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 each battery is maintained correctly. Once the cells of any individual battery are fully charged, the BMS will prevent further charge going into that battery, and once one battery in a series connected chain stops allowing current to flow through, the current flow through the whole series connected chain of batteries will also stop.Electrically, with series connected batteries there is no mechanism how the individual batteries can “self equalise” with each other, such as what is possible (to a limited degree) in parallel connected systems through a battery at a higher voltage feeding energy to another battery at a lower voltage. Thus, any voltage imbalances that develops between the individual battery modules, will remain unless each battery is individually charged / balanced again to the same voltage. A charger that is typically then connected across the series connected bank, can only provide a charge current across the whole series connected bank, most chargers cannot detect the individual voltage and/or state of charge of the individual batteries, and the charger will behave based on the cumulative voltage of the entire series connected battery bank, being unaware of the individual battery voltages. Even if the charger had a mechanism to detect the individual battery voltages, there is no way for most single higher voltage chargers to feed a separate charge current to each individual battery. Therefore, the charger cannot detect or respond to 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 voltage imbalances can develop and are exacerbated over time since during each charge/discharge cycle the issue repeats and can cause greater imbalances as time goes on.
Solutions for series / high voltage battery requirements
Fortunately there are solutions to the above issues with series connected battery banks.
- Depending on the battery voltage required, the simples approach is to opt for battery modules that are designed to operate at the voltage that your system requires, e.g. a dedicated 36V battery for a 36V application can be selected rather than connecting 3 x 12V batteries in series. The great advantage is that with all the individual cells are being monitored together by the internal BMS, that can correctly manage the charge & discharge to & from each cell which will allow each individual cell to be balanced and this will provide the best reliability relating to the problems mentioned in the previous points. If a larger capacity is required than what an individual battery can provide, then more than one battery at the target voltage can be connected in parallel, which does not suffer from the specific problems mentioned above.
- In the event that a battery voltage & capacity is required for which there are no suitable batteries available for the application, or where you have a situation where the same batteries will be used at different times for different voltage application (e.g. you need to use the same batteries at times for a 12V application and other times for a 24V application), you really have no alternative but to series connect a bank of batteries. In this situation, there are still options to address the above issues with a series connected set of batteries:
- One option is to charge the batteries with multiple identical but separate chargers – one charger connected to each battery instead of charging with a higher voltage charger connected across the whole series connected bank. This will ensure that on each charge cycle, each battery is getting individually fully charged with the effect that the battery bank is re-balanced on each charge cycle. However, this can be inconvenient from an installatation perspective.
- Another option is to add an external battery equalizer device. This device does the same equalizing function as the internal BMS/BPS however, this equalizer is monitoring & equalizing the batteries in a series system individually. The way it works is that it has individual connection to each battery to monitor the battery voltages. When it detects a voltage difference develop between the battery modules, it will then draw power from the highest voltage battery and feeds it to the lowest voltage battery, and it continues with this until the battery voltages of the batter bank are within a narrow range, at which point the device will go back to sleeping/monitoring mode, to be re-activated if a the voltage differences increase again. This device can play a vital role in balancing each battery module equally in the series connected system. So to sum this up your BMS/BPS is monitoring the battery module internal cells (Layer 1) & the external battery equalizer is monitoring across the battery modules 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.