LiFePO4 Series Connected Battery modules

We often get the question from customers, can you connect multiple LiFePO4 batteries in series to achieve a desired working voltage outcome. This topic is not well understood by many consumers including many LiFepo4 battery suppliers who claim series connection systems are a reliable & functional practice. It is most important to understand the practical & scientific limitations associated with connecting LiFePO4 batteries in series using multiple battery blocks & here is why. 

• Firstly & most important when Lithium battery packs are constructed, the cell matching process will carefully select cells with very similar characteristics and group them together. A battery pack is then built using sets of matched cells. There will be minor differences in the sets of cells, which therefore means there are minor differences in each battery module, even from the same age and same manufacturer. When you connect multiple separate individual battery modules together, there will be differences between the battery modules vs the cells in an individual battery pack. These differences contributes to imbalances between the battery modules over time.
• For example, when connecting 4 batteries in series, you end up with 4 independent internal Battery Management Systems (BMS’s) that can each only manage the set of cells internal to each battery. The BMS in each LiFePO4 battery balances the cells and performs other tasks to ensure that the batteries are maintained correctly. Once the cells of any individual battery are fully charged, the BMS will prevent further charge going into that battery. However, since the internal BMS’s in the individual batteries do not communicate with each other, there is no feedback to the charger to determine when all the battery cells in each battery pack are fully charged. Therefore, the charger cannot detect a situation where one battery is full or overcharged vs another battery that still needs to be charged further – therefore the charger will complete the charge cycle with some battery modules still being slightly under charged, resulting in a series connected battery pack system that is no longer equally balanced. These imbalances are exacerbated over time since during each charge/discharge cycle the issue repeats and can cause greater imbalances as time goes on.
• Depending on the battery voltage required, the most effective way to get around these issues is to have battery modules with a single Battery Management System / Battery protection System that is designed to manage the internal battery cells together. The great advantage is that with all the individual cells being monitored together, the BMS/BPS can correctly manage the charge & discharge to & from each cell which will allow each individual cell to be balanced and this will guarantee the best reliability relating to the problems mentioned in the previous points. This ultimately will result in a guaranteed longevity of the battery pack solution.
• In the event a battery voltage & capacity is required & there are no suitable LiFePO4 batteries available for the application you really have no alternative but to series connect a bank of batteries. In this situation it is highly recommended you install a battery external equalizer relay system. This device does the same equalizing function as the internal BMS/BPS however, this equalizer relay system is only monitoring & equalizing all the battery blocks in a series system individually & plays a vital roll in balancing each battery block equally in the series connected system.  So to sum this up your BMS/BPS is monitoring the battery block internal cells (Layer 1) & the external battery equalizer is monitoring the battery blocks as a second layer (Layer 2). The external monitoring & balancing relay system will give the series connected battery bank the best chance of an extended life as apposed to connecting your batteries in series without the layer 2  protection.