Whether you spend $400 or $20,000 on your batteries, without proper care they may be headed to the scrap yard sooner than you would like. Here are some expert tips on battery installation, charging, and maintenance to make sure your investment is long-lived.
Proper housing. Make sure your batteries are housed in a safe, easily accessible place. Most batteries require an enclosure that is lockable, sealed, insulated, and vented outdoors. Small details such as sloped covers (so things are not piled on the box), clear viewing windows (for easy inspection), and a removable side (for ease in replacing batteries) can make a big difference.
Batteries are dangerous (see “Safety!” sidebar), and they should not be accessible to anyone unaware of proper safety protocols. But we also want the batteries to be accessible when they need maintenance. Cell caps on flooded batteries, and terminals, should be easily reachable. Consider battery layout, as it is preferable not to lean over one battery to reach another—making access easy reduces the chance of accidental shorting.
Interconnections. Some industrial batteries come with bus bars for making intercell connections, but most battery banks need cables for series and parallel connections, as well as cables to connect to an inverter or DC load center. Battery cables should be large enough to handle their maximum continuous current, and be protected with fuses or circuit breakers rated for high amp-interrupt current. Cable size is determined from the inverter specs and/or DC loads that come off the battery bank. For residential-sized systems, 2/0 or 4/0 cable is common.
Using welding cable for batteries was once a common practice, as listed cable was not available and it is relatively inexpensive, flexible, and can handle lots of current. However, it is not designed for this application and is not listed by the National Electrical Code for use in battery systems. Flexible, UL-listed, NEC-approved battery cable is now readily available, and should be used for all battery wiring.
Keeping batteries healthy requires equal charging and discharging across all cells—differences in resistance within a battery bank can lead to premature failure. Poor lug crimps, loose terminal connections, unequal parallel cable lengths, and small wire gauge can all affect the equal treatment of cells.
Wiring. Electrons can follow numerous paths when entering or leaving a battery bank with multiple parallel strings, so it’s critical to minimize the number of parallel connections and ensure they are equal in length. When wiring parallel strings, always make series connections first. Next, parallel the positive ends of the strings, and then connect the negatives. Inverter cables should be connected on opposite corners of the battery bank to keep electrical paths between strings as equal as possible.
State of Charge
A battery’s lifespan is affected by how deeply it is discharged before getting charged back up, and how long it stays in that discharged state. A battery’s state of charge (SOC) is the amount of energy remaining in the battery. The lower the SOC is allowed to drop, the shorter its lifespan will be. Sizing an off-grid battery bank for 50% SOC is common, but remote systems with no backup power sources may be designed to maintain SOC at 75% or above to extend battery life—the fewer times a heavy, unwieldy battery bank needs to be replaced, the better. Backup power systems are often designed to go down to 20% SOC since they are rarely discharged.
Many methods use voltage to determine SOC, although it’s not the most accurate measure. The voltage of a battery at rest can tell us SOC, but in an RE system, they are nearly always charging or discharging, so attaining this rested state is difficult.
The most accurate way to measure SOC on flooded batteries is by checking the electrolyte’s specific gravity (SG). Hydrometers are the most common tool used to measure SG, but handheld refractometers can also be used. Be sure to choose one that is accurate to at least three decimal places, and follow the instructions for your model. The “SOC” table gives approximations, but the actual specific gravity and voltages will vary for battery makes and models.
An amp-hour meter is a more common and fairly accurate way to keep track of SOC. These meters require a shunt to measure the current going into and coming out of the batteries, and some can keep track of more than one charging source (PV, wind, generator, etc.). Some have built-in alarms, generator start relays, and data logging capabilities. With proper setup, they can give you a much better picture of the SOC.
Every battery has its own charging specifications. Chronic under- or overcharging is one of the most common ways to shorten the life of a battery bank. Undercharging can cause sulfate crystals to build up on the plates, reducing the battery’s capacity and shortening its life. Overcharging leads to excessive gassing, lowered electrolyte levels (which cannot be replaced in sealed batteries), and more wear and tear on internal plates.
Generally, manufacturers give both voltage and current charging specifications. Check with the manufacturer for their recommended maximum charge rates. Most PV arrays are not large enough to supply that much current, so it is not usually an issue, but generator and utility charging can be. Sophisticated chargers have programmable maximum charge current.
RE system batteries should be charged with a three-stage charger. During the first stage (bulk), all available charging current is sent into the batteries until they reach a specified voltage. Once they reach this voltage, they are about 80% charged, and the second stage (absorb) starts, and the current decreases just enough to keep the voltage stable. The absorb cycle has either a time and/or current end point (see “Absorption Time” sidebar). When this is met, the batteries should be full. The charger then enters the “float” stage with a slightly lower voltage and a trickle charge keeps the batteries full. Occasionally, the battery bank will need to be equalized to remove imbalances between batteries and cells. This is accomplished by intentionally overcharging the bank.
Reliable, programmable charge controllers will ensure batteries do not get overcharged, and a low-voltage disconnect (LVD) protects them from being overly discharged. Most residential inverters have a built-in, programmable LVD, but beware of smaller units with a set LVD. These often trigger at a very low voltage (down to 10.5 V), not to protect your batteries, but to protect the inverter. An inverter LVD can only protect the battery bank from AC loads, not any DC loads. For small systems, some PV charge controllers add this function—for larger systems with DC loads, a dedicated controller may be needed for LVD.
Another common problem is a lack of adequate charging capacity. Batteries should be brought to 100% SOC at least once a week—and more often is better. Be sure your charging source can deliver enough energy to replace daily usage and to catch up after any periods of days without input. For example, if you’ve designed a battery-based PV system to have three days of autonomy, but your PV array only produces enough energy to replace 1 to 1.5 days of energy use, you’ll need another charging source to “catch up” after three cloudy days or your batteries will spend too much time in a discharged state, shortening their life. This is one of the main reasons why many off-gridders have backup generators.
Battery voltage is temperature-sensitive, and you’ll need to ensure that batteries are not overcharged when they are hot and not undercharged when they are cold. Most chargers have an optional remote temperature sensor that’s placed in the middle of the battery bank (adhered to the side of a battery). Since a battery’s temperature compensation requirement varies, always check manufacturer’s specs and program your charger accordingly.
Even if a battery bank is working well, performing routine maintenance can prevent future problems. All batteries should be thoroughly inspected at least once or twice annually. The cases and terminals should be kept clean and corrosion-free. While flooded batteries typically show corrosion buildup around the terminals from gassing, it should be minimal. Sealed batteries should not have any buildup.
To neutralize any escaped acid, wipe battery cases and terminals with a clean cloth or soft brush dipped in a baking soda and water solution (1 pound of soda to 1 gallon of water), make sure vent caps are on and securely tightened. A bubbling solution is a sign that some acid is present; wait until bubbling stops and then wipe with clean water and dry. Be careful not to let anything into the battery through the vent caps.
Once the cases and terminals are clean, check the terminal connections and inspect the battery cables for any wear or loose crimps, and replace if necessary. Clean and recoat terminals and lugs with a thin layer of anticorrosion treatment (petroleum jelly works). Always leave your batteries clean so you can easily see any future corrosion or acid leakage.
Flooded batteries need to have their electrolyte levels checked every one to two months. Even if batteries only need to be filled every six months or so, checking water levels more often is recommended to ensure the plates are never exposed. If this happens, that part of the plate will quickly oxidize and block the chemical process there. Even if this only happens to one cell, it creates resistance and unequal charging throughout the battery bank and can reduce the whole system’s efficiency and battery bank life.
Electrolyte should completely cover the plates, but be about 1/4 inch below the cell fill tube. Overfilling is a common mistake. If you often find acid on your batteries, check that you aren’t overfilling the cell. Shining a small flashlight into the cell can help you see the electrolyte level.
Only use distilled water for filling. Tap water, even filtered, can contain impurities that will harm your batteries. Use a funnel to help avoid spills. Hydrocaps and watering systems can help you keep batteries filled, but they are not a substitute for regular inspection. Also note that some of these devices need to be removed during equalization.
Signs of a Bad Battery
While an almost unlimited number of things can go wrong in a battery bank, there are a few signs and symptoms common in RE systems:
- The batteries complete a bulk/absorb charge cycle, but voltage plummets as soon as you stop charging and add a load, which indicates reduced capacity. Sulfate buildup has occurred and the batteries may be nearing the end of their life.
- One cell is different (voltage or SG readings; corrosion; appearance) from the rest often indicates that it is failing.
- Bulging cases are typically a sign of flooded batteries that have frozen. The frozen electrolyte expands and causes the cases to bulge, often forcing terminals and plates to warp, and cases to crack.
- Overheating or overcharging sealed batteries can cause cases to cave in. Excess pressure builds up inside the battery, and gasses escape through the safety valve covers. When the battery cools, some of the electrolyte is missing (from off-gassing) and the decreased pressure causes the cases to cave in significantly.
- Melted lugs/battery terminals could be the result of resistance from loose connections or corrosion buildup.
One of the best ways to track your batteries’ health is to keep regular, precise records. During your maintenance checks, measure individual battery or cell voltages, and check specific gravity for flooded batteries. Ideally, readings should be taken after the batteries have been at rest for 12 to 24 hours and are fully charged, but this is generally impossible in an off-grid situation. Checking after 30 minutes of rest (no loads, no charging) will still give you good information.
These checks can alert you to bad cells, or let you know if the entire bank may be on its way out. Any differences in cell voltages or SG indicate you may have a failing (or failed) cell, and checking your readings against the expected SOC will tell you if they are losing capacity. The sooner you spot a problem, the more likely you will be able to fix it.
- Absorption Time
Ideally, there would be enough energy put back into the battery to bring it up to 100% SOC each day. Sounds simple, but loads and available charging vary almost constantly. An amp-hour meter can help, since it keeps track of the net energy in the batteries, but many charge controllers only have an absorb “time limit” function.
Manufacturers have a formula that can estimate this time limit based on expected SOC, available charging current, and battery capacity—but it is still just an estimate. Since PV systems often have relatively low charging current, erring on the longer side for PV charger absorption times is appropriate for most off-grid systems.
For example, Surrette Battery Co. provides the following formula:
Absorb time = (0.42 × battery’s 20 hr. capacity) ÷ charge current
If there’s 800 Ah capacity, and 75 A of charge current:
(0.42 × 800 Ah) ÷ 75 A = 4.5 hrs. absorb time
More sophisticated charge controllers also have a current trigger to end an absorb cycle. The higher the SOC, the less current is needed to keep the battery at absorb voltage. Since most PV charge controllers start a new bulk/absorb cycle each day, this is a great way to ensure batteries are getting just enough of a charge—even when they start at different SOCs. For example, a vacant off-grid cabin will not need much PV absorb time, since the batteries will be relatively full each morning. When the cabin is being used and the batteries are being discharged more deeply, PV absorption should be maximized.
- Battery Cell Anatomy & Chemistry
Lead-acid battery cells consist of lead and lead-oxide plates surrounded by an electrolyte, a mixture of sulfuric acid and water. Taking electricity out of the battery (discharging) causes the plates to change to lead sulfate, and dilutes the electrolyte. Putting electricity into the battery (charging) forces the sulfate coating off the plates and back into the electrolyte, making it more concentrated, and the plates return to lead and lead oxide. Hydrogen and oxygen gas are released during charging as some of the water molecules in the electrolyte break apart from electrolysis.
- Battery Wiring
SOC for Generic Batteries
- Three-Stage Charging
- Typical Charging Voltages
Lithium Ion Battery
Lithium ion (Li-ion) battery packs are an advanced chemistry that provides increased performance over nickel or lead based chemistry at a price premium. Li-ion and lithium polymer batteries can have as much as twice the energy density of NiCd (nickel cadmium), have relatively low self-discharge rates, and are capable of high discharge currents.
The lithium ion chemistry is available in several variations. Lithium cobalt oxide provides the greatest energy density; lithium manganese offers greater safety with lower energy density than lithium cobalt oxide; lithium iron phosphate provides high discharge rate capabilities, as well as long cycle and calendar life. Mixed cathodes can be used to optimize lithium rechargeable cells’ specific performance characteristics.
- Cell voltage: 3.2 – 3.7V (nominal)
- Capacity: 500mAh to 5000mAh
- Energy by volume: 270 Wh/L to 324 Wh/L
- Energy by weight: 105 Wh/kg to 130 Wh/kg
- Cycle life: 300 – 1000 cycles
- Self discharge rate: approx. 10% per month
- Operating temperature range: -20°C to 60°C
- Preferred charge method: constant voltage/constant current
- Size: variable
- Applications: military, industrial, medical, consumer electronics
Because of lithium’s inherent volatility and lithium ion’s high energy density, every lithium ion battery pack and cell requires a special control circuit to manage its upper and lower voltage thresholds and to regulate its operating current and temperature during charge and discharge.
Lithium ion battery technology is always evolving. Better manufacturing techniques continue to bring the cost of lithium ion cells down, and new electrode materials provide improved capacities.
In general, lithium ion charge circuits are of two-stage constant current/constant voltage design. Fully discharged cells take constant current charges at a rate of 1C (1x the rated capacity), until reaching their set voltage of between 3.6 and 4.2 volts (depending on specific chemistry), a process which takes approximately one hour. The batteries’ chargers will then switch to constant voltage, with the current tapering off until the batteries are fully charged—approximately another two hours. The entire charging process is a fairly fast one, at about three hours. Lithium ion batteries may be removed from charge before they’re fully charged, with no side effects. Li-ion batteries cannot be trickle charged, as they do not tolerate overcharge conditions.
Discharge Characteristics for a Typical Li-ion 18650 Cell
Discharge Vs. Temperature Characteristics for a Typical Li-ion 18650 Cell
High Rate Discharge Characteristics for a Typical Li-ion 18650 Cell
Voltage Vs. Current Discharge Characteristics for a Typical Li-ion 18650 Cell
Lithium Polymer Battery Packs
Lithium-ion-polymer batteries differ from lithium-ion batteries only in construction—it is not a unique and different chemistry. Li-polymer can be created in an array of chemistries, the mo
st widely used of which is Li-cobalt. The difference in construction over conventional li-ion cells allows for lower cost, safer operation, and flexible packaging options; cells as thin as a credit card are possible. The combination of these factors makes custom lithium polymer battery packs and battery assemblies an increasingly popular choice for a wide range of applications.
- Cell voltage: 3.2 – 3.7V (nominal)
- Capacity: 500mAh to 3000mAh
- Energy by volume: 250 Wh/L
- Energy by weight: 120 Wh/kg
- Cycle life: 500+ cycles
- Self discharge rate: approx. N/A
- Operating temperature range: -20°C to 60°C
- Preferred charge method: constant voltage/constant current
- Applications: audio-visual equipment, cellular phones, office automation, notebook PCs, PDAs, handheld devices