How to Check a Battery Storage Warranty: Degradation, Throughput and Test Conditions

Short answer. A storage warranty has two layers: a defects warranty (workmanship and materials for a period of years) and a performance warranty (the battery will still hold a stated share of its energy after a stated time or a stated throughput). The second layer is where buyers lose money, because the number that matters — remaining usable energy — depends on how and where it is measured, how cycles are counted, and which operating conditions void the promise. This guide shows what each term means, using public definitions and one published warranty as a worked example. It does not rate suppliers.

Which standards the terms come from

The performance language in warranties borrows from testing standards. IEC 62933-2-1:2017 (edition 1.0, published 13 December 2017) focuses on unit parameters and testing methods of electrical energy storage systems; the storage devices and technologies themselves are outside its scope. IEC 62619:2017 (published 13 February 2017) specifies requirements and tests for the safe operation of secondary lithium cells and batteries in industrial applications, including stationary applications such as electrical energy storage systems. In China, GB/T 36276-2023 "Lithium ion battery for electrical energy storage" was issued on 28 December 2023, took effect on 1 July 2024, is in force and is managed by the China Electricity Council. A warranty that cites a test standard should say which edition and which clauses apply to its capacity test; a warranty that cites none should include its own full procedure.

Six definitions to pin down

The U.S. DOE / PNNL 2022 cost and performance assessment gives compact definitions that work as a reference vocabulary:

PNNL also makes a point that matters for every datasheet comparison: developer specification sheets report Li-ion cycle life from 1C tests and extrapolation, while most warranties limit the user to one full 100% DOD-equivalent cycle per day, so reaching the quoted cycle count would take far longer than Li-ion calendar life and the datasheet cycle life would have to be derated. In other words, a datasheet cycle number and a warranty cycle cap are not the same quantity. For NMC systems, PNNL's cost model also assumes an initial warranty included with each storage block that expires four years after purchase, after which an annual warranty cost is incurred — and again after each DC rack augmentation; it does not expect LFP to incur an additional warranty cost. A warranty that ends before the financing term means a budget line, not a gap to ignore.

What a procurement template asks for

Sandia National Laboratories' report SAND2016-8544, "Energy Storage Procurement Guidance Documents for Municipalities" (printed September 2016, prepared with Clean Energy Group / Clean Energy States Alliance), contains request-for-proposal templates that show which warranty information a buyer is expected to ask for. The template requests typical degradation-curve information for the proposed battery system and, where cell change-out is recommended, a battery replacement schedule with replacement costs and escalation factors. It requires at minimum a five-year warranty for all BESS components with a separate cost breakdown for additional years, and prices an extended-warranty offering beyond the five-year base warranty as a line item. It states that BESS maintenance cost and performance degradation over project life are considered in selection, as a total-cost-of-ownership process.

The template's warranty clause itself is short: the vendor warrants that equipment and the completed BESS are fit for purpose and free from defects in workmanship and materials for five years after acceptance, and must state life expectancy for the discharge profiles given in the RFP. Note the structure: the performance expectation is tied to the buyer's own discharge profiles, not to a generic cycle count. The template also asks bidders for a list of similar projects from the past three years including warranty claims and uptime percentage, with client contacts the buyer may call. Warranty claims history is itself a due-diligence item.

A published warranty, read clause by clause

Utility-scale warranties are negotiated and rarely published. To show how the clauses are written, we use a residential document that is public: the Sungrow HV Battery Limited Warranty (V7.0, 202312). It applies only in Australia to the listed SBR/SBH battery models and is given to the original purchaser — or, where that purchaser is a distributor, retailer or accredited installer who on-supplies the battery, to the end user; it is otherwise not transferable. It is used here only as an evidence sample of warranty structure.

Two warranties, two clocks. The document warrants freedom from defects in material and workmanship for ten years and, separately, that the battery keeps at least 60% of initial capacity for ten years (120 months) from installation — no more than 126 months from manufacture — or until a minimum energy throughput is reached, whichever comes first, under the stated capacity test conditions. Three features are worth copying into any checklist: the retention threshold (60% here — the same number PNNL uses as an end-of-life example), the dual trigger (time or throughput, whichever first), and the manufacturing-date cap that starts the clock before installation.

Throughput cap. The table lists, for example, 9.6 kWh (SBR096) with a minimum energy throughput of 40.32 MWh and 10.0 kWh (SBH100) with 42.00 MWh. Dividing throughput by nominal energy gives 4,200 full-capacity-equivalent cycles in both cases (CNWattHub arithmetic; the document states no cycle count). A throughput cap is a cycle cap expressed in energy, and it is the first place to look when a warranty seems generous on years.

The capacity test. The warranty defines its own test: ambient 25–28 °C; discharge at constant current to end-of-discharge voltage or self-protection; wait 10 minutes; charge at constant current / constant voltage to full; wait 10 minutes; discharge again recording current, voltage and time; remaining usable energy is the integral of discharge time, current and voltage. This is the clause that decides disputes. A test defined at battery terminals at 25 °C will not reproduce what a site meter sees at the AC side in summer; the buyer needs to know which one the threshold refers to.

Data access. The warranty requires internet access to the product's performance data and log history to assess battery condition. Remote data access as a condition of coverage is common; it should be matched against the owner's own cybersecurity and data rules.

Chinese project rules that touch warranty practice

The NEA's interim rules for new-type storage require main equipment to be tested and certified by qualified bodies, and require the project owner to monitor operating state and, when design life is reached or safe operation is no longer assured, to organise assessment and rectification — and, if the requirements are still not met after rectification, to decommission the project and report to the filing authority. "Design life" in a Chinese storage filing is therefore an operational trigger, not only a warranty parameter, and the two should be aligned.

Seven questions to put to a supplier

1. Port. Is the guaranteed energy measured at DC battery terminals, at the AC terminals of the converter, or at the point of interconnection?
2. Conditions. At what temperature, C-rate, SOC window and rest times is the capacity test run, and who runs it?
3. Metric. Is the threshold a share of nominal energy, of usable energy at delivery, or of a measured baseline?
4. Caps. What is the throughput or cycle cap, how are cycles counted, and what is the assumed DOD?
5. Exclusions. Which operating profiles, ambient conditions or firmware states void the performance warranty?
6. Data. What remote data access is required, and what happens if it is interrupted?
7. Remedy. Repair, augmentation, replacement or payment — and what if the product line is discontinued?

Scope limits

One residential warranty, one procurement template and one national-laboratory report are used as evidence; none of them is a utility-scale warranty, and no degradation curve or cycle-life number for any utility-scale product is given. Standard texts were read only at abstract level. This is not a supplier recommendation.