Battery Sizing Calculation

How to determine the Size of the Battery array that will work well for your needs: The first thing that you will have to decide on is the operating voltage of your system, whether a 36Volt of 48Volt system. The higher voltage systems is slightly more effective, but a little more expensive. We found that independent home owners mostly prefere the 36Volts packages, while the communication industry rather the 48Volts systems. Sizing your battery bank and inverter is elementary math's. Power is measured in Watts. The formula to determine watts is as follows: (Watts = amps x volts. ) Appliances wattage is usually listed on the manufacturer's label. After you've collected this information about all the items that you want to power off your system, you are ready to determine the battery size you will need. STEP 1: Determine your daily energy budget. Make a list of all the appliances that you want to serve with power. List their Watt ratings and list an estimate of the number of hours that each item will be used per day. Multiply the watt ratings with the hours used per day, to determine the daily watt-hours per items. Add these values together, to arrive at a total budgeted watt-hour needed per day. STEP 2: Multiply total daily Watt hours needed by the number of anticipated days of autonomy, to determine you basic battery size requirement. (For excellent wind conditions choose 1. For poor wind conditions choose 3.) This figure we call you basic battery size. STEP 3: Multiply this basic battery size by 2, to determine safe battery size. STEP 4: Now, convert this safe battery size, to amp-hours as follows: Safe battery size expressed in Amp-hours = Watt hours / DC volts. (DC volts is the operating voltage you've chosen for the battery bank. For small systems it is normally either 36 volts or 48 volts. For larger system is can be 110 Volt, 240 volt or 600 volt.) With this figure for a Safe battery size, expressed in amp=hours, you can go and shop around for a suitable battery bank. STEP 5: To determine the correct inverter size, total the wattage requirements for all the appliances you plan to run simultaneously. Add at least 25% to this perceived requirement. The final check is to look for surge watts of any item of you appliances that might exceed your inverter size. Choose an inverter size to suite this requirement.. and if in doubt, go for one size up.

Introduction

Stationary batteries on a rack (courtesy of Power Battery) This article looks at the sizing of batteries for stationary applications (i.e. they don't move). Batteries are used in many applications such as AC and DC uninterruptible power supply (UPS) systems, solar power systems, telecommunications, emergency lighting, etc. Whatever the application, batteries are seen as a mature, proven technology for storing electrical energy. In addition to storage, batteries are also used as a means for providing voltage support for weak power systems (e.g. at the end of small, long transmission lines).

Why do the calculation? Sizing a stationary battery is important to ensure that the loads being supplied or the power system being supported are adequately catered for by the battery for the period of time (i.e. autonomy) for which it is designed. Improper battery sizing can lead to poor autonomy times, permanent damage to battery cells from over-discharge, low load voltages, etc.

When to do the calculation? The calculation can typically be started when the following information is known: • • •

Battery loads that need to be supported Nominal battery voltage Autonomy time(s)

Calculation Methodology

The calculation is based on a mixture of normal industry practice and technical standards IEEE Std 485 (1997, R2003) "Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications" and IEEE Std 1115 (2000, R2005) "Recommended Practice for Sizing NickelCadmium Batteries for Stationary Applications". The calculation is based on the ampere-hour method for sizing battery capacity (rather than sizing by positive plates). The focus of this calculation is on standard lead-acid or nickel-cadmium (NiCd) batteries, so please consult specific supplier information for other types of batteries (e.g. lithium-ion, nickel-metal hydride, etc). Note also that the design of the battery charger is beyond the scope of this calculation. There are five main steps in this calculation: 1) Collect the loads that the battery needs to support 2) Construct a load profile and calculate the design energy (VAh) 3) Select the battery type and determine the characteristics of the cell 4) Select the number of battery cells to be connected in series 5) Calculate the required Ampere-hour (Ah) capacity of the battery

Step 1: Collect the battery loads The first step is to determine the loads that the battery will be supporting. This is largely specific to the application of the battery, for example an AC UPS System or a Solar Power System.

Step 2: Construct the Load Profile Refer to the Load Profile Calculation for details on how to construct a load profile and calculate the design energy, , in VAh. The autonomy time is often specified by the Client (i.e. in their standards). Alternatively, IEEE 446, "IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications" has some guidance (particularly Table 3-2) for autonomy times. Note that IEEE 485 and IEEE 1115 refer to the load profile as the "duty cycle".

Step 3: Select Battery Type The next step is to select the battery type (e.g. sealed lead-acid, nickel-cadmium, etc). The selection process is not covered in detail here, but the following factors should be taken into account (as suggested by IEEE): Physical characteristics, e.g. dimensions, weight, container material, intercell connections, terminals • application design life and expected life of cell • Frequency and depth of discharge • Ambient temperature • Charging characteristics •

• • • •

Maintenance requirements Ventilation requirements Cell orientation requirements (sealed lead-acid and NiCd) Seismic factors (shock and vibration)

Next, find the characteristics of the battery cells, typically from supplier data sheets. The characteristics that should be collected include: • • • • •

Battery cell capacities (Ah) Cell temperature Electrolyte density at full charge (for lead-acid batteries) Cell float voltage Cell end-of-discharge voltage (EODV).

Battery manufacturers will often quote battery Ah capacities based on a number of different EODVs. For lead-acid batteries, the selection of an EODV is largely based on an EODV that prevents damage of the cell through over-discharge (from over-expansion of the cell plates). Typically, 1.75V to 1.8V per cell is used when discharging over longer than 1 hour. For short discharge durations (i.e.