Battery Backup Estimator

50 % DOD is the standard long-life design point for lead-acid in standby/UPS applications. This maximises cycle count — typically 200–500 full cycles to 50 % DOD. For occasional emergency backup where batteries rarely discharge, 80 % DOD is acceptable. Never design for 100 % DOD; you will damage the cells and void the warranty.

Yes — add 20–25 % margin beyond the calculated Ah requirement. This accounts for load growth, actual vs rated capacity variation between battery units, battery replacement scheduling and worst-case temperature or aging coincidence. It is also common to round up to the next standard battery string configuration.

Battery capacity degrades over time — lead-acid typically to 80 % of rated capacity after 3–5 years, and IEEE 485 defines end-of-life as 80 % of original capacity. An aging factor of 80 % means you size the bank to deliver the required runtime even at end-of-life, so the bank will be oversized when new. This is good practice for mission-critical installations.

Common DC bus voltages: 12 V and 24 V for small consumer UPS systems. 48 V is the standard for telecom, small data centre UPS and modern home energy systems — lower current for the same power, smaller cable cross-section. 120 V DC is used in utility and large industrial UPS systems (IEEE 485). 240 V DC in large data centre UPS (matching the rectifier bus).

Never mix batteries of different Ah ratings in the same parallel string — weaker cells will be overloaded during charging and discharge. Mixing old and new batteries in the same bank causes premature failure of the new cells. Replace battery strings as a complete set or full bank.