Active Management: Expect More From Your BMS

In today's age of lithium batteries, battery management is a familiar concept. Every battery has some form of internal or external battery management system (BMS), which plays a crucial role in ensuring the safety and performance of lithium batteries. At its core, the BMS acts as a protective mechanism, safeguarding the battery cells from external influences such as excess voltage, current, and temperature. However, the functions of a BMS can go well beyond simple protection.

A BMS includes internal sensors that measure a wide variety of parameters; in the case of a communicating BMS, this information is made available to external systems, typically an inverter or charger. This intra-system communication allows us to optimize our loads and charge currents based on the battery's needs for maximum longevity and efficiency. (For more background on the difference between open-loop, closed-loop, communicating, and non-communicating lithium batteries, read this article).

The next evolution of the internal BMS 

Pylontech stands out among communicating BMSs, as they have added another layer to the BMS concept: active management. Pylontech batteries with active management reach beyond the traditional protection role and can control the current entering and exiting each battery. In a parallel configuration, one battery is designated the Master battery, responsible for observing and managing the other batteries, referred to as the Follower batteries. The Master battery takes all the information from the Followers and determines the appropriate current flow into or out of each battery. This active management is crucial because batteries connected in parallel will balance themselves automatically, sometimes violently, and to the system's detriment.

Figure 1. State of charge (SOC) versus open-circuit voltage for a lithium-iron phosphate battery cell.  

The voltage curve of a lithium battery shows that it rises quickly at the start, levels off, and then spikes up again as it approaches a full state of charge (SOC). The voltage remains relatively stable when the battery is in the middle 80% of the SOC range. However, voltage rises and falls significantly in the outer 10% segments. How does this result in a potentially dangerous situation? The automatic balancing that occurs between parallel connected battery modules is voltage-dependent. In this case, think of voltage as a difference in pressure between two buckets that are connected at the bottom by a hose. Slight differences in voltage will result in small balancing currents; significant differences could result in large currents. In the voltage curve shown above, we can see that the difference in voltage between a battery at 70% SOC and one at 40% is relatively small. In an uncontrolled circumstance, this would result in a small trickle of current between the batteries, bringing them into balance. However, a major issue arises when the difference is between a 5% SOC battery and one at 20% (or a battery at 75% and one nearing 90%). Now, the difference in voltage "pressure" is far more significant, and the current rush when the two are connected could be far more than the BMS, wires, etc., can handle.

This balance issue is why most battery manufacturers advise against adding a new battery to an older, well-used bank. As lithium batteries age, they lose capacity. The generally held principle is to retire a battery once it reaches 80% of its original capacity. This means that if a battery with a clean bill of health is connected in parallel with a battery that is only halfway through its lifespan, retaining 90% of its original capacity, we have the potential - on the low end of the discharge cycle - to encounter a significant difference of voltage, should the older battery reach  5% capacity while the newer one remains above 15% due to its higher initial capacity.

Pylontech's BMS prevents this from ever being a problem by actively managing the current flow in each battery, ensuring that all batteries in a parallel string reach the same SOC simultaneously, regardless of their relative capacity or starting point. This capability is what allows Pylontech batteries of different ages and capacities to be combined in a single string without a problem. For example, a new 4.8 kWh US5000 Pylontech battery can be combined with an older 3.5 kWh US3000 battery, and the active management system will bring each battery to full charge and discharge without imbalances. This function is illustrated in the following charts, where two Pylontech batteries are brought smoothly into balance on both the charge and discharge cycles. (Read more about adding capacity to a Pylontech lithium battery bank here).

On the surface, this is a unique feature that allows homeowners with budget constraints to increase their capacity as resources become available gradually.  However, the real advantage becomes evident on a commercial scale. For a large system specifically designed to meet an industrial need, the failure of a single battery module could result in a permanent reduction in capacity without the advantages of active management. If 1 out of 60 batteries goes offline 2 years into a system's lifespan, most battery options would require replacing all 59 remaining batteries along with the faulty unit. With a Pylontech installation, this would be a simple matter of replacing the single unit without interrupting the operation of the larger system.

Another common scenario where active management is beneficial is in adverse conditions. If one battery in a string is exposed to colder temperatures (such as nearer an uninsulated exterior wall exposed to sub-zero temperatures than the rest of the battery bank) and reaches a point where it cannot accept or discharge a charge, this battery will isolate while the rest of the bank can continue to operate normally. When the temperature rises, and the affected battery becomes operational again, it will not be at the same SOC as the rest of the bank because it has been offline for a day or two. Pylontech's BMS can control the charge and discharge currents of the reactivated battery, bringing it back in line with the others.

For many manufacturers, battery balancing is a major concern (you can see this in a manual's discussion on cable lengths and balancing procedures). It's not uncommon for an entire bank to shut down when one battery isolates itself due to extreme temperatures or to prevent an imbalance. In contrast, a Pylontech battery can limit current flow and bring itself back into spec with the rest of the bank once it comes back online.

This level of active management within a power system adds value to smaller systems but is certainly magnified as a system's size increases. In large commercial installations where downtime is unacceptable and comes at a high organizational cost, this feature becomes invaluable. With the added ability to replace damaged or aging units with new batteries, commercial and industrial clients can rest easy knowing that single-point failures will not compromise their operations or add layers of hidden costs to their investment when systems don't work as they should. 

The hidden cost of a non-communicating battery bank

What are the pitfalls of using a non-communicating battery in a power system?

Let's consider a recent, real-world example: 

A Victron system consists of a large Quattro 48/10000 120V Inverter/Charger, Cerbo GX + GX Touch, a SmartSolar 250/100 MPPT solar charge controller, a SmartShunt, and a bank of 3 non-communicating 48V rack-mount batteries. One of the batteries, due to some small internal balancing issue between cells, suddenly shuts down all charge/discharge. The two remaining batteries, which were comfortably within their capacity for charge/discharge as a string of three, are suddenly receiving all of the charge from the MPPT and the Quattro attached to a generator. They operate fine for a few minutes, no change is really seen anywhere else in the system. As heat builds due to the high amperage passing into each of the remaining batteries, one of them reaches a critical point and also shuts down its charge circuit, leaving the last battery to suddenly be blasted with twice the charge current it can handle; it also shuts down. Suddenly the system, which had been seemingly operating optimally only a few minutes before, has nowhere to direct the 100A of charge coming from the charge controller - the system doesn't know the batteries are about to shut down entirely and is unable to react quickly enough to shut off the charge from the MPPT before it short circuits the DC bus of the Quattro 48/10000 Inverter/Charger, resulting in a great deal of smoke and the lingering smell of burnt componentry. As a result of this short circuit, the Quattro needs to be sent for advanced repairs, which costs a few hundred dollars and takes nearly two weeks for it to be shipped, repaired, and returned. In the meantime, it requires extensive trial and error and testing of multiple components to determine the root cause of the original issue and its starting point. Ultimately, the Quattro needs to be uninstalled, leaving the power system non-operational for over two weeks and relying solely on the generator. 

How would this scenario have played out when using the same capacity of Pylontech US5000s, a fully communicating battery? If one battery starts experiencing balancing issues between cells internally, that information is passed along to the Master battery. In response, the battery with cell balancing issues is allowed to lower its charge rate to a trickle while the cells balance out. The Master battery makes a decision about how much charge amperage it now needs to charge optimally and passes that information to the Victron Cerbo GX device. The Cerbo GX decides that it no longer needs the charge coming from the generator via the Quattro 48/10000 and uses its Generator Start/Stop relay feature to shut down the generator and use only the charge from the SmartSolar MPPT 250/100. Charging continues for several hours while the unbalanced battery makes adjustments to its internal balance. Meanwhile, the other batteries continue to charge at an optimal rate. After several hours, the battery with balance issues is now ready for normal operation; the remaining two batteries have neared 95% SOC and need far less amperage, so they limit their charging, and all the excess charge from the MPPT is directed to the newly balanced battery which is at a lower 65% SOC. That battery charges quickly while the remaining two batteries continue to fill up with a trickle of charge until all three batteries sync up at 99% SOC. The system owner sees only a small note in his VRM data about a high/low cell voltage difference that occurred while they were away at work for the day - upon returning home, the system is balanced, and all is well. 

Even if there had been a much more significant issue with the battery and a shutdown was necessary, the Cerbo would have been informed of a loss of communication with the battery bank, and all charge would have been immediately shut down, likely saving many hundreds of dollars and preventing damage to the rest of the system. The issue with the batteries could be identified, and a solution could be determined without damaging further equipment.

Conclusion

Pylontech's active management system is a significant advancement in lithium battery technology. It allows for precise and automatic control of current flow, enables the combination of batteries of differing ages and capacities, and ensures balanced operation even in adverse conditions. This is where a Pylontech BMS's ability to actively manage the system internally is the most valuable advancement in lithium technology that we have seen in today's market. Can your BMS do this? If not, we'd love to connect with you and discuss how Pylontech batteries could help take your project to the next level. Send a message here or reach out directly to mike@icmontana.com.