A list of useful terms & specifications related to battery storage
–Nominal capacity: The total amount of energy that the battery can hold at a time, usually described in kilowatt-hours (kWh). Sometimes the nominal capacity of a battery is the same as the usable capacity, but not always.
–Usable capacity: The amount of energy that a battery can hold after depth of discharge (see below) is taken into account, in kWh. We believe that all manufacturers / retailers should be using this term to sell batteries.
–Depth of discharge (DoD): The percentage of the battery’s capacity that can be safely discharged without significantly damaging the battery or reducing its lifespan. Lithium batteries can typically be discharged to 80% or 90%, while lead batteries will only go down to 40% or 60%. Flow batteries like RedFlow’s ZCell or Imergy’s ESP5 can be discharged to 100%. Sometimes – especially for lead batteries in off-grid situations – battery banks may be deliberately sized to have a ‘shallow’ DoD to extend their lifespan.
–Maximum power: The maximum/peak amount of power that a battery can generate at a given time, for a short period (usually 3-30 seconds), in kilowatts (kW). (Read more about energy vs power batteries.) An example of when maximum power might be required is when you switch on a blender or vacuum cleaner, both of which cause a sudden surge before leveling off.
–Continuous power: The amount of power the battery will generate in normal, non-peak operating conditions – the amount of power you can ordinarily expect out of the battery.
–Roundtrip storage efficiency: The efficiency of charging and discharging the battery – how many kWh do you get out for every kWh you put in? For most batteries this actually varies depending on how the batteries are used (e.g. fast discharge can result in poorer efficiency), but you can usually get an average figure. Remember that this is only for the battery – the inverter will also reduce efficiency.
–Cycle life: The number of charge/discharge cycles a battery has before it is considered to have reached its ‘end of life’. Some battery manufacturers give cycle life as full cycles, while others may count even a partial cycle as one full cycle. Lithium batteries will typically have a cycle life of 4,000-10,000 cycles, while a lead battery might be as low as 1,000 cycles. For lead batteries, cycle life can be extended by designing the system to have a lower depth of discharge.
-Design life: Sometimes it can be unclear if cycle life refers to warrantied cycle life or design cycle life. Design life is the number of cycles/years you can expect the battery to operate in a given application (is it for an off-grid system? grid-connected system? multiple daily cycling? etc). Beware that a battery bank’s design life may be much longer than its warrantied life.
–End of life (EoL): End of life is the point at which the battery would ordinarily be replaced. In most cases, end of life is associated with a decreased ability to hold a charge – which is why you’ll see figures like ‘80% retained capacity’. Flow batteries (Redflow & Imergy) will retain all of their capacity (100% EoL), while some lithium batteries may retain as much as 90% (e.g. Enphase and Sonnen). LG Chem and the Powerwall have a 60% EoL figure.
–Megawatt-hour throughput (MWh throughput): The total amount of energy you can expect the battery to ‘pass through’ over its lifetime, in MWh (1,000kWh = 1MWh). This is the best way to compare value across battery products, but is not a specification that all manufacturers give (although they really should). Sometimes you can find a throughput figure in their warranty documents, but not on their spec sheets. Redflow’s ZCell advertises a 30MWh throughput warranty, for example. LG Chem’s RESU has throughput details in the warranty document, but not spec sheets. Tesla’s Powerwall has a nominally ‘unlimited’ cycle life and no MWh throughput specified in its warranty, but the battery’s end of life is specified at 60% retained capacity, and it’s really only designed for once daily cycling. Solar Choice has estimated the Powerwall’s warrantied lifetime throughput as 18.3MWh. (If you’d like to learn more about battery throughput (and compare a range of battery products, you should check out Solar Choice’s Battery Storage Product Performance Comparison Tool.)
Ambient temperature range rating: Batteries are sensitive things and will not operate as well as they could if the outside temperature gets too hot or too cold. Operating outside of the prescribed temperature range may damage the batteries (thus shortening their life), reduce their efficiency or both (read more on Battery University). Fortunately, most battery management systems are designed to keep batteries operating optimally – which means your battery may not operate if ambient temperatures go outside the bounds of what is prescribed in the spec sheet. (Also keep in mind that warranties can be tied to the ambient temperatures that the batteries are exposed to.)
Product warranty: This is the part of a warranty that covers all parts of the battery unit – including the cabinet/casing (if applicable) and things like the interface (e.g. LCD interface). Basically, the purpose of a product warranty is to cover everything to do with the unit. Product warranties are usually shorter than performance warranties (see below).
Performance warranty: This is the amount of time you can expect the battery to continue to operate – within specified parameters, of course, or it will be replaced. Most battery storage systems for home use have 5-10 year performance warranties. Remember to check to the terms to be sure of what costs would fall on you in the event of a warranty claim (e.g. would the warranty cover a full replacement, or would you be responsible for some of the shipping & installation costs?)
Indoor or outdoor rating: Depending on the battery product you’re looking at, there may be a requirement that it be installed indoors, outdoors or either. It’s easier to find batteries to be installed indoors, while batteries that are rated for outdoor installation need an Ingress Protection (IP) rating. You can read more about IP Ratings here.
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