Five Questions to Ask Your BESS Supplier Before Signing

Most procurement failures in battery storage happen before commissioning — at the RFQ stage, when buyers ask the wrong questions. Price per kWh is easy to compare. What is harder to compare, and what matters far more, is whether the supplier can actually deliver what the spec sheet describes. Here are the five questions that separate a serious supplier from a vendor with a good PDF.

1. Where are your cells from, and can you prove cycle-life data at that exact chemistry and format?

The cell is the most critical and least visible component in a BESS container. Suppliers often specify "LFP prismatic" without naming the cell manufacturer, capacity, or format — because cells are swapped between batches depending on availability and margin.

Ask for the specific cell part number and the third-party cycle-life test data for that exact cell. The test should show capacity versus cycle count through at least 1,000 cycles at the rated depth of discharge, with temperature conditions stated. A supplier who cannot produce this document either does not know what cell they are shipping or does not want you to know.

For reference: a well-manufactured 314 Ah LFP prismatic cell from a reputable tier-1 manufacturer will show less than 3% capacity fade after 1,000 cycles at 25°C and 100% DoD. The data will exist. If it does not, the cell quality is unknown.

2. What is your first-pass yield, and where in the production process do you measure it?

First-pass yield — the fraction of units that pass quality inspection without rework — is the most honest single number a manufacturer can give you. An 85% first-pass yield means 15 out of every 100 cells, modules, or packs required rework before proceeding. That rework introduces handling, introduces re-weld joints, and introduces human error.

The follow-up question is where yield is measured. Measuring only at system-level FAT hides defects that accumulated earlier. A serious line measures at cell intake (sorting), at module EOL, at pack EOL, and at system FAT — and can show you the data for each gate. Industry benchmark for a mature lithium-ion line is 98–99% first-pass yield at cell/module level. Our Mandvi line runs 99.7%.

3. What does your warranty actually cover, and who bears the cost of a cell-level replacement?

BESS warranties are written to protect the supplier. Read the exclusions before you read the guarantees.

Key questions: Does the capacity warranty apply to the full system or individual racks? What is the degradation curve the warranty is written against — calendar years, cycles, or both? If a rack underperforms, does the supplier replace cells, packs, or the whole rack? Who pays for mobilisation, labour, and crane time? What is the response time commitment, and is it contractually binding?

A warranty that guarantees 80% capacity at year 10 but excludes labour, mobilisation, and "acts of ambient temperature" is not a warranty in any practical sense. Get the warranty document in full before you sign the supply agreement, not after.

4. Can I walk the factory floor — and what will I see there?

A factory visit tells you more in two hours than a supplier questionnaire tells you in two months. The things that matter are: Is the cell intake process automated and documented? Does the line have in-line weld inspection, or are welds visually checked by hand? Is the pack EOL test a full capacity cycle or a spot-check? How are rejects handled — are they segregated and traced, or reworked on the line without logging?

A supplier who declines a factory visit, or offers only a virtual tour, is a supplier with something to hide. We invite qualified buyers to walk the Mandvi floor every Wednesday.

5. Who is your service engineer, and where are they based?

A BESS system that runs without incident for five years and then has a BMS fault at 2 a.m. on a Monday requires a service engineer who knows the product, has access to spare parts, and can reach your site within a defined SLA. Ask for the name, location, and certification of the service engineer who will be responsible for your project. Ask what the spare parts holding looks like — specifically, whether your system's BMS firmware version is still supported, and whether replacement BMS boards and pyro fuses are held in country.

Offshore support desks and 72-hour response windows are not acceptable for grid-connected storage assets. If the supplier cannot name a local service contact, the service model is not built.

These five questions are not adversarial. They are the minimum due diligence that any serious BESS procurement should include. A supplier with a real product and a real manufacturing operation will answer all five without hesitation.

Talk to our team about your project — we are available for pre-bid technical discussions.