BMS telemetry: the battery passport that knows the real condition.
A battery passport with factory data alone is, after two years of operation, a snapshot from yesterday. This guide explains what BMS telemetry is, how SoH and SoC differ, which condition data the EU Battery Regulation wants to see in the passport - and how the API connection works in practice.
- BMS telemetry transmits the battery management system's condition data (SoH, SoC, capacity, power, cycles) machine-to-machine to other systems - such as the digital battery passport.
- SoC is the fuel gauge, SoH the ageing certificate: SoC says how full the battery is right now; SoH how much of its original capacity permanently remains.
- The EU Battery Regulation makes condition data mandatory: Annex XIII item 4 lists parameters on state and lifetime, Art. 14 obliges access to BMS data for determining SoH.
- Live streaming is not required - but currency is: a lean ingest API that reports readings to the passport regularly is the practical route instead of manual maintenance.
What is a BMS - and what is BMS telemetry?
Every modern lithium battery has a battery management system (BMS): electronics that monitor cell voltages, currents and temperatures, balance the cells and protect the battery against deep discharge, overcharging and overheating. In passing, the BMS is the best witness to the battery's condition - it continuously measures what happens to it.
BMS telemetry means: these readings do not stay inside the battery but are reported outward through an interface - to an energy management system, a fleet platform or the digital product passport. Typical paths are CAN bus or Modbus from the BMS to a gateway and from there HTTPS/JSON to the cloud. For the battery passport only the last step matters: a machine-readable reading, cleanly assigned to a passport.
SoH, SoC & co.: the condition values at a glance
The two most important metrics are constantly confused: the State of Charge (SoC) is the current charge level - the fuel gauge. The State of Health (SoH) is the ageing certificate: the ratio of the capacity still usable today to the rated capacity from the factory. A battery can be 100% charged (SoC) and still be at 78% health (SoH) - then its "full" charge only delivers 78% of the original amount of energy.
| Parameter | Unit | Why it belongs in the passport |
|---|---|---|
| State of Health (SoH) | % | Remaining capacity relative to rated capacity - the single most important ageing indicator. 100% = as-new; below ~70-80% the second-life or recycling window opens for many applications. |
| State of Charge (SoC) | % | Current charge level. For the passport it mainly matters as context for a measurement (SoH readings depend on SoC), not as a continuous value. |
| Remaining capacity | kWh / Ah | Absolute usable energy. The trend over time (fade curve) shows how fast the battery is actually ageing. |
| Power | kW | Available charge/discharge power. Power fade is the second ageing path alongside capacity fade - especially in high-current applications. |
| Cycle count | count | Full-cycle equivalents since commissioning - the reference figure for warranty terms and expected remaining life. |
| Temperature window | °C min/max | Extreme temperatures of the period. Heat and frost events explain accelerated ageing and are valuable context for assessors. |
| Negative events | event | Deep discharge, overheating, accident - Annex XIII requires information on events that materially affect the battery's condition. |
Why the battery passport needs condition data - and what the EU really requires
The EU Battery Regulation 2023/1542 does not treat the passport as a static data sheet. Annex XIII item 4 requires information on battery state and lifetime - including parameters such as capacity and power over time as well as negative events (e.g. accidents or deep discharges). Art. 14 additionally obliges manufacturers to give parties with a legitimate interest access to the BMS data from which SoH can be determined.
Important for context: a live stream is not mandated. But a passport whose condition data is never updated misses the purpose of the regulation - recyclers, second-life operators and buyers of used batteries should be able to read from the passport what the battery can still do today. These are exactly the groups who decide on reuse, repurposing or recycling based on SoH; for fleet operators and insurers the documented condition history becomes a value lever at resale.
How the BMS telemetry API works at Batteriepasswerk
Batteriepasswerk accepts condition data through a lean ingest API - built for BMS gateways, fleet backends and energy management systems, not for people with Excel sheets. One record per measurement, authenticated by a company-owned API key:
In the dashboard under Integrations → BMS telemetry, create a key (bpk_…). It is shown exactly once; only a SHA-256 hash is stored - just like a password.
The BMS, a fleet gateway or the energy management system sends a small JSON record per measurement over HTTPS to the ingest endpoint - identified by serial number or passport ID.
The key determines the company, the serial number the passport - readings can never land in the wrong tenant or on the wrong passport. Invalid values are rejected instead of silently swallowed.
Each measurement appears as a data point on the passport: current SoH/SoC at a glance, and the capacity fade chart shows ageing over years - verifiable for every authorised party.
Security is not an afterthought here: keys are stored only as a SHA-256 hash, every request runs against rate limits per IP and per key, and readings outside plausible ranges are rejected. Telemetry thus fits into the same integrity architecture that protects all other passport data. The BMS telemetry API is included in the Pro and Enterprise plans.
Frequently asked questions
What is BMS telemetry?
What is the difference between SoC and SoH?
Does the EU Battery Regulation require live data in the passport?
Who needs a battery's condition data?
How does BMS data reach the Batteriepasswerk battery passport?
Which plan includes the BMS telemetry API?
How often should a battery send telemetry?
Sources & further reading
From factory data sheet to a living battery passport.
In the Pro plan your BMS reports condition data straight to the passport - connected in minutes via an API key, with SoH history and fade chart on each passport.