Understanding Batteries Used in PV Systems

If you’ve done your homework before purchasing batteries for your solar PV system you’ve likely purchased some sort of a deep-discharge lead-acid type. Your batteries may have been designed for golf-carts, fork-lifts, or perhaps the kind used to run a trolling motor and lights on a fishing boat. If your budget didn’t allow you to buy the best available batteries, you might have chosen batteries with a shorter life expectancy. Whatever the type of lead-acid batteries you’ve chosen, they all have similar characteristics. If you want to know the condition of your batteries at any given time, you need to understand some basic electrical principles and some battery quirks. That is the purpose of this paper.

A battery can be thought of as a storage medium for electrical energy. Just as a glass of water can be nearly full or nearly empty, a battery can be fully charged, fully discharged, or anywhere in-between. If we were to measure the amount of water in a glass with a ruler, it would be easy to determine how full (or empty) the glass is. Since we can’t see electrical energy, we need to use instruments to measure the state of charge in a battery. A hydrometer is the most accurate instrument for state-of-charge measurements, but a voltmeter can also be used.

A hydrometer measures the density of the fluid, known as electrolyte, in a battery. The heavier the fluid, the greater the state of charge. To perform a test, a little of the liquid is drawn from the battery into the hydrometer. A float and a scale within the instrument indicate the specific gravity (density), of the liquid. With that reading, and through the use of a chart, the state-of-charge can be determined. Obviously, a hydrometer test cannot be done on a sealed battery.

Although not as accurate as a hydrometer, a voltmeter can be used to measure the state-of-charge of a lead-acid battery. However, there are a few things you need to know in order to make accurate measurements.

* Batteries have internal resistance.

When a battery is a part of a complete circuit, that is to say one in which current flows, the battery voltage will decrease. The voltage drop caused by the load on the battery will result in an inaccurate state-of-charge determination. Measure battery voltage with no load connected.

* Batteries are charged by applying a voltage greater than the rating of the battery across the terminals.

To charge a 12-volt battery, for example, 14.4-volts dc may be applied to the battery terminals. You cannot determine the state-of-charge of a battery when a charging voltage is applied. Some charge controllers provide pulses instead of a steady voltage, making dc-voltage readings inaccurate.

* The voltage reading across a recently charged battery may be far in excess of the battery’s rated voltage.

This phenomenon is known as a surface charge. To accurately measure the state-of-charge it is necessary to wait until this charge dissipates, or to burn it off by applying a load to the battery for a short time.

Interpreting Battery Voltage Readings:

Intuitively, one might expect the voltage reading of a fully-charged 12-volt battery to be exactly 12-volts. Likewise, one might expect the voltage reading of a half-discharged 12-volt battery to be 6-volts. However, this is not the case. Keeping in mind the information presented above, the chart below can be used to determine the percentage of charge for a 12-volt battery.

Battery Voltage / Percent of Charge

12.70 Volts = 100
12.58 Volts = 90
12.46 Volts = 80
12.36 Volts = 70
12.28 Volts = 60
12.20 Volts = 50
12.12 Volts = 40
12.04 Volts = 30
11.98 Volts = 20
11.94 Volts = 10
11.90 Volts = Discharged

Understanding that overcharging and over-discharging can damage or shorten the life of a battery, this information is very important. Fortunately, your charge controller prevents overcharging. It does so by removing charging current to the battery-bank once it senses that the batteries are fully charged.

Typically, inverters include the ability to turn themselves off when the battery voltage drops to preset level. As a result, the inverter protects the battery from over-discharging. Systems that do not include an inverter should include some method of removing the load when battery voltage drops to a dangerous level. Some charge controllers provide this functionality.

More Battery quirks:

* Battery capacity is affected by temperature.

A cold battery is not able to supply as much power to the load as a warm battery can.

* The discharge rate affects battery efficiency.

As the rate of discharge increases, battery efficiency decreases. An over-sized battery array is more efficient than a smaller-capacity array.

* Battery charging is more efficient at a lower rate of charge.

Just as batteries are more efficient at lower discharge rates, charging efficiency is greater at lower charge rates. In other words, a battery more efficiently stores energy pumped into it when the charge rate is relatively low.

* Batteries are not 100% efficient.

Expect losses in batteries, and other system components for that matter.

* A strange battery quirk.

When connecting a load to a fully charged battery the voltage initially drops as expected, but then begins to increase over time. This phenomenon may be due to the chemical reactions that occur when current flows within a battery.

* Chronically undercharging a battery will result in shorter battery life.

A PV array that does not provide enough charging-power to keep the battery-bank fully charged will result in reduced battery life. If your PV system is unable to maintain a fully-charged battery bank, add solar panels to your array or reduce the load on the battery.

* Allowing a battery to remain in a discharged state for an extended period of time will shorten its life.

Hard crystals of lead sulfate will form on the plates over time, reducing the capacity of the battery. To prevent this, don’t let a battery remain in a discharged state for an extended period of time.

* Avoid battery stratification.

Because battery acid (sulfuric acid) is heavier than water, it tends to settle at the bottom of the battery. This causes a build-up of lead sulfate on the plates near the bottom. This can be prevented by applying an equalizing charge of 14.4 volts to a fully charged battery for a short time, at least once per month. The elevated voltage causes bubbling, mixing the electrolyte. This function is built in to many modern charge controllers.

* Allow a battery-bank to age gracefully.

Do not add batteries to an existing battery-array. If you do, the life-expectancy of the newer batteries will be adversely affected.

* Overcharging can be dangerous.

Batteries produce hydrogen gas when charging. Overcharging, or charging at a high rate, causes an excess of hydrogen gas production. For safety, the battery-bank should be vented to the outside. Since hydrogen gas is lighter than air, an effective system includes a hood above the battery array, and stove-pipe or pvc tubing rising from the highest point.

Unless your batteries are sealed, check fluid levels at least twice per year. Excessive fluid use can be an indication that batteries are being charged excessively. If that’s the case check the rest of your system, particularly the charge controller, to be sure everything is working properly.

Battery bank capacity testing:

The capacity of a battery bank can be measured by conducting a discharge test. At night, when no charging current is present, connect a constant load to the battery bank. Using an accurate digital voltmeter, measure battery bank voltage at regular intervals and record the results. Terminate the test when the battery bank is 50% discharged. The test results can be used as a baseline to be compared with those of future tests. When the capacity of the battery bank falls to an unacceptable lever replace the entire battery bank.

It is also important to note that one defective battery in a battery array can create a load on the rest of the batteries within the array. To locate a defective battery it is necessary to disconnect batteries one at a time from the array and test each battery individually. The defective battery will probably have a significantly lower voltage than the others.

Summary:

The best way to check the state-of-charge of your batteries is through the use of a hydrometer. It is easier to measure the state of charge with a voltmeter, but you need to avoid measurement pitfalls in order to ensure accuracy.

Avoid battery problems by conducting periodic state-of-charge checks, monitoring the fluid levels, and by avoiding stratification through the application of an equalizing charge periodically. In addition, keep the tops of the batteries and battery terminals clean. Make sure that terminal connections are tight.

To design a PV system you’ll need to understand electrical principles beyond those presented in this paper. For the sake of clarity, technical terms and formulas were avoided.

Solar John