Lead Acid Runtime Guide
Why Runtime Changes
Lead acid batteries are simple, strong, and widely used. They power inverters, alarm systems, pumps, telecom panels, and emergency lights. Runtime is not found from amp hours alone. The load, battery age, discharge limit, temperature, wiring loss, and inverter loss all change the final time.
A fresh battery may deliver its rated capacity at the test rate. Many labels use a twenty hour rate. A 100 Ah battery can deliver about five amps for twenty hours during that test. A larger current gives less usable capacity. This is called Peukert effect. Flooded cells usually lose more runtime at high current. AGM and gel batteries often perform better, but they still need derating.
Derating and Battery Life
Depth of discharge is also important. A battery may run longer when deeply discharged, but service life can fall. Many standby systems use fifty percent or less. Critical systems may keep an extra reserve. Cold weather reduces chemical activity. Old batteries also store less energy. This calculator lets you model each condition before a real outage happens.
The result should be treated as a planning estimate. Real runtime depends on battery brand, plate design, cable size, terminal condition, and cutoff voltage. Inverters may be less efficient at very small loads. Motors and compressors can draw surge power. For important equipment, test the system under the real load. Replace batteries that fail a timed load test.
Planning Better Backup
Use the adjusted runtime to compare options. Add parallel strings for more capacity. Raise system voltage to reduce current. Reduce nonessential loads during backup operation. Keep batteries charged, clean, and ventilated. Record every test date. A simple maintenance log makes future estimates much better.
Small changes can create large gains. Replacing a wasteful load may add hours. Cleaning terminals can reduce voltage drop. Choosing a higher voltage bank can lower heat in cables. Matching batteries by age and model keeps strings balanced. These habits make backup systems easier to trust.
For safety, follow the battery maker’s instructions. Use correct fuses and cable sizes. Keep sparks away from flooded batteries. Ventilate charging areas. Do not mix old and new batteries in one bank. Good design protects equipment, batteries, and people. Review settings whenever the load or battery bank changes during future maintenance work.