2.5 Current Limits

This page explains how the BMS protects the battery against too much charge or discharge current, and how to choose the limits and delays on the LiMon™ Current tab from the ratings of the cells and the power path.

Applies to

Boards: BMS1820, BMS3610, BCU2002, BMS1601, BMS1810.

2.5.1 How It Works

Every limit is compared with the pack current of 2.4 Current Measurement. Enter all limits as positive amperes; the BMS applies the discharge limits to discharge current and the charge limits to charge current.

  • Warning: the current stays at or above the warning limit for 1 s. The BMS reports a WARNING event; no path is switched off. The warning clears as soon as the current is below the warning limit.
  • Discharge 1st stage (slow): the discharge current stays at or above DCHG O.C. Protection for DCHG O.C.P Delay. The discharge path switches off. Keep the warning limit at or below the protection limit.
  • Discharge 2nd stage (fast): the discharge current stays at or above DCHG O.C. Protection (2) for DCHG O.C. Delay (2). The discharge path switches off. This stage has no warning.
  • Charge: the same as the discharge 1st stage, with CHG O.C. Warning, CHG O.C. Protection and CHG O.C.P. Delay. The charge path switches off.
  • Peak: the discharge current reaches Max. Allowed Peak Current (event Short Circuit Protection). The BMS reacts practically at once and switches off both paths.

Table.2.5.1: Current Events

Event Error bit Class Path switched off Releases after
Discharge Over Current Warning 13 WARNING none current below the warning limit
Discharge Over Current Protection 14 PROTECT discharge 15 s
Discharge Over Current 2nd Protection 15 PROTECT discharge 15 s
Charge Over Current Warning 16 WARNING none current below the warning limit
Charge Over Current Protection 17 PROTECT charge 10 s
Short Circuit Protection 18 PROTECT charge and discharge 60 s

A PROTECT event releases by itself after the time in the table, and the path switches on again. Repeated trips are counted against Error Recovery Attempts; 3.1 Protection and Recovery explains what happens when the count runs out.

Fig.2.5.1: Trip and Recovery of a PROTECT Event

1 Over the limit current at or above a protection limit 2 Delay still above when the delay has passed? 3 Trip path OFF, trip count + 1 4 Wait release time: discharge 15 s, charge 10 s, short circuit 60 s 5 Retry? trip count below Error Recovery Attempts? yes no: the delay timer starts again Path ON again a new trip counts again Path stays OFF User Attention Required, until the BMS restarts yes no DCHG O.C.P Delay DCHG O.C. Delay (2) CHG O.C.P. Delay Error Recovery Attempts Warnings switch nothing off; they clear as soon as the current is below the warning limit. Error Recovery Attempts = 0: trips are not counted and the BMS switches the path on again without end.

The BMS does not replace a fuse

The current limits are a BMS function that acts on the measured current. A real short circuit can rise faster than the BMS can react. Protect the power path with a fuse rated for the cells and the cables.

2.5.2 Settings

Table.2.5.2: Current Limit Settings (LiMon™ Current and Peripheral tabs)

Setting (LiMon™ tab › name) What it does How to choose Unit Range Default
Current › DCHG O.C. Warning Discharge warning after 1 s. A little below DCHG O.C. Protection. A 0…5000 20
Current › DCHG O.C. Protection Slow discharge limit. Continuous discharge rating of the cells and the power path. A 0…5000 25
Current › DCHG O.C.P Delay Time above the slow limit before it trips. How long the cells may carry currents up to the 2nd limit. ms 0…60000 5000
Current › DCHG O.C. Protection (2) Fast discharge limit. Short-time (pulse) discharge rating. A 0…5000 50
Current › DCHG O.C. Delay (2) Time above the fast limit before it trips. Shorter than DCHG O.C.P Delay. ms 0…60000 3000
Current › CHG O.C. Warning Charge warning after 1 s. A little below CHG O.C. Protection. A 0…5000 10
Current › CHG O.C. Protection Charge limit. Charge rating of the cells; above the highest current of the charger. A 0…5000 15
Current › CHG O.C.P. Delay Time above the charge limit before it trips. Long enough for normal charger start-up peaks. ms 0…60000 5000
Current › Max. Allowed Peak Current Highest discharge current; switches off both paths practically at once. Above DCHG O.C. Protection (2), below the current the sensor can measure. A 0…5000 200
Peripheral › Error Recovery Attempts Trips before a protection stays off. Default 5. 0 retries without end, which wears the contactors. — 0…30 5

The LiMon™ reference describes every field of the screen: 7.4 Current Settings and 7.5 Peripheral Settings.

2.5.3 Choosing the Values

  • Order: warning ≤ 1st stage < 2nd stage < peak, and DCHG O.C.P Delay > DCHG O.C. Delay (2).
  • Weakest part: use the lowest rating among cells, contactors, cables, busbars and the fuse.
  • Sensor range: the BMS cannot read more current than the channel in use can measure (2.4). A limit above that range never trips. With a LEM DHAB S/133 the high channel reads up to ±750 A.
  • Loads with a start-up peak (motors, inverters): set the 2nd stage and its delay so that the normal start-up peak passes. The peak limit stays above any normal current.

Calculation example: 16S LFP, 100 Ah cells, LEM DHAB S/133

These are chosen values to show the method; use the datasheets of your own cells and parts.

Cell rating: 1C (100 A) continuous discharge, 2C (200 A) for 10 s, 0.5C (50 A) charge.

Table.2.5.3: Example Settings

Setting Value Reason
DCHG O.C. Warning 90 A early notice below the continuous rating
DCHG O.C. Protection 100 A 1C continuous
DCHG O.C.P Delay 10000 ms currents up to 2C for at most 10 s
DCHG O.C. Protection (2) 200 A 2C
DCHG O.C. Delay (2) 1000 ms above 2C only briefly
CHG O.C. Warning 45 A early notice
CHG O.C. Protection 50 A 0.5C charge
CHG O.C.P. Delay 5000 ms charger start-up
Max. Allowed Peak Current 400 A above the 2nd stage, well inside the sensor range

Fig.2.5.2: Discharge Trip Map of the Calculation Example

0 100 200 300 400 500 0 1 5 10 Discharge current (A) Time until the path switches off (s) no action warning after 1 s (90 A) 1st stage: 100 A, 10 s 2nd stage: 200 A, 1 s peak: 400 A, practically at once outside the example cell rating DCHG O.C. Warning DCHG O.C. Protection DCHG O.C.P Delay DCHG O.C. Protection (2) DCHG O.C. Delay (2) Max. Allowed Peak Current BMS trip time with the example settings beyond 1C continuous / 2C for 10 s warning band

2.5.4 Checking the Result

  • Under a known load the current in Live View matches a reference meter (2.4), so the limits act at the right current.
  • The warning appears in the events when the current passes the warning limit for more than 1 s.
  • After a protection, the path switches on again after the release time in Table.2.5.1.

2.5.5 Common Mistakes

  • Negative limits: enter all limits as positive amperes.
  • Warning above the protection limit: the protection then acts at the warning limit instead of its own limit.
  • Peak limit above the sensor range: it never trips.
  • 2nd stage delay longer than the 1st stage delay: the fast stage trips later than the slow one.
  • Charge limit below the charger current: the charge path switches off again and again until the retry count runs out.
  • Error Recovery Attempts 0: the BMS switches a faulty load on again without end.