Renewable energy sources, such as wind and solar, are inherently unpredictable. In use cases where the grid is not available to augment the renewable generation, batteries and/or generators are frequently installed to ensure energy is available when the sun isn’t shining or the wind isn’t blowing.
Lead acid batteries are most common energy storage solution for these applications, which means the systems are sized to ensure the batteries discharge roughly 50% of their capacity. This is done to increase the lifetime of the battery. However, in certain failure scenarios, the state of charge of an off grid battery pack can fall well below 50%. These scenarios include:
- Failure of the backup system: Off-grid systems that include diesel gensets to supply some of the daily load or to provide emergency backup can have failures. Generators have moving parts and must be regularly maintained to ensure dependable performance. If diesel supply runs out, it may be several days before it is replenished, particularly in remote locations, such as islands. This is especially true in remote areas of the developing world.
- Long stretches without sunlight: If there is no backup system, battery installations are often sized to ensure several days of power with no sun available. However, in the rare instances where clouds or inclement weather impede sunlight for an extended period of time, the battery may reach beyond the recommended depth of discharge.
- “Oops” Moments: Outside of these scenarios, there are often instances where a battery will accidentally discharge further than intended. Whether this is because the battery’s state of charge was measured incorrectly, or because the backup power isn’t turned on in time, batteries may inadvertently stay partially charged for an extended period.
Demonstrating The Performance of Lead Acid Batteries
Standing at a partial state of charge, even if the only a few hours, is extremely detrimental to a lead acid battery and repeated occurrence destroys the lifetime of battery. To demonstrate this we recently performed a partial state of charge test on a high-end lead acid battery and compared this to our Aqueous Hybrid Ion (AHI) battery. We tested a scenario in which the battery charged for six hours, simulating a charge by a solar array, discharged to 80% DoD for six hours, simulating a steady load, and then stood at a partial state of charge for six hours, simulating a scenario in which the battery was unable to recharge.
We repeated this test continually for 60 cycles. The lead acid batteries began at approximately 300 Wh (nominally 190 Ah at a 5 hour rate) and the AHI batteries began at 140 Wh.
The lead acid battery dropped to half its usable capacity (150 W), after only 30 cycles, while the AHI battery remained constant. In cases of unreliable or nonexistent backup, or when “oops” moments are frequent, this data indicates lead acid batteries will rapidly deteriorate. Though no one would ever design a system that purposely held a lead acid battery at a partial state of charge, off grid systems are not always used as they were intended, and accidents happen. To guarantee worry-free performance, a different solution is needed.
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