity that is removed during a discharge is called the depth of discharge (DOD). Typically, if you regularly, completely discharge (100% DOD) the battery it will only have a functional life of 50 to 300 cycles. If you discharge the battery 80% (80%DOD) you get about 400 to 800 cycles; at 50% DOD you get around 800 to 1200 cycles.

As the number of cycles increases, for a given DOD, the battery capacity slowly decreases. By industry standards, the functional life of the battery is over when, at full charge, the battery only retains 80% of its original capacity.

How fully a battery is recharged has a profound effect on battery capacity and battery life. Lead acid batteries need to be recharged 100% on a regular basis to insure they retain their capacity and expected life. If the battery is regularly only recharged to 90% or 95%, a chemical reaction occurs called Sulfation. As a battery is discharged, sulfur from the acid reacts with the lead in the battery plates and forms lead sulfate, and releases electricity. When you recharge the battery, you input electricity to reverse the process and breakdown the lead sulfate attached to the lead plates. If you only recharge the battery 95%, you are leaving 5% of the lead sulfate crystals attached to the lead plates. Over time these lead sulfate crystals increase in area and become strongly bonded to the lead plates, this is Sulfation. The portion of the lead plates that has the bonded sulfate crystals becomes chemically inactive and this functionally reduces the battery capacity. Chronic, incomplete recharging can cause a major reduction in capacity and battery life. Most batteries do not wear out, they die from sulfation.

The state-of-charge (SOC) of a battery tells you both how deeply you are discharging the battery and how completely you are recharging it. Determining the SOC of a battery can be as elusive as defining the battery capacity. The simplest and least accurate method of determining SOC is using battery voltage. The only way to get an approximate SOC reading using voltage is to let the battery sit for at least four hours with no charging or discharging, battery at rest, before taking the voltage readings.

A much more accurate way to measure SOC is to use a quality amp-hour meter such as the E-meter or Trace TM500. These devices measure not only battery voltage, but also measure all the amp-hours flowing into and out of the battery. They will accurately indicate both the SOC of your battery and when your battery is full recharged. They can compensate for charging inefficiencies and battery temperature. A quality amp-hour meter is a wise investment for any large battery system.

For flooded lead-acid batteries, check the specific gravity of the electrolyte. Use a quality hygrometer with a temperature compensation scale. Most flooded lead acid batteries have a specific gravity of 1.250 to 1.280 when the battery is full recharged. You will need to know the correct specific gravity for your batteries. Remember, you are working with sulfuric acid, use all necessary safety precautions including a face shield.

As you can see, battery capacity is an ever-changing value depending on the size of the load, battery temperature, age, and cycle history. You have to estimate a typical average for these factors to determine your battery capacity. The SOC is constantly changing with the varying loads and charge sources. For most battery configurations used with independent power systems, it is recommended having 3 to 5 days of battery capacity (72 hours to 120 hours). This not only allows the battery to operate at its slower, higher rate capacity, but also permits a much shallower average DOD. Your system should be designed to limit the maximum battery DOD to 80%. Many systems are configured to limit the typical DOD to 50% and only use the 80% DOD for extreme circumstances.