Lithium-ion Batteries & Battery Packs Interview Questions & Answers

eMobility Academy — Lithium-ion Batteries & Battery Packs interview questions

1. Q1.Explain how a lithium-ion cell works.

   Fresher

   A lithium-ion cell has a cathode (a lithium metal oxide or phosphate, such as NMC or LFP), an anode (usually graphite, sometimes blended with silicon), a porous separator, and a liquid electrolyte of lithium salt (typically LiPF6) in organic carbonates. During charging, lithium ions leave the cathode, travel through the electrolyte and separator, and intercalate into the graphite, while electrons flow through the external charger. During discharge the process reverses and electrons flow through the load. No metallic lithium is meant to form; the ions shuttle between host structures, which is why it is called a rocking-chair battery. A thin SEI (solid electrolyte interphase) layer forms on the anode in the first cycles and is essential for stability.

2. Q2.Compare LFP and NMC chemistries.

   Fresher

   LFP (lithium iron phosphate) has a nominal voltage of about 3.2 V (charge limit about 3.65 V), lower energy density (roughly 160 to 200 Wh/kg at cell level), excellent cycle life (often 3,000+ cycles), lower cost, no cobalt or nickel, and much better thermal stability. NMC (nickel manganese cobalt) has a nominal voltage of about 3.6 to 3.7 V (charge limit about 4.2 V), higher energy density (roughly 220 to 280 Wh/kg), typically 1,000 to 2,000 cycles, and lower thermal runaway onset temperature. LFP dominates Indian three-wheelers, buses and many cars because of cost, heat tolerance and safety. NMC is used where weight and volume matter most, such as premium scooters and longer-range cars. LFP's flat voltage curve makes SoC estimation harder.

3. Q3.What are the common cell formats and their pros and cons?

   Fresher

   Cylindrical cells (18650, 21700, 4680) are mature, cheap, mechanically robust and easy to cool, and a fault in one cell involves little energy. But a pack needs many of them and many welds. Prismatic cells have a rigid aluminium can, large capacities (50 to 300 Ah), fewer interconnections and good volumetric packing; swelling must be constrained with end plates, and a single cell failure releases more energy. Pouch cells give the best gravimetric energy density and flexible shapes, but need external compression and structural support, and can swell with gas generation. Indian two-wheelers often use 18650 or 21700 cylindrical NMC cells, while three-wheelers and cars commonly use prismatic LFP.

4. Q4.What is C-rate?

   Fresher

   C-rate expresses current relative to the cell's capacity. 1C is the current that would fully charge or discharge the cell in one hour; for a 50 Ah cell, 1C is 50 A, 0.5C is 25 A, and 2C is 100 A. A 2C discharge empties the cell in about 30 minutes. Datasheets specify maximum continuous and pulse charge and discharge C-rates, which usually depend on temperature and SoC. Higher C-rates increase heat generation (I²R) and accelerate ageing, and charging at high C-rates, especially at low temperature, risks lithium plating on the anode. Power cells tolerate high C-rates at the cost of energy density; energy cells are the opposite.

5. Q5.How do you calculate the series-parallel configuration for a 48 V, 2.5 kWh pack using 3.6 V, 5 Ah 21700 NMC cells?

   Fresher

   For voltage: a 48 V class pack with 3.6 V nominal cells needs 13 cells in series (13 × 3.6 = 46.8 V nominal, 54.6 V at 4.2 V per cell). Each cell holds 3.6 × 5 = 18 Wh. One 13S string holds 13 × 18 = 234 Wh, so for 2.5 kWh you need 2500 / 234 ≈ 10.7, round up to 11 in parallel: a 13S11P pack, 143 cells, about 2.57 kWh and 55 Ah. Then check that the peak discharge current divided by 11 is within each cell's rating, and that the module fits the space. Some designs use 14S to raise voltage. You can check variations with the battery pack calculator.

6. Q6.What is the difference between energy density and power density?

   Fresher

   Energy density is how much energy a cell or pack stores per unit mass (Wh/kg, gravimetric) or volume (Wh/L, volumetric); it decides range for a given weight or space. Power density is how fast that energy can be delivered per unit mass (W/kg), deciding acceleration and fast-charge capability. Cells are designed toward one or the other: thicker electrodes give more energy but higher resistance and less power; thinner electrodes give more power. Pack-level figures are always lower than cell-level figures because of enclosure, busbars, BMS and cooling, so a pack might reach only 60 to 75% of the cell's gravimetric energy density depending on design. Cell-to-pack designs aim to improve that ratio.

7. Q7.What is thermal runaway and how is it prevented at the pack level?

   Fresher

   Thermal runaway is a self-accelerating reaction in which a cell's internal heat generation exceeds its heat dissipation. Triggers include internal short circuits from manufacturing defects or dendrites, overcharge, external short, crush or penetration, and overheating. SEI breakdown, separator melting, and cathode oxygen release then feed each other, and the cell can vent flammable gas, catch fire or explode. Prevention works in layers: cell quality and screening; a BMS that enforces voltage, current and temperature limits; fuses and contactors; thermal management; and propagation barriers such as mica sheets, aerogel pads or spacing between cells so one cell failure does not cascade. Packs also need venting paths. AIS-156 includes a thermal propagation requirement and a warning to the rider.

8. Q8.What does state of health (SoH) mean and how does a battery degrade?

   Fresher

   State of health compares the battery's present condition with its new condition, usually as remaining capacity (for example 85% of rated Ah) and sometimes as resistance growth. End of life for automotive use is often defined at 70 to 80% capacity. Degradation has two components. Calendar ageing happens with time, faster at high temperature and high SoC. Cycle ageing depends on depth of discharge, C-rate and temperature. Mechanisms include SEI growth that consumes lithium, loss of active material, particle cracking, and lithium plating during cold or fast charging. Practical advice: avoid storing at 100% in heat, avoid frequent fast charging when cold, and keep the pack in a moderate temperature range.

9. Q9.Why are cells graded or matched before pack assembly?

   Fresher

   Even cells from the same batch vary slightly in capacity, internal resistance and self-discharge. In a series string, the weakest cell decides when charging must stop and when discharge must stop, so a mismatched cell limits the whole pack's usable capacity. In parallel groups, cells with different resistance share current unevenly and age at different rates. Grading means measuring each incoming cell's OCV, AC internal resistance (typically at 1 kHz) and capacity, then binning them so cells in a pack are within tight tolerances. Checking OCV again after a rest period also catches high self-discharge cells, which can indicate internal micro-shorts. Good grading reduces balancing load and early field failures.

10. Q10.What are the main components of a battery pack besides the cells?

    Fresher

    A pack contains cell holders or module frames, busbars or nickel strips to connect cells, the BMS (master and slave boards, voltage and temperature sense wires), current sensor (shunt or Hall), contactors and pre-charge circuit (or MOSFET switches in low-voltage packs), a main fuse, a manual service disconnect in larger packs, thermal management (cooling plates, thermal interface material, heaters), insulation and fire barriers, the enclosure with gaskets meeting IP67, a pressure vent for gas release, and HV and LV connectors. Each needs to be specified for vibration, ingress, thermal and electrical safety, which is why pack engineering is as much mechanical and thermal as it is electrical.

11. Q11.What is internal resistance and why does it matter?

    Fresher

    Internal resistance is the opposition to current inside a cell, combining ohmic resistance of electrodes, electrolyte and current collectors with charge-transfer and diffusion effects. It is measured as AC impedance at 1 kHz for quick QC, or as DC resistance from the voltage drop under a current pulse, which is more relevant to real loads. It matters because it causes voltage sag under load, which reduces available power and triggers low-voltage cutoffs early, and because it generates heat proportional to I²R. Resistance rises as cells age and as temperature drops, which is why EVs feel weaker and charge slower in cold conditions. Rising resistance is often an earlier ageing indicator than capacity loss.

12. Q12.How do you choose between laser welding, ultrasonic welding and resistance spot welding for cell interconnects?

    Experienced

    Resistance spot welding of nickel or nickel-plated steel strips is cheap and common for cylindrical cells in two-wheeler packs, but has limited current capacity and higher joint resistance, and the heat input must be controlled to avoid cell damage. Laser welding works with aluminium and copper busbars, gives low and repeatable joint resistance, high throughput and low heat input, and is the standard for automated prismatic and cylindrical lines, though equipment cost is high and it needs good fixturing and weld-depth control. Ultrasonic wire bonding or welding suits pouch tabs and fine wire bonds that act as cell-level fuses. Selection depends on current per joint, materials, cell format, volume and inspection capability, including pull tests, resistance measurement and vision inspection.

13. Q13.What does AIS-156 require for two-wheeler traction batteries?

    Experienced

    AIS-156 covers L-category electric vehicles. After the 2022 fire incidents its amendments, applied in two phases from December 2022 and March 2023, added significant battery requirements. Highlights include: cells to be tested to the IS 16893 series; an IP67 rating for the pack; thermal propagation testing, with an audible or visual warning to the user if a thermal event starts; BMS requirements including cell-level monitoring and protection against overcharge, over-discharge, over-current and over-temperature; a fuse or circuit breaker; requirements on the charger interface; and mechanical and environmental tests such as vibration, mechanical shock, thermal shock, fire resistance, external short circuit and overcharge or over-discharge protection. It also requires traceability and data logging. Always quote the current revision from the ARAI site in an interview, since amendments keep coming.

14. Q14.How do you design the thermal management for an LFP pack in a three-wheeler operating in Indian summers?

    Experienced

    Begin with the heat load: use the duty cycle current profile and DC resistance to compute I²R heat per cell, including high-temperature and end-of-life resistance. Ambient can be 45°C in north India, so passive air cooling must keep cells below the chemistry's limit, typically about 55 to 60°C maximum operation, with a target near 35°C for life. For moderate power LFP three-wheelers, conduction to a finned aluminium base plate with good thermal interface material is often enough; for higher C-rates or fast charging, a liquid cold plate with a chiller or radiator is needed. Keep temperature spread within the pack to about 5°C, because hot cells age faster. Validate with CFD, then thermocouple-instrumented drive-cycle tests in a climatic chamber, and set derating curves in the BMS.

15. Q15.What is lithium plating and how do you avoid it?

    Experienced

    Lithium plating happens when lithium ions arrive at the graphite anode faster than they can intercalate, so they deposit as metallic lithium on the surface. It is driven by charging at low temperature, high charge C-rates, high SoC, and anode overhang or design issues. Plated lithium causes irreversible capacity loss, increases resistance, and can grow dendrites that pierce the separator and cause internal shorts. Avoidance is a BMS and charging strategy problem: derate charge current below about 10 to 15°C and block charging below about 0°C unless the pack is heated, taper current at high SoC, and use cell-specific charging maps from the cell supplier. Detection methods include analysing voltage relaxation after charge and differential voltage analysis.

16. Q16.How do you validate a new pack design before production?

    Experienced

    Validation runs through a DVP&R (design verification plan and report). Electrical tests: capacity and energy at multiple temperatures, power capability (HPPC), insulation resistance, and BMS function and fault injection. Environmental and mechanical tests: vibration and mechanical shock, thermal shock and humidity cycling, IP67 ingress, salt spray, drop and crush where required. Abuse tests: external short circuit, overcharge, over-discharge, over-temperature, and thermal propagation. Then durability: accelerated cycle life on representative profiles and calendar ageing. For India, the homologation tests come from AIS-156 (L-category) or AIS-038 Rev 2 (M and N), and transport needs UN 38.3 for cells and packs. Results feed DFMEA updates before PPAP and SOP.

17. Q17.What is cell-to-pack (CTP) design and what are its trade-offs?

    Experienced

    Cell-to-pack eliminates or minimises the module level, placing large cells directly into the pack enclosure, as in BYD's Blade LFP pack or CATL's CTP designs. Benefits: higher volume utilisation and pack-level energy density, fewer parts, fewer interconnects and lower cost, which is what made LFP competitive for long-range cars. Trade-offs: serviceability drops because you can no longer swap a module; thermal propagation design becomes harder because there are fewer barriers; the pack structure must carry mechanical loads, sometimes integrating with the body (cell-to-body or cell-to-chassis), which complicates crash design; and manufacturing tolerances on large cells matter more. For Indian two-wheelers with cylindrical cells, structural cell holders achieve a similar effect.

18. Q18.A pack shows one parallel group consistently lower in voltage at end of discharge. How do you investigate?

    Experienced

    A group that drops faster at end of discharge has lower capacity or higher resistance than the others. Steps: check the BMS logs for that group's voltage over full charge and discharge cycles and its balancing history. If it is low only under load and recovers at rest, suspect high resistance: a bad weld, loose busbar, or a cell with rising impedance, so measure joint resistance and look for hot spots with thermal imaging. If it is low at rest too, suspect lower capacity or high self-discharge: possibly a disconnected cell in the parallel group (a failed weld or open fuse link), which effectively reduces that group's capacity. Disassemble only with HV precautions; measure individual cells, compare against incoming QC data, and feed findings into supplier and process corrective action.

19. Q19.What is UN 38.3 and why does it matter?

    Experienced

    UN 38.3 is the section of the UN Manual of Tests and Criteria that lithium cells and batteries must pass before they can be transported by air, sea, road or rail as dangerous goods. It has eight tests: altitude simulation, thermal cycling, vibration, shock, external short circuit, impact or crush (cells), overcharge (batteries) and forced discharge (cells). It matters because cells imported into India, packs shipped to customers or service centres, and warranty returns all need compliant documentation and packaging, and failed packs are classified differently for transport. It is a transport standard, not a vehicle safety certification, so it does not replace AIS-156 or AIS-038 Rev 2 testing.

20. Q20.How would you approach second-life use or recycling of EV batteries?

    Experienced

    When a pack falls to about 70 to 80% SoH it may still be valuable for stationary storage, such as solar buffering or backup power, where weight does not matter and C-rates are low. A second-life process needs grading: capacity and resistance testing, checking cell imbalance and history, and screening out cells with high self-discharge or swelling, then re-packaging with a new BMS. LFP is particularly suited to second life because of its cycle life and stability. Packs that are not reusable go to recycling through hydrometallurgical or pyrometallurgical processes to recover lithium, cobalt, nickel and copper. In India the Battery Waste Management Rules, 2022 place extended producer responsibility on producers, so OEMs must plan collection and recycling targets.

More free EV career resources: emobility.academy/career-resources