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Independent Power - More About Batteries

Lead acid batteries are presently the most common type of deep cycle battery used in Independent Power Systems. In order to make accurate decisions in selecting and caring for these batteries, a basic understanding of this battery type, and battery terminology is helpful.

Efficiency

The efficiency of a battery is defined as the ratio of the electrical energy delivered by the battery, to the electrical energy consumed by the battery. The energy conversion efficiency of most lead acid batteries ranges from 80% and 85%.

Factors Affecting Battery Performance

If a battery is discharged slowly it releases more power than if the battery is discharged quickly. For example a 12-volt battery rated at 100 amp hours, can deliver 5 amps per hour for 20 hours before it can no longer deliver power. However, the same 12-volt battery can deliver only a maximum of 70 amp hours if discharged in 5 hours.

The actual depth (percent of capacity) of discharge will have a strong influence on the life of a battery. For example, a battery that has a cycle life rated at 10% discharge per day for 5 years may only last a little more than two years, if discharged 20% per day.

Operating Temperatures

Battery Capacity As A Function of Temperature
Temperature (F°)
Percentage of Capacity
120
110
85
105
80
100
60
90
40
77
20
63
0
49
-20
35
-40
21
-60
9

You can see from the chart above that it makes sense not to let batteries get too cold.

On the other hand higher temperatures result in an increase in capacity, but above 85° the life of the battery is reduced. If batteries are to be operated at high ambient temperatures to improve life expectancy, a lower concentration of electrolyte is recommended. This is called tropical electrolyte.

Stratification of Electrolyte

If a battery is maintained at it's float voltage and never charged above this voltage, two problems can occur.

  1. The electrolyte may stratify. This means the acid part of the electrolyte will be concentrated at the bottom of the battery, with the water at the top. This causes poor performance and makes the battery more susceptible to freezing, causing permanent internal damage.

  2. In a system where several batteries are connected in series it is possible that a single battery may lose it's capacity before the rest. This will cause poor performance for the entire battery bank.

Overcharging the batteries (commonly called Equalization) periodically can help mitigate these issues. This means that the battery voltage is raised above the normal float voltage for a while. Once the battery has reached a full charge the voltage is dropped to minimize excess 'gassing' (bubbling). This is called a finishing charge. Thankfully, most of the good inverter/chargers and solar cell regulators perform this charge requirement automatically. Proper charging is critical in extending battery life.

Discharged Batteries

A battery that has been discharged should be recharged as soon as possible. Prolonged periods of a low state of charge will result in severe shortening of battery life. The battery will either not regain full capacity or may not be able to be recharged at all.

Battery Past History

Used batteries seldom perform well. As the age of the battery increases the capacity decreases. The actual amount of decrease depends on battery construction. A really impressive looking battery may not produce much so be wary of used battery 'deals'.

Battery Additives

Adding anything other than clear DISTILLED water to a battery will cause shorter life, and lower capacity. Battery additives claim to improve battery performance or increase capacity should be carefully scrutunized for independent documentation of the claims.

Self-Discharge Rate

All batteries continually lose some of their stored energy because of internal reactions, even if they are not being discharged to power motors or lights. This is called "self-discharge", usually expressed in terms of amp hours lost per month. This may be stated as the percent capacity lost per month. Self-discharge increases with battery temperature and age. This rate can, in extreme cases, reach 40% to 50% of capacity. This means that any alternative energy source will have to supply the battery with the normal charging current, and additionally an amount equal to the self-discharge rate.

Electrolyte Specific Gravity

The specific gravity of a battery cell electrolyte is an indicator of the charge of that cell. At full charge the specific gravity is high (1.250-1.300), and the acid concentration is high (40%). Most lead acid batteries are considered to be completely discharged if the specific gravity is 1.100.

This is measured with a battery cell hydrometer. This instrument should be cleaned with water after every use, and should never be used to test different type os batteries. Specific gravity measurements are affected by temperature. As a result, in order to obtain an accurate value, you must first measure the temperature of the electrolyte then record the specific gravity reading and adjust it to the temperature corrected value. Temperature correction tables usually come with the hydrometer kit.

In Summary

Batteries are critical to living off the grid, so learning more about them is a great advantage in building a truly great system. Bu