Heavy and expensive batteries can be the weakest link in a renewable energy system. And if you abuse them, they can wear out fast. Here’s how to get the most out of your investment.
Take the initial cost of your battery bank, and divide by the number of years until it needs replacement. That’s your annual “battery bill.” If you can stretch battery life to eight or 10 years, the bill is minimal. If you ruin them in a year, that’s a big bill, and you probably were not paying much attention to them. Overcharging, undercharging, and high and low temperatures can all count as “abuse.”
Determining if your batteries are being used—or abused—is where battery monitoring systems come in. Professional RE installers and troubleshooters, and system owners alike, can analyze the stored data for clues as to what went wrong—or right—with a system. And battery-destroying problems, like loose connections or chronic undercharging, can be detected and nipped in the bud.
Accurately determining how full your batteries are (state of charge; SOC) is a complex problem. When you fill up your car with gasoline, all the fuel you pump into the tank stays there until you are ready to use it. Filling batteries is different—more akin to a having a small hole in the bottom of the gas tank. Batteries lose a little electrical “fuel” each day, even if unused—called “self-discharge.” Plus, a portion of the energy that’s pumped in never even gets stored, because battery-charging efficiency is never 100%. To further complicate matters, the faster the rate at which that stored energy is used, the smaller your tank becomes. Finally, temperature also affects battery storage capacity and longevity.
The most accurate way to determine SOC is to measure the electrolyte’s specific gravity for each battery cell. In a fully charged lead-acid battery, the electrolyte is a strong mix of sulfuric acid and water; in a fully discharged battery, the mix is mostly water. A specific gravity measurement compares the electrolyte’s density to that of water at the same temperature.
A hydrometer is the standard tool used for measuring specific gravity and typically costs about $30. The denser the liquid (and thus the higher the SOC), the higher the float rides. Choose a hydrometer that includes a thermometer. Colder electrolyte is denser—without temperature compensation, a hydrometer will show an inaccurately high SOC. To use the device, first put on your protective eyewear, rubber gloves, and old clothes. Then, using the bulb, fill the hydrometer full of electrolyte. Record both the number showing at the liquid level and the temperature reading. Do this for every cell of every battery in the bank, then compute the SOC for each cell.
This is a time-consuming and potentially back-breaking task, with the prospect of a mess at any moment. Thankfully, there are simpler (and more automated) solutions available for battery monitoring. But make no mistake—a good hydrometer is an essential item in any battery tool kit. It provides the bottom line on determining battery SOC.
The simplest and cheapest way to monitor a battery bank is by simply reading its voltage with an accurate voltmeter. A variety of products are available—some show the voltage reading directly; others using a “gas gauge” format, showing voltage on a scale from “full” to “empty.” Most cost less than $50.
However, this technique works only under certain conditions, and the range from empty to full covers only a short range of voltages, so accuracy is compromised. For voltage monitoring to accurately assess SOC, the battery bank must be “at rest” for at least two hours, with no energy moving in or out. A voltage reading during charging can confirm that a battery bank is full, but offers no other information. When the household is using energy from the battery, the voltage reading will be artificially low; when the battery is charging, it will be artificially high. For a PV system, checking battery voltage is best done during early morning—before loads are in use and before PV modules start sending energy to the batteries.