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.
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.
When purchasing rechargeable batteries, especially those for solar power storage, the depth of discharge becomes an essential qualifier of performance. You may see battery labeling showing a range of lifecycle options such as 25,000 cycles at 30% DoD or 1,000 cycles at 75% DoD.
Cost Implications of Depth of Discharge for Solar Storage
When you shift to stored solar power for your home or business, you’ll likely want the option of a deeper discharge. Why? Because you’ll need access to as much stored energy as possible to keep lights, appliances, and other devices fully functioning. But remember, drawing down the battery deeply in the short run will reduce the number of cycles the battery operate effectively.
The result is a higher cost per kWh over the shortened lifespan of the battery. For example:
Let’s say your 10-kWh lithium-ion battery costs $6,000 and promises 1,000 cycles at 80% DoD. That means you’ll have 8,000 kWh across its life (10 kWh x 1,000 cycles x .8), and you’ll pay $0.75 per kWh ($6,000 / 8,000).
If you run the same battery at 20% DoD, you may see 10,000 cycles or 20,000 kWh across its life – and only pay $0.30 per kWh. Unfortunately, you may not be able to power all your appliances or lights when you need them.
The Vanadium Advantage
Vanadium flow batteries and battery life are different than traditional lithium-ion batteries. A vanadium battery uses a liquid, non-flammable electrolyte solution to store energy, enabling it to deliver at 100% depth of discharge without degrading capacity over time. This means a StorEn* vanadium battery provides the full power you need for thousands of cycles and many years – keeping the cost per kWh for solar storage lower than other options. Furthermore, the electrolyte is 100% reusable in a new battery, which means there is no need to mine new vanadium.
You can find out more about StorEn’s products and invest in their reliable, cost-effective technology by visiting their investment campaign.