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101 renewable - Battery sizing guidelines (from Trojan)

Article Details

Last Updated
6th of October, 2018

(21 votes)
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General Guidelines:-

  >_Calculate the load in Watts-hours per 24-Hr Day (Wh/d) as accurate as possible,
  >_Include system losses due to efficiencies of power conditioning (inverter, DC/DC converters),
  >_Include the appropriate factors: Temperature, autonomy, design margin, and depth of discharge (DOD),
  >_Consider shallow daily DOD (max 20% recommended) and occasional deeper DOD (max 80%) during cloudy days,
  >_Use the correct battery rating (100-Hr), or a battery rating that approximates the actual autonomy hours for the system load,
  >_Select highest battery capacities per unit to reduce the number of battery strings in parallel for better charge balance,


The recommended maximum number of strings in parallel is 6.

Load calculation:-

     Load Description



Hrs of Operation

Inverter Efficiency

Energy Consumption
Per 24-Hr Day(Wh/d)
(A x B x C) / D

      Total DC Energy Consumption Sum1

This table applies for resistive loads. For motor applications, please consult with manufacturer for the applicable power factor. Inverter efficiency shall be the average during the day, not the maximum efficiency. If not known, please use 0.80.

Battery bank Capacity B(Ah)
  >_Aha = Ahd x TC x DA x DM / DOD
  >_Aha: Amp-hours adjusted,
  >_Ahd: Amp-hours day (from chart Total Energy Consumption Calculation),
  >_TC: from chart Temperature Correction Factor,
  >_DA: Days of Autonomy (from chart Adjustment Factors),
  >_DM: Design Margin (from chart Adjustment Factors),
  >_DOD: Depth of Discharge (from chart Adjustment Factors),  

Load Description:-

Load Description



       DC Load

Hrs of Operation

Inverter Efficiency

Energy Consumption
Per 24-Hr Day(Wh/d)
(A x B x D) / E

      Total DC Energy Consumption Sum1

Common voltages: 12 Volt, 24 Volt or 48 Volt. For other voltage, consult Trojan Battery for advice. If & when DC-DC converter is required, please use 0.85.

**If not required, please use 1.00.

Total Energy consumption Calculation:-

Description Calculations Steps Results (units)
Total AC Consumption SUM 1 (Wh/d)
Total DC Consumption SUM  2 (Wh/d)
Total Consumption Watt-hours Per 24-Hr Day SUM 1 + SUM 2 = SUM 3 (Wh/d)
Battery Bank Voltage Common Voltages: 12V, 24V or 48V (V)
Total consumption Amp-hours Per 24-Hr Day SUM 3 / Battery Bank Voltage =  (Ah)

Battery bank capacity (Amp-hour):
The battery bank capacity, B(Ah), is the capacity of the battery bank required to run the daily load under normal condition (Total consumption Amp-hours Per 24-Hr Day - see table above). This battery bank capacity shall be calculated to account for system operational parameters. The following is a list of battery bank capacity adjustment factors that will serve to calculate the adjusted and final battery capacity required to run the system appropriately. 

Adjustment Factors:-

Synbol Parameter Range Comments
TC Temperature Compensation Flooded Lead Acid See Temperature Correction Table
DA Autonomy days 2  10* System Dependent
DM Design Margins 1  -1.25 System Dependent

* For stand alone solar, recommended value is 5 days. For hybrid, recommended minimum value is 2 days,
** Recommended value is 1.10, and typical value is 1,
*** Recommended value is 0.2, and typical value is 0.5

Temperature Correction Factor:-

(^F) (*C) Flooded AGM GEL
77 25.0 1.00 1.00 1.00
50 10.0 1.19 1.08 1.11
32 0 0 1.20 1.25
14 -10 -10 1.35 1.42

Battery configuration:-

The battery bank capacity can now be used to determine the amount of batteries needed for the installation.

Description Calculation Steps Results
Number of Strings Divide   the B(Ah) by   the battery’s 100-hour  rate
 (if   5   days   autonomy   selected)  
or rate that approximates 
the actual  autonomy hours for  the sys.
Number of Batteries Cells / Monoblocks per String Divide the battery bank   voltage by the voltage of   the chosen battery   cell   monoblock
Total Batteries Cells / Monoblocks A x B
Battery Model Refer to Trojan Battery's Renewable Energy Brochure or www.trojanbatteryre.com

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