By Dean Middleton, vice president of sales at SimpliPhi Power
With new storage options frequently coming to the market, understanding and comparing them can be confusing. But contractors can learn a lot by inquiring about the right things. Here are 10 questions solar installers should ask manufacturers before buying a battery.
Dean Middleton
1.What is the battery chemistry?
Knowing the battery chemistry and the cell level form factor (the physical shape of the battery and its terminal) is the first step in understanding the relative strengths and weaknesses of different batteries and their performances. Specifically with lithium-ion chemistry, form factors range from cylindrical and prismatic to pouch. Each form factor creates a different layer of specifications in combination with the chemistry that impacts the efficiency rate, available power capacity, cycle life, depth and rate of discharge, operating temperature and a host of other performance parameters. The internal architecture and power electronics built into some batteries serve to leverage or mitigate the intrinsic qualities of the chemistry and form factor. Different project requirements, such as peak shaving, back-up power, load shifting, frequency and voltage regulation impact the relative efficacy of one battery type over another for the application at hand.
2. Does the battery chemistry contain cobalt?
Cobalt is a toxic material. Cobalt oxide type cathodes are also what generate the heat commonly associated with lithium-ion batteries. Unmonitored, cobalt can lead to thermal runaway and fires. Thermal runaway can take place regardless of the internal battery management system, ventilation and/or cooling built into a battery. The external environmental and ambient temperatures where the batteries are housed must also be monitored and controlled. This adds to the complexity and costs associated with lithium-ion with cobalt battery installations, particularly within the context of the levelized cost of energy. Although the earlier generation lithium-ion batteries using cobalt are fundamentally more energy dense per lb/kg on a chemical level, the heat mitigation techniques that must be incorporated to manage and safeguard against heat build-up, thermal runaway and fire offset this fundamental advantage and create lithium-ion cobalt batteries that are heavier, larger and more costly over time.
3. What is the cycle life?
Cycle life is the number of times the battery can be fully charged and discharged before it runs the length of its useful life. Each time a battery is charged and discharged, chemical or physical changes occur within the battery (also known as the memory-effect). Cycle life varies among chemistries and form factors.
4. What is the energy density?
Energy density is the amount of energy stored in a given battery per unit volume. Ask questions related to how much of the total energy stored in the battery is actually available at any given time for use (the actual power capacity) versus nameplate capacity, which would be the maximum rated output.
5. What are the charge and discharge rates?
Ability to withdraw power from the battery at both low current and high current rates is critical to getting the maximum use out of a battery. Some batteries provide low current, long duration discharge but fail in high current, short duration applications. Ideally, look for a battery that provides the widest variety of charge and discharge rates without the concomitant degradation of capacity and cycle life over time. This ensures the batter can be used across a range of applications that might require both long duration and short duration discharge and charge profiles, without damaging its overall performance and longevity.
6. What is the round-trip efficiency?
A battery’s round-trip efficiency rate indicates how much of the energy generated on site is captured or lost in a “round trip” between the battery being fully charged and fully discharged to meet electrical loads. Efficiency losses are generated by the internal resistance of the chemistry, form factor and power electronics, but they can also come from heat mitigation measures, ventilation or cooling. Depending on the use cases and various charge rates and depths of discharge or duty cycles, the battery’s efficiency can vary.
7. How many products does a company have in the field, for how long and what is the failure rate?
Products at work in the field are a critical measure as to the credibility of a technology beyond specification sheets and product presentations. Learning the answer to these questions can provide insights into the number of successful installations, how long they have been selling into actual markets and the relative success rate or efficacy of their storage technology.
8. Is the battery management system (BMS) included in the manufacturer’s ROI calculations?
The BMS can erode or improve the efficiency rate. Often the BMS is a separate unit, for a price additional to the cost of the battery unit itself. Asking about the BMS can open the door for learning about other hidden costs and equipment a given battery might require.
9. Can you connect with someone who has installed a system?
Installations are not always as straightforward as sales people would have you believe. See if they can connect you with someone in your area who has installed their batteries. Talk with that installer about the project. Was technical support required and/or provided, and how helpful were they?
10. What is the true cost of the energy storage system?
Very often the low up-front price point or cost of some batteries is significantly reduced or entirely eliminated by the battery’s installation, shipping and logistics. This includes many of the elements discussed in this article, such as weight and square footage requirements, ventilation and cooling systems, efficiency rate, short cycle life and name-plate versus usable power capacity. The impact of these elements in the overall efficiency of the battery impacts its weight and footprint. Thus, a battery which maximizes efficiency while maintaining lower weight and footprint stores more energy per dollar spent.