High-Capacity Battery Banks

Product Selection and System Design Fundamentals

Due to significant decreases in PV module cost and the rising cost of fuel for backup generators, many off-grid renewable energy systems are increasing in size. Advancements in power conditioning equipment, such as higher power inverter/ chargers and PV array charge controllers, and the increased availability of high-capacity batteries are also driving this trend. As remote, stand-alone systems with large PV arrays become more common, the demand for high-capacity battery banks is growing. Designing and installing a high-capacity battery bank involves more than simply adding strings of batteries to increase storage capacity. High-capacity banks require proper battery selection and a careful analysis of the available products and design trade-offs to achieve a durable, high-performance system with a long service life.

For the purpose of this article and the accompanying product specifications tables, I define a high-capacity battery bank as one with 50 kWh or greater of energy storage. A system of this size typically provides 1,000 amp-hours (Ah) of total energy storage at 48 Vdc nominal. In the US, a 48 Vdc configuration is the standard for large battery-based systems due to code and safety issues associated with higher battery system voltages. Outside of the US, nominal battery system voltages higher than 48 Vdc may be more common. I focus specifically on lead-acid batteries and related product selection and system design considerations. While additional battery technologies are commercially available, including lithium ion, they are prohibitively expensive for use in stationary applications at this time.

HIGH- VERSUS LOW-CAPACITY BATTERIES
Designers and installers who work on off-grid applications have significant experience with 12 V or, more commonly, 6 V lead-acid batteries with capacities ranging from 100 Ah to 400 Ah. These low- and medium-capacity batteries are typically made up of individual 2 V cells that are manufactured as complete units, often referred to as monoblock batteries. Monoblock batteries do not allow for the removal of individual cells and do not provide access to individual cells for electrical testing. When high-capacity battery banks are required, a common error that inexperienced system designers make is to connect these 6 V or 12 V batteries in series and parallel to achieve higher bank voltage and capacity. However, configuring multiple strings of batteries in parallel often results in poor performance and shorter-than-expected service life due to unequal charge and discharge rates between the individual battery strings. Multiple series strings also lead to an increase in ongoing maintenance time and cost.

In contrast, well-designed high-capacity battery banks are typically configured using between one and three series-connected strings of individual 2 V lead-acid cells, with one series string being the ideal design goal. Designing the system around a single series string of batteries is generally the optimal approach, but very large systems may require paralleling of series strings to reach the required storage capacity. The high-capacity batteries that are listed in the accompanying specifications tables (see "Inside This Article") have amp-hour capacities that range from 500 Ah to over 5,000 Ah at the 20-hour rate. Due to their high capacities, these batteries can often eliminate the need to connect series strings in parallel. Some manufacturers group individual 2 V cells in sets of six or more in large steel cases or trays to protect the more fragile plastic containers of the individual cells. In many instances, these industrial batteries are designed to allow individual cells to be removed from the steel case, making transportation, installation and servicing easier.

Historically, integrators have favored using low- and medium-capacity 6 V or 12 V batteries wired in series and parallel to form a larger battery bank because it is often easier and less expensive upfront than using higher-capacity industrial batteries. Low- and medium-capacity batteries are often available off the shelf and locally in stock, which eliminates the shipping costs that the higher-capacity models incur. In addition, these lower-capacity models usually weigh 125 pounds or less and feature durable cases with handles that make them easy for one or two people to move. Specifying 2 V cells or other large batteries manufactured as 4, 6, 12 or even 24 Vdc units often requires ordering the battery with enough lead time to have it shipped from the manufacturer. Handling high-capacity cells or steel-cased grouped cells can be difficult. While smaller 2 V cells with 500 Ah capacities can weigh as little as 57 pounds, the highest-capacity individual 2 V cell listed in the specifications tables weighs close to 800 pounds. Steel-cased batteries formed from groups of 2 V cells can weigh more than a ton. A sturdy forklift is usually required for the unloading and installation of these industrial batteries, and some remote sites with limited or challenging vehicular access may limit or eliminate their use entirely.

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