Putting Batteries in Parallel? Better Watch Out for These Failure Modes

As the demand for increased energy storage capacity grows, engineers are frequently challenged to place multiple batteries in parallel. Using multiple batteries can offer extended runtime, enhanced reliability, and the ability to carry energy to different locations that may not have charging capabilities. With these benefits come certain complications. Battery management systems (BMS) are essential for safeguarding lithium batteries from potential abuse by monitoring their voltage, current, and temperature, and controlling the flow of power. However, when batteries are not specifically designed for parallel connections, nuisance tripping issues often arise. This article aims to explore the problems encountered by designers when placing batteries in parallel and examine potential solutions to eliminate these challenges.

Parallel batteries and swappable batteries provide unique advantages to certain applications. Several factors drive the need for additional capacity and flexibility, including the desire for enhanced reliability (e.g., N+1 redundancy systems), extended runtime, the ability to transport stored energy to different locations, and the need for speedy recharging. Some industries that use multiple batteries in their applications include telecommunications, portable hospitality carts, education, industrial operations including factories and warehouses.

All lithium batteries must have a Battery Management System (BMS) to ensure safe operation, but this same system can cause some headaches in multiple battery systems. All BMS perform the same core functions of voltage, current, and temperature monitoring. If any of these parameters exceed safe operating values, the BMS will disconnect the battery from the system. Typically, the BMS uses a contactor or MOSFET to disconnect the system from the battery. If these BMS aren’t designed to consider the transients and energy balance conditions in a parallel battery system, they may cause a nuisance by tripping off the power.

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Figure 2: Typical Safe Operating Range of Lithium Ion Cells

Nuisance tripping refers to the undesired activation of the battery management system, resulting in the disconnection of batteries from the load, other batteries or charger. Several mechanisms can lead to nuisance tripping in parallel battery configurations. Firstly, overcurrent and high capacitance can trigger the BMS to shut down the output. This can occur during start up, initial connection of the battery, or when loads are switched on and off. Secondly, when hot swapping batteries with different states of charge (SOC), the BMS may sense the high SOC battery charging the low SOC battery as a fault and initiate a trip. Lastly, inadequate load balancing among parallel batteries can lead to variations in current flow, causing the BMS to erroneously trigger a shutdown. If the impedance of the path to each battery isn’t equal the current won’t be distributed equally.

To overcome these challenges associated with nuisance tripping, engineers have devised various solutions that address the specific issues encountered in parallel battery configurations. Some potential approaches include:

·       Diode OR: Implementing diodes in the circuit can allow the batteries to share the load while preventing reverse current flow (one battery charging another). This solution ensures that the batteries discharge at the same time, and the higher SOC battery will discharge first to equal the other batteries. This does have a few disadvantages including poor current sharing between different SOC batteries, and you will have to charge the batteries using a separate current path. This is usually not the best solution.

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Figure 3: Two batteries connected by a Diode OR

·       Chipsets/Switching: Utilizing specialized chipsets or switching mechanisms enables the batteries to be connected and disconnected seamlessly without triggering the BMS to nuisance trip. These solutions can provide more precise control over current flow and control hot swapping. Multiple manufacturers produce IC’s specifically for this use case. Some examples are MAX1773A and the MCP73871. The drawbacks to these chipsets are typically a lack of flexibility for custom applications. Increasing the current, voltage, or number of batteries is typically not possible.

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Figure 4: MAX1773 Typical Application for two battery applications

·       DC-DC Converters: Introducing DC-DC converters in parallel battery configurations helps regulate voltage and current distribution among the batteries. These converters can equalize the SOC and minimize variations, thereby reducing the likelihood of nuisance tripping. This will add cost, and more points of failure within the system.

·       DC-DC Converters within Batteries: Some battery designs incorporate built-in DC-DC converters, which eliminate the need for external converters. These integrated solutions enable efficient management of individual battery voltages and currents, contributing to better load balancing and reduced nuisance tripping occurrences. This does add some cost, but move the failure points to the batteries, which will wear out and need to be replaced anyway.

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Figure 5: DC-DC converters placed in parallel to load balance batteries

·       CANBUS Communication: Implementing a CANBUS (Controller Area Network) communication system between chargers, loads and the multiple BMS facilitates real-time monitoring and coordination. This communication protocol enhances load sharing and allows the BMS to make informed decisions, minimizing nuisance tripping. This can control all components in the system including Diodes, DC-DC and switching chipsets.

More manufacturers are requiring battery swapping, or multiple battery systems to meet their customers’ needs. In order to ensure a seamless customer experience engineers must ensure the batteries can be placed in parallel, charged, and discharged without any nuisance tripping. By implementing some of these solutions in your next design you can ensure your customers have uninterrupted power when and where they need it. Some manufacturers now offer off the shelf solutions for swappable energy storage that engineers can integrate with little integration effort. The Green Cubes Swappable Industrial Battery has been designed by the leading swappable battery engineers to prevent any nuisance tripping and can be mixed and matched to fit many applications.

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