Energy storage is critical for solar energy systems to store power when it is not needed and to distribute it when production falls short of demand. And to ensure a seamless transition from live to battery power in your solar energy system, you need to make sure your batteries are in prime working condition.
Keeping batteries in peak condition
Two of the most common types of batteries used for solar power storage are lead-acid and lithium-ion. The most important part of maintaining either type is to make sure the battery is operating at acceptable temperatures, from 10 to 60°C (50 to 140°F). Too cold and the electrochemistry slows. Too hot and the cathode material breaks down. Either extreme can reduce the life of your battery. In cold climates, it is a good idea to use a battery heater or thermal insulation. In hot climates, batteries should be stored in shaded and well-ventilated spaces.
With lithium-ion batteries, you also must contend with thermal runaway that can occur in hot temperatures. Heat causes the cathode to decompose, releasing oxygen that burns the electrolyte, and further increases temperature and pressure until, if not checked, the battery explodes. Thermal runaway is a dangerous feedback loop, often resulting in a fire.
In addition to temperature, it is vital to ensure batteries have the appropriate input voltage and current. If the voltage is too low, the charge state will be low. For lead-acid batteries, it is critical to ensure a high state-of-charge (SOC) because they have a low depth of discharge. Low depth of discharge means that only a small amount of their total capacity should be used.
An SOC below 50 percent in these batteries can lead to sulfation — a buildup of small sulfate crystals on the terminals. This reduces the battery’s output, decreases its life and increases maintenance costs.
Sulfation on the terminals can be minimized by keeping the SOC high. If the SOC drops too low, you can use a desulfating battery charger to dissolve the sulfates with high frequency electronic pulses. You can also connect an alarm with a visual and audible alert that activates when the SOC falls below 50 percent.
Corrosion also can occur on the terminals of lead-acid batteries when hydrogen gas in the battery acid reacts with other substances or when the electrolyte leaks from overfilling with water or overcharging. To remove corrosion, use antioxidant materials according to the manufacturer’s recommendation.
Building a battery maintenance routine is highly recommended to ensure longevity of the battery. This includes following the best practices outlined by IEEE for the appropriate battery.
Assessing battery health
It is important to understand the SOC of a battery as well as the state of health (SOH). The state of charge is like the “fuel gauge” for a battery. It indicates the amount of energy remining in a battery compared to its total capacity to provide the user with an estimate of how much longer a battery can perform before being recharged. SOC is an instantaneous indicator of the battery capabilities.
SOH is a longer-term indicator of battery capabilities and tells the user the performance of the battery as well as life left in the battery.
Following the appropriate IEEE standard depending on the type of battery being used is important to understanding the health of the battery.
To get a more comprehensive view of the health of individual stationary batteries or battery strings, you can use a battery analyzer. Look for a portable battery analyzer with a CAT III 600V, 1000 Vdc rating for safe measurements around battery power supply equipment. The analyzer should measure both dc and ac voltage and current, plus:
- dc voltage, internal resistance and temperature.
- Battery internal resistance to indicate the health of the battery’s electrochemistry
- Temperature, which is crucial for lithium-ion batteries that are prone to overheating and thermal runaway
- Strap resistance testing to determine if the interconnections need to be replaced or the screws tightened for better contact.
- Ripple voltage to reveal unwanted residual ac component of the rectified voltage in dc charging and inverter circuits — one of the root causes of battery deterioration
- Include an angled probe for hard-to-reach places inside a cabinet.
- Enable profile creation, so you can share battery management profiles making tests repeatable, reduce setup time and organize your historical measurements.
The battery analyzer should be able to test all aspects of a single cell or a string of batteries. Automatic sequence testing of voltage, resistance and temperature will help expedite verifying the health of the large battery banks common in large solar power systems.
Lithium-ion batteries require less maintenance, but you still need to regularly check their SOC and keep them as close to 25°C as possible. They have a higher depth of discharge, so much of their charge can be depleted before recharging, which makes them a good choice for variable solar energy storage systems.
Sean Silvey is a product application specialist with Fluke Corp., a manufacturer of electrical test and measurement tools.
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