Question: Is it important to have a BMS or SBM installed on the battery pack?

Answer: Simply said: Yes!

It is important to install and to use a protection of the lithium batteries.  The batteries need to be protected against deep discharge and the overcharge of the individual cells.  In general there are many ways to protect the cells or packs of the cells against improper use.  Whatever system is used, the main function of the system is to disconnect the battery from the load to prevent over discharge, or disconnect the battery pack from the charger to avoid over charge. The primary reason for the protection is the relatively high cost of the lithium cells and quite easy way to damage their performance or destroy them completely. In view of the expected lifetime (up to 30 years) and number of cycles (thousands) it is worthy to have a measure of protection installed.

Installing a complete Battery Management System (BMS) is the most advanced way of protecting the cells and the battery pack. At this moment there are about 50+ different BMS systems on the worldwide market. Check these links to the BMS selector and BMS option selector. (There are many other systems not listed on this page, and new BMS systems are being developed and produced by more and more companies.) The BMS monitors the voltage of each cell and based on the voltage of each cell, the BMS sends information to the charger and the discharger to protect the battery.

GWL/Power does not endorse any specific BMS model against others. We let the decision to be made by each customer. Based on our long-term commitment to the customer support, we provide an additional support for the RT-BMS system and the BMS123 system.

The cell protection can be done in a simpler way using the Simple Battery Management Board (SBM) or monitoring equipment like CellLog or Akumon (for 12V packs). To allow for an extra safety, multiple „layers“ of protection are suggested to be installed, especially for application critical installations.

** The overview of related links:

The Simple Battery Management Board (SBM) - a single board solution protect the battery pack
http://gwl-power.tumblr.com/tagged/SBM

The Real-time BMS (RT-BMS) system – a complete BMS with multiple functions and ballancing currents up to 10 Amp
http://gwl-power.tumblr.com/tagged/RTBMS

The BMS123 system – an innovative BMS solution with simple installation directly at the cells. (Commercially availible from Sept. 2012, testing units available now)
http://gwl-power.tumblr.com/tagged/BMS123

The Cell Ballancing Module (CBM) to allow the balancing of single cells
http://gwl-power.tumblr.com/tagged/CBM

The CellLog and PowerLog tools to monitor the cells in small installations
http://gwl-power.tumblr.com/tagged/CellLog
http://gwl-power.tumblr.com/tagged/PowerLog

The Akumon – battery monitoring and balancing board for 12V packs.
http://gwl-power.tumblr.com/tagged/Akumon

** Check following FAQ published previously:

• Why is the BMS needed for a LFP battery pack consisting of many cells?
• Safe steps to use the LP/LFP batteries
• Battery protection against discharge

An Example of a multiple CBM installation

A multiple CBM installation to in crease the ballancing currents.

How does the CBM work if the charging current is higher than the balancing current?

The balancing current of the CBU module is about 1.7A. If the charging current is higher than the balancing current, the voltage will continue to increase gradually.  If the charging current is lower than the balancing current, the voltage will decrease slowly.

The diagram shows that the balancing of the CBU starts at 3.67V and the voltage is reduced to 3.64V and does not keep increasing as fast as without the balancing. However the charging current is 2A while the balancing current is 1.7A so the voltage starts increasing slowly again. This is a proper way the CBM operate when the balancing the cells.

Installing battery packs

Photos of examples of two battery packs:  48V/20Ah consisting of 16 3.2V cells in 16S configuration.  The other pack is 24V/400Ah, consiting of 16 cells in 2P8S configuration. This pack has CBM boards installed to allow for ballancing of the cells during the final part of the charging stage.

Cell Ballancing Module (CBM) - example of installation

An examples of CBM installation. The cable hooks are soldered on to the PCB directly.

The diagram showing how the CBM unit balances the voltage of a cell when voltage level has reached over 3.6V.  Check CBM price here.

The application of the Cell Ballancing Module

16 CBM units are used to balance the final stage of the charging of the EV scooter GWL/Scooty. The 16 lithium cells make the total of 48V (nominal voltage). Each cell using one CBM for balancing.

The demonstration of function of the CBM boards

The CBM boards (Cell Ballancing Module) will start balancing of the cell as soon as the full charge level is reached. The full charge level is is at 3.60V for LiFePO4 cells.

The diagram shows the charging of a 4-cell battery pack with following configuration:

Cell1 - no CBM board installed
Cell2 - one CBM board installed (balancing up to 1.7A)
Cell3 - two CBM boards installed (balancing up to 3.4A)
Cell4 - three CBM boards installed (balancing up to 5.1A)

The first part of the diagram shows the charging with 1A current. The Cell1 (with no CBM) goes over the 3.8V level and the voltage would increase very rapidly. Cells2, 3, 4 are ballanced with the CBM modules and the voltage is kept bellow 3.6V.

The second part of the diagram shows the charging with 3A current. The Cell1 (with no CBM) goes over the 4.4V level and is OVERCHARGED. The Cells 2 (with one CBM) is balanced and the voltage increases slowly. However ONE CBM (balancing up to 1.7A) is not enough for the 3A charging current. As a result the voltage would also increase above 3.8V, if not stopped. Only Cell3 and Cell4 (with 2 and 3 CBM boards) are ballanced sufficiantly and the voltage is kept bellow 3.6V.

This demonstration shows that the CBM boards can balance the LiFePO4 celss easily, however for higher currents, more CBM boards needs to be connected together to increase the total balancing current.

Tip: you can check the CBM pricing here.   Support: You can download the test data source here

Testing the CBM performace

We tested the CBM boards before releasing them to our customers. You can see the voltage level of the 3 cells (on the left side) kept bellow 3.6V, while the right cell is not ballanced and the voltage increases above 3.9V.

CBM - Cell Balancing Module - detailed photo

Cell Balancing Module (CBM) - easy installation

The photo shows a four-cell testing pack with CBM installed. On the right there is a single CBM - balancing up to 1.7A. On the left, there is a cascade of 3 CBMs that increase the balacing current to up to 5A (3x ~1.7A).

Cell Balancing Solved! - The CBM1 module is here

GWL/Power is introducing the easiest solution to keep your LiFePO4 cells balanced. The CBM module provides the cell balancing whenever the voltage increases over the 3.60V cell balance detection voltage.

Product specifications
Cell Balancing Module to limit the overcharge of single LiFePO4 cell (3.2V)
Balancing detection voltage: 3.60 V (+/- 0.05V)
Balancing release voltage: 3.50 V (+/- 0.05V)
Maximal voltage: <5 V
Balacing current: 0 ~ 1.7 A
Self consumption: < 20 microA
Working temperature: -40 to +125 deg. C
Temperature of resistors: up to 85 deg. C
Dimensions: 55 x 55 x 8 mm

Cell Balancing Module (3.60V- 1.7A) product information