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.
Specific Gravity
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.
Monitoring by Voltage
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.
Counting Coulombs
Using specific gravity to determine SOC is messy and time-consuming, and can't easily be automated. Voltage readings are woefully inaccurate. So what's left? Coulomb counting.
A coulomb is the amount of electric charge that is transported in 1 second by 1 amp. Devices that count and total coulombs are called amp-hour meters or watt-hour meters. A shunt-a high-power, precision resistor that is not affected by temperature-is used. The meter measures the voltage drop across the shunt and, using Ohm's law, calculates the amps and/or watts going into or out of the battery bank. The meter also tallies how long this current is moving in either direction, giving you amp-hours or watt-hours.
More sophisticated amp-hour meters may use multiple shunts, so you can separately monitor your RE energy inputs-like from a PV array and a wind turbine. Many include a battery temperature sensor, which improves accuracy.
Coulomb counting is not infallible, as charging efficiency and self-discharge will both change as a battery ages. But the technique is very convenient and gives you a darned good estimation of battery SOC. Reading an amp-hour meter is so easy that anyone can learn to do it, and then start the backup generator before the SOC is low enough to cause battery damage.
Amp-hour meters are generally set up to display a simple "percent of full" reading, but they store much more data. When set to show amp-hours, the meter reads "0" when the batteries are full. As energy is used from the battery bank, the meter counts down in negative numbers; as the bank is charged, it counts up. A positive number indicates incoming energy that was not stored because the batteries were already full. Even the most basic of these meters stores some historical data, like the maximum depth of discharge since the last reset and the number of hours the battery bank was under a certain set critical voltage. This data can be used to troubleshoot a system or watch for problems.
Meter Choices
There are a variety of options available with amp-hour meters, including their ability to read multiple shunts, and their capability for having remote displays, data storage, computer interfaces, and even Internet monitoring via smartphones or websites.
Internal meters. If your battery-backup grid-tied PV system is simple, your battery monitor will be simple, too. Stored energy from your batteries is used only during a utility outage, and grid energy is usually used to quickly charge them again after power is restored. Some newer battery-based grid-tied inverters already have their own metering for the battery bank, which you can monitor through the system status display and built-in computer interface.
System-integrated meters. Many newer inverters and charge controllers can be networked together using special hubs and routers for monitoring. Battery monitors can be easily added to that network, so that all of the data from every device can be read on the system status display. This puts the entire system's performance at your fingertips. Even wireless links to the monitor are possible.
In some cases, you'll have to purchase extra equipment, like hubs and displays, for this networking capability. Also, the communications protocols used by equipment manufacturers are proprietary, so if your inverter, charge controller, and battery monitor are made by different companies, you'll need a laptop computer to integrate the data.
Stand-alone meters. These include the original amp-hour meters, and are the most versatile. No matter what sort of system you have, what resources provide your energy, or who designed your power system how long ago, you can monitor your battery bank with a stand-alone meter, and there are models that can monitor multiple shunts for more detailed data.
Display Choices
Stand-alone meters can be mounted some distance from the shunt in a convenient and easily visible place for viewing the display. Integrated and internal meters may have a simple display at the unit, with the rest of the details available through the remote monitor display.
Even the simplest and least expensive of modern amp-hour meters have serial data output, and the more advanced models make it easy for you to connect the system to your PC. You'll need a PC interface and software from the meter manufacturer. Note that to collect detailed historical data, a computer that's running all the time is required. Some enthusiasts use an old laptop that's been retired from daily use just for these logging operations. You can watch your system performance live with a "dashboard" application, analyze your data with a spreadsheet, or even send data to your website or smartphone.
Some system monitors are designed to communicate directly with your home computer network, either through a LAN cable or by wireless. For this data communication, your Internet service and/or routers must be powered on, but your PC doesn't have to be running. Unless you are a computer programmer, you'll also need to subscribe to an Internet system monitoring service (which in some cases is free) to establish a secure site to log into your meter, and a way to build monitoring "widgets" to insert into your website or blog.
Your Internet service must include a static IP address to use any of these web-monitoring features, and that may cost extra. With some satellite Internet services, putting a system monitor online may not be allowed-be sure to check with your Internet provider first.
Installation & Setup
Amp-hour meters are easy to install, but be sure to follow the manufacturer's instructions. Before you start, shut down your entire power system using the main DC disconnect. Turn off the PV array breaker and disconnect the battery output cabling in the battery box. You'll need to find a good location for the shunt, keeping in mind that all energy moving into or out of the battery bank must pass through it on the main negative wire. A good location is inside the inverter power panel; many panels have spaces reserved near the main negative buss just for this purpose. You'll need a short cable with lugs on both ends that's the same wire gauge as your battery output cable (since this is part of the negative-side circuit pathway to/from the battery bank).
Next, mount the display panel in a convenient and visible location, and run the cable from the shunt to it. Most displays mount in standard electrical boxes. The cable used is a special multiwire, shielded, twisted-pair bundle that should be purchased directly from the meter manufacturer to make sure it is the right type. If you are installing a system-integrated meter, things are even easier-the special cable from shunt to monitor box will be short, and then you simply plug the monitor into the system network hub with a LAN cable or wireless-to-LAN interface.
After meter installation and powering up your system, your new meter will need to be programmed with your battery bank's amp-hour capacity, the system voltage, and the voltage level at which your charge controller considers the batteries to be full. Then, you must charge the battery bank to full capacity, usually with a generator, until that "full" voltage level is reached. At that point, your meter takes over, and you are in the monitoring business!
A simple amp-hour meter and shunt costs about $200-a small price to pay considering the total cost of an entire renewable energy system and the relatively fragile nature of its battery bank. If you monitor your battery bank regularly and react accordingly, you can extend its life by many years before replacement is needed.