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BMS design: requirements and features

A project log for Universal Battery Module

The goal is to create a bulletproof 18650 based battery solution applicable to a wide variety of tasks; solar, utility vehicles, EVs, boats.

Michel KuenemannMichel Kuenemann 03/19/2016 at 07:232 Comments

Regarding the Battery Management we need following functions :

At cell group level (one group = 24 cells wired in //)

  1. Voltage monitoring with UV and OV detection - 6 Volts full scale - 2 mV resolution
  2. Temperature monitoring UT and OT detection - -40°C / + 85°C (-49 °F / +185°F) scale - 0.1 °C (0.2 °F) resolution.
  3. Passive balancing - Power: To Be Defined

At stack level ( = complete stack of 14 groups in series)

  1. Bidirectional current measuring of battery current with SOC calculation - +/- 200 A - 100 mA resolution
  2. Unidirectional current measuring of charging current (MICHEL) - + 100 A - 50 mA resolution
  3. Voltage measuring - 80 Volt FS - 20 mV resolution
  4. Main CB control
  5. Inverter control (MICHEL)
  6. CAN bus Communication (MICHEL)
  7. Serial bus for lighting control (MICHEL)
  8. Long term storage of all battery parameters and important events
  9. Embedded webserver for user interaction

Consumption during active mode:

During active mode, the BMS monitors the battery parameters, logs them and insures battery safety.

During active mode, the consumption of the BMS shall be less than 50 Wh per day.

It shall be possible to disable / enable the BMS without having to disconnect wires. When disabled, the BMS is in storage mode.


Consumption in storage mode

During storage mode, the BMS does not perform any measurement / logging of the battery parameters.

When storage mode, the consumption of the BMS shall be less than 1 Wh per day. During storage mode a 4 KWh battery looses less than 10 % of its capacity when left sitting uncharged during one year (To Be Confirmed).


Safety

The main concern regarding Lithium batteries is overcharging. Overcharging can lead to degasing and even fire. Overcharging is avoided thanks to the passive balancing feature of the BMS. If this feature fails unexpectedly, one or more cell groups may get overcharged without CB tripping or warning. This would lead to a dangerous situation.

Following unexpected events may lead to battery overcharging / overheating

  1. Undetected erroneous cell group voltage measuring (voltage reading is too low).
  2. Undetected erroneous cell group temperature measuring (temp reading is too low).

In order to mitigate these risks, the design shall include redundancy and/or built-in test mechanisms.


Electrical Protections

The cell voltage measuring inputs shall withstand 60 VDC during 1 minute without degradation

The cell temperature measuring inputs shall withstand 60 VDC during 1 minute without degradation

The current measuring inputs shall withstand 60 VDC during 1 minute without degradation

The BMS main board power supply input shall withstand 60 VDC during 1 minute without degradation


Environmental

The BMS shall be fully functionnal between 0 °C and 70 °C (32°F / 160 °F)

The BMS shall be designed to function 24 hours/day - 365 days/year.

Design rules

The BMS is intended to be used by JAMES and MICHEL in their personal applications. The BMS will be used and maintained by them during several years. Following design rules apply:

NOTE: the requirements specific to Jame's application and Michel's application of this project are tagged (JAMES) / (MICHEL) respectively.

Discussions

Michel Kuenemann wrote 03/19/2016 at 15:17 point

I did not manage to download the data sheet of the MNEDC250RT. Could you please put it in the "file bag". 

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Ampeater wrote 03/19/2016 at 10:14 point

Agreed. The "remote trip" feature of the circuit breaker chosen (Midnite Solar MNEDC250RT) requires a 24vdc pulse on an auxiliary set of contacts. The stack level controller needs to be able to generate this pulse, and ideally it would log the reason for the trip. It may be preferable for the more power hungry circuitry to be powered on the load side of the breaker so a LVC, UT or OT fault reduces power consumption to the absolute minimum possible.  This would also make it easy to put a module into storage mode by manually tripping the breaker. 

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