Criteria for sizing battery banks

Solar Battery: Design it right.

In solar applications, a lengthy charge time is not desirable since we only have a maximum of 4-6 hours of peak sun each day due to limited daylight and varying weather conditions. To ensure that the batteries receive enough charge, batteries must be adequately sized to prevent deficit charging and premature battery failure.

There are two definitive types of battery-based systems used in solar applications; Off-Grid and Grid-Tied. Off-Grid systems are often used where a customer chooses not to connect or there is no available connection to the grid. This customer may live remotely and have chosen to install a solar system to generate and store adequate power in the battery to run all electrical requirements within the home.

With Grid-Tied systems, a customer typically lives in an urban area where they may experience infrequent power-cuts, say 2 hrs/day. The solar system with battery is used as a backup power supply, meant to supplement power during these brief 2 hr outages.

OFF-GRID SYSTEMS

When sizing a battery bank, it is important to determine the appropriate capacity requirement so as not to oversize the battery. A battery bank which is too large for solar to charge often leads to sulfation issues because the battery will never reach 100% SoC. With traditional off-grid systems, DOD is set to 50% to maximize the life of the batteries.

BATTERY SELECTION

It is important to select a battery model which offers enough capacity, but also minimizes the number of parallel strings required to accomplish the desired voltage and capacity. Systems with multiple parallel strings of batteries will often experience an imbalance of charge. These systems will also require additional maintenance as this increases the number of terminal connections requiring cleaning as well as the number of cells to water. When charge imbalance is not addressed through adjustments in charge time and periodic equalizations this will eventually lead to premature failure of the battery bank.

Determining the capacity requirement for off-grid systems is done by completing an audit of energy consumption based on actual load requirements. The load is calculated by the total amount of power necessary to support the load for a 1-day period and then factors in how many days it may be required before recharging. In a typical solar system, the 10 hr AH rate (C10) is used when sizing systems to cover 1-2 days of autonomy or less. The 20 hr AH rate (C20) is used when designing systems to cover more than 2 days of autonomy.

Most systems are designed for a 1-2 day rate, due to the cost of batteries. This is also most typical for solar applications as these systems allow the battery bank to be charged each day.

Deep cycle batteries will perform best when they are maintained at a full charge. By holding at a full state of charge, this will extend the overall life of the battery bank as this will prevent stratification and sulfation.

It is important to be aware that as load profiles change this too will affect how quickly battery capacity will be reduced. Often a backup system is sized to meet the load requirement at the time of install, but this demand will increase with every small addition to the load. End users will inadvertently add more loads after the installation causing problems with how often the battery bank needs to be recharged, thus increasing cycle life usage. This needs to be avoided, and can be by properly educating the customer at the time of sale.

GRID-CONNECTED BACKUP

In the event of a grid failure, the battery bank in a grid-tied system provides power for the critical loads during the outage. The battery bank should only be sized to handle the loads supported for the duration of a temporary outage. These systems are commonly used to run household necessities such as limited lighting, TV, fans, etc. It is important to limit the size of the supported load to essential service only as higher capacity battery banks require more maintenance as well as power usage to hold at full charge, reducing the amount of generated power which can be sold to the utility.

Typically, because outages are infrequent and occur only a few times per week in most regions, these systems are designed for greater depth of discharge than off-grid to lower initial installation costs. In some cases, they may be designed to discharge as much as 20% state of charge over a relatively short period of time. Cycling with grid-connected systems is significantly less frequent than with off-grid, where this may occur daily, a lower DOD is acceptable as long as the customer understands that the overall cycle life is affected when they cycle their battery bank beyond a 50% state of charge.

In areas where the grid may not be reliable and extended or intermittent outages occur frequently, it would be necessary to size the battery bank and charging source(s) to support a larger load over a longer period of time. This would prevent the battery bank from over-discharging and/or insufficient charging between cycles.