Lithium Battery OEM ODM: Custom Voltage, Capacity and BMS
Lithium Battery OEM ODM: Custom Voltage, Capacity and BMS
When you brief a lithium battery manufacturer for a custom pack, three parameters decide whether the result is a product or a problem: voltage, capacity and the battery management system. Get these right and the pack drops into your device; get them wrong and you ship a field-failure line item. This guide is for OEMs and ODM buyers who need to specify a pack without leaving the factory room to guess.
What “custom voltage” really means
Pack voltage is set by how many cells you stack in series. A 48 V LFP pack is 16 cells at 3.2 V; a 36 V NMC pack is 10 cells at 3.6 V. The BMS must match the series count exactly, and the charge cutoff must match the chemistry—51.2 V for a fully charged 16S LFP, ~54.6 V for a 15S NMC. A common mistake is specifying “48 V” and assuming any 48 V BMS fits; the cell count and cutoff differ by chemistry, and a mismatch faults the inverter or overcharges the cells. State the series configuration, not just the label, and the ambiguity disappears.
Capacity: Ah vs Wh, and how to specify it
Ampere-hours describe charge; watt-hours describe usable energy, and the distinction decides your real runtime. A 100 Ah LFP cell at 3.2 V holds 320 Wh; a 90 Ah NMC at 3.7 V holds 333 Wh. Specify capacity at the C-rate you actually draw, because a cell rated at a gentle 0.2C looks bigger than the same cell at 2C. Always state “100 Ah at 1C, 80% DoD” rather than a bare “100 Ah,” or the factory will quote the friendliest number. For a battery application solutions program, normalize every quote to Wh at the duty cycle so you compare like with like.
The BMS: topology, comms and protection
The BMS is the pack’s brain. At minimum it must provide over/under-voltage, over-current, short-circuit and over-temperature protection plus cell balancing. Topology matters: a centralized BMS suits small packs, while a distributed (slave-per-module) design scales to large banks. Communication is where OEMs stumble—specify CAN J1939 for vehicles, RS485/Modbus for stationary, SMBus for portable computing. A BMS that cannot report state-of-health over the bus is a black box you will regret during a field failure. Insist the supplier runs its own BMS firmware team so the logic can be tuned to your load, not a generic preset.
Matching the BMS to chemistry
LFP and NMC need different charge cutoffs, balancing strategies and thermal models, so the BMS is not interchangeable between them. LFP’s flat voltage curve demands coulomb-counting for accurate state-of-charge, while NMC’s sloping curve tolerates voltage-based estimation but needs tighter thermal margins. A home energy storage system built on LFP wants a BMS tuned for 6,000 shallow cycles; a drone pack on NMC wants one tuned for high-C bursts. Tell the supplier the chemistry first, then the numbers; reversing the order invites a mismatched BMS.
Common spec mistakes
| Mistake | Consequence | Fix |
|---|---|---|
| “48 V” without cell count | BMS/charger mismatch | State series config + cutoff |
| Capacity without C-rate | Overstated runtime | Specify Ah at draw C-rate |
| Implicit communication bus | Host can’t read SoC | Name CAN/RS485/SMBus |
| No thermal model for NMC | Thermal event risk | Require redundant sensing |
| Design to dimensional limit | No service clearance | Leave 5–10 mm margin |
The RFQ block that prevents loops
Send suppliers one consistent block: “Application: ___; Chemistry: ___; Series config: ___S; Voltage (nominal/cutoff): ___; Capacity: ___ Ah at ___C; Continuous/peak current: ___/___ A; Comms: ___; IP: ___; Target certs: ___; EAU: ___ units.” Suppliers answering precisely are the ones worth a second call. A vague RFQ is the number-one cause of sampling loops, and every loop is a week of calendar you cannot bill back. Treat the RFQ as a contract artifact, not a casual email.
OEM vs ODM revisited
In this context, “OEM” usually means you own the electrical and mechanical design and the factory builds to print; “ODM” means the factory owns reference designs you private-label. For a custom voltage/capacity/BMS pack, a true ODM adds value by owning cell grading, BMS tuning and enclosure tooling—but it also owns the IP, so clarify who carries the certification burden and who must re-qualify when a cell model changes. A distributor wanting speed leans ODM; an OEM with a patented device insists on OEM-to-print to protect its design.
Roadmap: semi-solid and sodium options
Two chemistries are worth a line in your spec. A semi-solid-state battery could let a weight-sensitive product run longer per charge with better thermal stability than NMC, though at a premium. A sodium-ion battery may cut cell cost for large stationary banks where weight is irrelevant. Neither is a drop-in today, but specifying with an eye to them—and asking your supplier for a re-cell path—prevents a costly redesign when the economics cross over. Treat chemistry flexibility as a design requirement, not a footnote.
Prototype-to-production recap
Custom packs move through requirement locking, EVT, DVT, pilot and mass production. For a bespoke voltage/capacity/BMS build, the pilot run is non-negotiable—20–50 units through your real duty cycle surfaces integration bugs at 1% of the risk of a full PO. Hold a first-article inspection on initial production and apply statistical process control on weld resistance, capacity and balance. The discipline is what separates a smooth ramp from a recall, and it is cheap insurance on a custom design.
Sizing the pack to the application: a worked example
Suppose a mobile lighting tower needs 1 kW for 8 hours = 8 kWh usable. At 80% DoD that is 10 kWh nameplate. At 48 V that is roughly 208 Ah; at 51.2 V fully charged, about 195 Ah. The continuous current is 1000 W / 48 V ≈ 21 A (0.1 C), trivial for LFP, but the inrush when the ballast strikes may spike to 3–4×, so the BMS and connector must be rated for the peak, not the average. Specify both numbers or the pack will trip at first strike. A battery application solutions engineer will size the peak automatically; a vague spec leaves it to chance, and chance fails in the field. Always state average and peak separately—they are different designs.
Thermal and enclosure design
Lithium prefers 15–35 °C, and an enclosure in a hot engine bay or a sun-baked cabinet ages faster than its spec. Match the enclosure material to duty: flame-retardant ABS for light duty, powder-coated steel for industrial, die-cast aluminium where you need heat sinking. Pair material with the right IP rating—IP54 keeps dust and splashes out; IP67 survives temporary immersion. Design the thermal path before the cosmetic shell; a pack that looks rugged but has no heat escape will quietly halve its cycle life. Ask the supplier to document the thermal simulation, not just claim a rating.
Certification planning by market
Plan certifications from your destination, not the factory’s default. North America leans on UL 1642 (cells) and UL 1973 (systems); the EU wants CE plus the relevant EN standards and RoHS; IEC 62619 governs stationary industrial cells and IEC 62133 portable/medical. UN38.3 is mandatory for transport everywhere. Request the actual test reports naming your exact model, and build a compliance dossier you can fork per lane. A home energy storage system sold into the EU needs the CE/EN path; the same pack into North America needs UL—designing for both from the start is cheaper than re-qualifying later. Keep one master dossier and version it.
Validation and pilot discipline
For a custom voltage/capacity/BMS pack, the pilot run is non-negotiable. Build 20–50 units and run them through your real duty cycle, not a bench spec, then hold a first-article inspection on initial production and apply statistical process control on weld resistance, capacity and balance. A pilot surfaces integration bugs at 1% of the risk of a full PO, and it is where you confirm the BMS actually reports state-of-health to your host. Skipping it to hit a launch date is the classic cause of “it worked in the lab but not in the product.” Treat the pilot as a gate, not a formality, and document the pass criteria before you build.
Frequently asked questions
Can I just say “48 V” and let the factory decide? No—state the cell count and cutoff, or you risk a BMS/charger mismatch that faults in the field.
Why does C-rate matter for capacity? A cell’s stated Ah is only valid at the test C-rate; draw harder and delivered capacity drops.
Do I need my own BMS firmware? Not necessarily, but the supplier must be able to tune theirs to your load; a fixed generic preset rarely fits.
How do I keep the design exclusive? OEM-to-print plus a mutual NDA and non-resale clause; verify reference designs aren’t sold to a rival.
Written by Karl at China Battery Technology. Send us your voltage, capacity and BMS requirement and we will return a tuned design—request a quote for a reviewed proposal within 24 hours.
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