Date: Aug 13, 2018
The idea that batteries have a ‘life’ is familiar. We’ve all experienced a ‘dying’ cell phone battery with its charge draining, usually at the most inconvenient time. And you might be curious about how this affects long-duration energy storage. To fully understand battery life, let’s start with a few fundamentals.
Battery Fundamentals
A battery stores energy in chemical form, then converts it into electrical energy. Battery ‘life’ refers to three characteristics: performance, longevity, and capacity.
Let’s explain the semantics of these words a bit further:
- Performance life is the run time of a battery on full charge.
- Longevity refers to the number of charge cycles a battery can take before it no longer charges.
- Capacity means that a new battery will charge up to 100% but an older battery will charge possibly up to 70%. For example, the Tesla Powerwall has a warranty of ten years at 70% capacity. Tesla recognizes that the battery will lose 30% or more in capacity over time. High DoD also affects capacity negatively.
Rechargeable batteries have a finite life. Every time you charge your phone, for example, small (and detrimental) changes occur to the battery’s electrodes. Eventually, these changes will kill the battery, preventing it from being able to charge or store energy.
Why ‘Depth of Discharge’ Affects Battery Life
The number of times you charge a battery affects its lifespan, but so does the depth of discharge (DoD) – how much energy of the total battery capacity is drawn off at a time.
You may have received instructions about your cell phone telling you to recharge the battery before it completely ‘dies.’ That’s because a 100% depth of discharge puts stress on a battery and shortens its lifespan. Think of it like driving an older car and letting the engine oil run out. You may be able to drive for several hundred (or thousand) miles, but eventually, the engine will stop working. A battery responds similarly. Consistently drawing a high level of energy per use disrupts the interior of the battery and affects performance.
