Battery degradation: is your check-up overdue?

Published February 2017

Battery financial models often assume 15+year project lifetimes. In this blog, Fliss Jones interrogates battery degradation risks, and seeks Doctor’s orders on how to protect battery health.

Dearie me, I’ve been having some health worries recently. Battery health worries.

You see, nearly all of the 500MW of the UK battery projects that secured Capacity Market contracts have committed to be operational out to 2035. This means that, unless they plan to trade their contracts on the secondary market, projects being built out this year need to remain operational at the grand old age of 17. Call me a hypochondriac, but I can’t help but worry: just how fit will these batteries be in their latter years?

Plagued with such health woes, it seemed prudent to consult a Doctor. So in this blog, I’ll draw on the fantastic work of Dr David Howey and his battery research group at Oxford University. I’ll infuse it with Everoze’s practical project experience as Technical Adviser to explain how geriatric batteries can retain a youthful glow.

Factors threatening battery health

The charts below are taken from an excellent study by the Oxford team, in collaboration with EnergyVille, Belgium:

  • Figure 1 shows calendar ageing – i.e. how a battery loses capacity if not being used.
  • Figure 2 shows cycle ageing – i.e. how a battery loses capacity if in continuous use.

Figure 1: Calendar ageing – experimental (marker) and simulated (lines) calendar ageing at various temperatures and state of charge (SoC).

 Figure 2: Experimental cycle ageing – residual capacity [%] vs. equivalent cycle number for an NMC Li-ion battery cycling at 45°C between various SoC windows.

Whilst the shape of these curves can vary with battery chemistry, the charts neatly illustrate a few variables of particular importance to long-term battery capacity:

  1. Time: Firstly, like all of us, batteries fade with the passing of time [Figure 1].
  2. Temperature: Secondly, high temperatures bring capacity fade – so cooling systems are particularly important [Figure 1].
  3. State of charge (SoC): Thirdly, a high state of charge can be taxing on batteries when they are not in use [Figure 1].
  4. Number of cycles: Fourthly, energy throughput vs capacity is key, normalised to a number of cycles. This means that every dispatch decision bears a degradation cost [Figure 2].

These are some main drivers, but it’s worth noting that degradation is an open research area, and other factors may also be important.

Your essential health check-up

So, how to guarantee performance in later life? Whilst none of us can dodge the passing of time, the recommendations of all health professionals are simple: diet, exercise and health insurance.

1. Diet – avoid extremes: Carefully consider the kWh consumption and discharge profile of your battery. Figure 2 hints that full charge and deep discharge ‘binges’ – such as triad-chasing from a 30-minute battery – could accelerate degradation. The Oxford team use a Belgian case study to show how maximising long-term profit sometimes means pursuing less valuable revenue streams, because this brings the benefit of lower battery capacity fade. In other words: keep your battery’s kWh diet within a healthy range. Resist regular full charge and deep discharge binges, unless gorging on the near-term revenue is justifiably delicious.

2. Exercise – make your software break a sweat: Demand that your software performs algorithmic gymnastics. As the Oxford team highlight, it’s critical to tightly integrate your cells, power electronics, battery management system (BMS) and dispatch system. Build in digital safeguards to protect your asset: you need your software to develop data-driven muscle tone which ensures a healthy operating profile.

3. Health insurance – invest in contractual protection for the future: This final point has many facets. Firstly, responsibility for the cooling/heating system must be carefully allocated given the high sensitivity of battery capacity to temperature. Secondly, make sure that incentives are well aligned between all parties – a common problem is aggregators being incentivised to ‘run a battery hard’ to maximise revenue, and not facing the associated degradation costs. Thirdly, futureproofed warranties with OEMs are important, as explored in our previous blog on revenue optionality.

On Doctor’s orders

So, if you want your battery to see old age, look after it. Factor battery degradation into your dispatch decisions, software set-up and contract design. Or if you prefer to live wild and run your battery hard, that’s fine too – but make sure your assumed project lifespan reflects this.