Sizing an Inverter Battery Bank for RV or Small Cabin

How Long Will my Batteries Last?

Unfortunately, this question cannot be answered without knowing the size of the battery bank and the load to be supported by the inverter. Usually, this question is better phrased as “How long do you want your load to run?”,  then specific calculations can be made to determine the proper battery bank size. Many manufacturers for off grid autonomous power generating equipment have fancy spread sheets (excel) that do the amp hour calculations based on usual electrical loads for various sizes of RV’s, mobile units etc., ambient temperature, regional solar irradiance with built in safety factors for reliability and availability so that they can automatically select their equipment and generate an install MTO (material take-off), these are helpful calculations and support the manufacturer’s warranty agreements but the do-it-yourselfer still needs to know what he or she is really getting based on technical ability / latest advancements and commercial monetary value.

 Formulas and Estimation Rules

  1. Watts = Volts x Amps
  2. Battery capacity is expressed by how many Amps for how many hours a battery will last – Amp-Hour (A.H.) capacity
  3. For a 12-Volt inverter system, each 100 Watts of the inverter load requires approximately 10 DC Amps from the battery
  4. For a 24-Volt inverter system, each 200 Watts of the inverter load requires approximately 10 DC Amps from the battery

The first step is to estimate the total Watts (or Amps) of load, and how long the load needs to operate. This can be determined by looking at the input electrical nameplate for each appliance or piece of equipment and adding up the total requirement. Some loads are not constant, so estimations must be made. For example, a full-sized refrigerator (750-Watt compressor), running 1/3 of the time would be estimated at 250 Watts-per-hour.

After the load and running time is established, the battery bank size can be calculated. The first calculation is to divide the load (in Watts) by 10 for a 12-Volt system or by 20 for a 24-Volt system resulting in the number of Amps required from the battery bank.

Example of Load Calculations

Suppose you were to run a microwave oven for 10 minutes a day, which draw about 1000 Watts, despite their size. To keep it simple, think of the inverter as electrically transparent. In other words, the 1000 Watts required to run the oven come directly from the batteries as if it were a 12 VDC microwave. Taking 1000 Watts from a 12-Volt battery requires the battery to deliver approximately 84 Amps.

A full-sized refrigerator draws about 2 Amps at 120 Volts AC. By multiplying 2 Amps x 120 Volts, you find out the refrigerator uses 240 Watts. The batteries will need to deliver 20 Amps to run the refrigerator (240 Watts/12 Volts = 20 Amps). Typically, refrigerators operate about 1/3 of the time (1/3 “duty cycle”), or 8 hours a day. Therefore, the A.H. drain will be 160 A.H. (8 hours x 20 Amps = 160 A.H.).

After the load and running time is established, the battery bank size can be calculated. The first calculation is to divide the load (in Watts) by 10 for a 12-Volt system or by 20 for a 24-Volt system resulting in the number of Amps required from the battery bank.

Example of Input Calculations

  1. Total Watts = 1000 W
  2. Amps from 12-Volt battery = 1000 ÷ 10 = 100 Amps DC
  3. Amps from 24-Volt battery = 1000 ÷ 20 = 50 Amps DC

Next, the number of DC Amps must be multiplied by the time in hours that the load is to operate.

Example of Amp-Hour Calculations

If the load is to operate for 3 hours:

For a 12-Volt battery: 100 Amps DC x 3 hours = 300 A.H.

For a 24-Volt battery: 50 Amps DC x 3 hours = 150 A.H.

Now, the proper type and amount of batteries must be selected. Traction batteries, (also called deep cycle or golf cart type), also AGM (Absorbed Glass Mat) type, should be used in order to be able to handle the repeated discharge/charge cycles that are required.

Choosing the Correct Number of Batteries

This is a little more difficult due to the rating method used by the battery manufacturers. Also, because of the nature of the battery, the higher the discharge rate, the lower the capacity of the battery.

See below configurations and reasons for doing so:

In Series configurations, done with batteries of the same size (Ah) and close manufacturing date, this will add the voltage of the two batteries but keep the amperage rating (Amp Hours) same. It is common to see 6V in series to produce 12V requirements.

Series_Batt

In Parallel connection, the current rating will increase but the voltage will be the same.

Para_Batt

Connecting the batteries up in series will increase both the voltage and the run time.

Series_Para_Batt

Batt_Eff

Most batteries’ A.H. capacity is stated for the 20-hour rate of discharge. This means that a battery has a 100 A.H. capacity if it is discharged over 20 hours, or at about 5 Amps-per-hour (100 A.H. / 20 hours = 5 Amps DC). However, this same battery would last only one hour if the discharge rate was 50 Amps-per-hour (50 Amps DC x 1 hour = 50 A.H.) because of the high rate of discharge. The chart above indicates that for 3 hours of discharge rate, the battery has only 70% capacity.  Therefore, we must have 428 A.H. of battery capacity. (Figured by dividing the A.H. capacity by the percentage of loss, or 300 A.H. [obtained from the Example of Amp-Hour Calculations above]  ÷ 0.7 (70%).

Therefore we would require 428 A.H. of batteries at a stated 20-hour rate. If the standard 12-Volt battery is 105 A.H., four batteries are needed in a parallel configuration.

Finally, two more items must be considered. The more deeply the battery is discharged on each cycle, the shorter the battery life will remain. Therefore, using more batteries than the minimum will result in longer life for the battery bank. Keep in mind that batteries lose capacity as the ambient temperature lowers. If the air temperature near the battery bank is lower than 77°F (25°C), more batteries will be needed to maintain the required capacity.

General Maintenance Rules for all types of stored energy batteries used with PV Arrays:

Store and operate your batteries in a cool, dry place. For every 10°C (18°F) rise above room temperature (25°C or 77°F), battery life decreases by 50%. Make sure that the charge controller or inverter system has a built in temperature compensation necessary because the charge voltage limit of a battery increases as temperature drops and decreases. Charge your batteries fully after each period of use. Allowing your batteries to sit in a low state of charge for extended periods will decrease their capacity and life. If you store your batteries for an extended period of time, be sure to charge them fully every 3 to 6 months. As a general rule of thumb, the total amps from your PV panels should be sized between 10% and 20% of the total amp-hours (Ah) of the battery pack. It is also important to consider charge controllers that have a charging algorithm that is configurable for the selected batteries (flooded/gel/AGM) utilizing a 3-stage Bulk, Absorption and Float settings , and specific to flooded acid, an Auto/Manual Equalization setting.

General Maintenance Rules specific to all flooded acid type batteries used with PV Arrays:

Lead acid batteries will self-discharge 5% to 15% per month, depending on the temperature of the storage conditions. Monitor battery voltage and specific gravity of the electrolyte regularly to verify full recharging. Many charge controllers have equalization settings that you can set to help ensure the health of your batteries. Equalize your batteries at least once per month for 2 to 4 hours, longer if your batteries have been consistently undercharged. Water your batteries regularly. Flooded, or wet cell batteries require watering periodically. Check your batteries once a month after installation to determine the proper watering schedule. Add water after fully charging the battery and use distilled water.