THE ADVANTAGES OF 2 V CELLS
In my experience, the hassle factor of a battery bank is proportional to the square of the number of paralleled strings. A two-string battery bank is four times more effort overall compared to a single-string battery bank. A three-string battery bank is nine times more effort overall. High-capacity battery banks should use paralleled series strings only when absolutely necessary.
There are numerous benefits to using individual 2 V cells or batteries composed of accessible 2 V cells. One benefit is that they are available in a wide variety of capacities ranging from under 500 Ah to just over 5,000 Ah. This range of capacities makes it possible for most 48 Vdc nominal systems to be configured with a single string of 24 cells in series, which results in the minimum number of cells in a system and minimizes the number of electrical connections and time required for maintenance. Since this design does not require paralleled strings, it eliminates the possibility of the battery bank becoming imbalanced due to unequal charge and discharge currents.
For very large battery banks over 175 kWh total capacity, paralleling two series strings is typically required. Battery cells larger than 3,500 Ah can be hard to source and even more difficult to transport and install. Even larger systems may require three battery strings in parallel. Designing large battery banks with more than three series strings is not advisable because it is difficult to keep the strings balanced and sharing the load equally. There are some alternative design strategies that can be used for these extremely high-capacity system requirements.
Another significant advantage of using individual 2 V cells is that it usually allows for measurement of each individual cell’s voltage. Typically, this is not possible with 6 V or 12 V batteries because the cell’s series interconnections are not accessible. The ability to check individual cell voltage is particularly useful when valve-regulated lead-acid (VRLA), absorbed glass mat (AGM) or gelled electrolyte (Gel) batteries are specified because unlike with flooded cells, it is not possible to use a hydrometer to measure the electrolyte’s specific gravity.
Having access to each individual cell’s terminals makes it possible to provide an individual cell with a corrective charge via a small portable power supply if a severe imbalance occurs. This can often be much easier and less stressful on the battery bank than performing an equalization charge that overcharges all the good cells to bring up any undercharged cells. Applying a corrective charge to an individual cell or cells is the most practical way to correct imbalances in a sealed AGM or Gel battery bank.
FLOODED AND SEALED BATTERIES
The increased availability of high-capacity 2 V sealed batteries makes them a viable option for large battery banks. In fact, the accompanying tables include slightly more sealed than flooded battery models. Sealed VRLA batteries can utilize either AGM or Gel construction. System designers should be aware that even though these two types of batteries are considered similar, the differences between them can make one type more suitable than the other for a particular installation. In addition, battery construction specifics vary from manufacturer to manufacturer.
Sealed batteries offer many advantages over flooded batteries, but also have several disadvantages that should be considered.
The advantages include:
- Less hazardous—no potential for spilled electrolyte during installation
- Easier to transport and install
- Not as likely to off-gas (although they will if overcharged)
- Less maintenance—no water to add and a significant reduction in terminal corrosion
- Less risk of damage from improper maintenance or adding poor-quality water
- No need to equalize—electrolyte does not stratify
- Some models can be mounted horizontally, reducing space requirements
- Can handle higher charging and discharging rates
The disadvantages include:
- Much more sensitive to improper charging and discharging
- Most cannot be equalize-charged to reverse damage
- Easy to overcharge and damage, especially in hot climates
- Require temperature-compensated charging sources
- Higher up-front cost
Sealed batteries are worth considering if the battery bank is part of a system that you expect to be minimally managed, particularly if there are concerns that the ongoing maintenance required by flooded batteries may not be done properly. However, if it is likely that the battery will be left at a low state of charge (SOC) level for long periods of time, then flooded batteries are probably a better choice. Many AGM and Gel batteries are designed primarily for float or shallow-cycle applications, but this does not mean that they are unsuitable for off-grid systems with a large PV array. Systems utilizing flooded batteries require rigorous training of on-site personnel, or, preferably, they should be covered by a maintenance contract.
On some systems, the combination of PV array output and the backup generator charger’s output may result in too high a combined charge rate for the size of the battery bank. For example, Surrette/Rolls recommends not exceeding a charging rate of 15% of the C/6 capacity rating for its flooded batteries. For a 100 kWh (2,000 Ah) battery bank, this would result in a limit of 300 A. If the system had a 14 kW inverter/ charger system with 180 amps of battery-charging capacity, this would allow for an approximately 8 kW nameplaterated PV array; this is an unrealistically small PV array considering the battery bank’s capacity. Many sealed AGM and Gel batteries do not have a specified limit on the charging rate since they tend to have lower internal impedance and can accept higher charging currents. This might be a significant reason to consider using a sealed battery when faced with this situation. Many manufacturers, including Surrette, now produce both flooded and VRLA batteries.
MINIMIZING REQUIRED BATTERY CAPACITY
System designers commonly select the size of the battery bank to limit the daily discharge to 20% and the overall discharge to 50% in poor weather conditions. This allows for good battery life in most systems. With a system that includes a large PV array and a large, high-quality backup generator, it may be difficult to justify the inclusion of a very large battery bank to handle extended periods of poor weather. If you oversize the PV array and limit the nighttime loads that are operated, a smaller battery may actually work better and can significantly reduce the initial cost for the overall system. The smaller battery bank becomes fully charged more frequently, allowing it to achieve a long service life.
Designing an optimized off-grid system requires an accurate projection of both the system’s electrical loads and the available charging sources. In recent years, software has made it possible to model the performance of an off-grid system and change key variables such as the size of the solar array, battery capacity and generator run-time. Modeling programs such as HOMER (homerenergy.com) and PVsyst (PVsyst.com) are fairly easy to learn and are faster and provide more comprehensive results compared to the repetitive, longhand calculations or complex spreadsheets that stand-alone system designers have historically used.
The most basic design variable is determining how large a battery bank is required to achieve the desired days of autonomy. The current standard design guideline of 3 to 5 days of autonomy that is often used during system sizing should be reexamined, with 1 to 3 days frequently being more realistic. The optimal number of days of autonomy is dependent on economic factors. Many fundamental design assumptions have changed significantly in the last few years: PV modules, racks and controllers have become less expensive while the cost of batteries, labor and transportation have increased.
For most off-grid PV systems, designing for too many days of autonomy will result in poor battery life. If the PV array is sized to meet the site’s loads on typical sunny days, then after a period of poor weather there usually is not enough PV production to both power the loads and recharge the battery. When poor weather occurs, the battery tends to become discharged and then remain discharged for long periods of time. Under these conditions, batteries become sulfated. The result is a loss of battery capacity and decreased operational life for the bank. To avoid this situation, a backup generator is typically incorporated into the system to power the loads during periods of poor weather once the battery has become discharged to 50% of its capacity.
With the reduced cost of PV modules, it is now more practical to significantly oversize the PV array. An oversized array makes it possible to both support daily electrical loads and quickly recharge the battery after a period of poor weather. This design strategy also ensures that ample solar power is produced during periods of partially cloudy weather. With an oversized PV array, it is often practical to reduce the battery capacity while still providing enough days of autonomy. A backup generator is usually still required, but it runs less frequently and the battery lasts longer since it is fully recharged on a regular basis.
A related and important factor is matching the battery capacity to the available ac charging source. Often the capacity of the inverter system’s ac battery charger or chargers creates a bottleneck when recharging the battery during backup generator operation. This results in the battery remaining at a low SOC during poor weather, even when the generator has been operating for long periods of time. The solution is to incorporate additional generator-powered battery chargers to allow more of the generator’s output to be stored in the battery. This often requires purchasing a larger inverter/charger system than was originally expected, since adding separate battery chargers can be difficult to coordinate with the chargers built into the inverter/charger system.
When calculating the required battery capacity, you must consider what portion of the loads being powered occur during the daytime, when the PV array is producing energy, versus at night, when the battery provides the energy. The daily depth of discharge (DOD) should be calculated using only the loads that occur at night, while the total daily load should be used when calculating the DOD for extended periods of poor weather. In some applications, it can be worthwhile to limit the loads that can be operated at night to force more loads to operate during the day when the solar array can power them directly. This allows for a smaller battery bank and also increases the overall efficiency of the system. Automated load control can be accomplished using a voltage controlled relay system that detects when the battery is being charged. This capability is built into many of the newer inverter/charger systems.