Batterie-Capacity SOC / Battery-Health SOH

02 May 2024

A BACS Specialty, and its relation to the requirements of the European Parliament EU BattG 2024 –
“Energy Throughput”

When BACS came onto the market 20 years ago, it quickly attracted great interest from customers with highly critical infrastructures. Within a decade, BACS developed into a key technology for stationary batteries in particularly critical areas and is now advertised by many international corporations as an “unofficial” standard for their systems. BACS has adapted significantly to the requirements of these highly critical customers and is now the undisputed market leader in the EU and number 2 in the USA in this sensitive market segment.

BACS has active battery management with a balancing function, a technology that must be used in lithium battery technology today to keep these cells stable. With the introduction of this technology on lead- and NiCd-based cells, similar positive effects were also observed after a few years. Balancing (or “equalizing”) also ensures the stability of lead/acid, NiCd and other types of chemistry whose cells are connected in series to generate high voltages. Just like with lithium-based cells, BACS ensures the “health” of the cells - SOH (State-of-Health) and determines the charge level SOC (State-of-Charge).

For our customers experienced in lithium technology: SOP (State of Power) is not measured by BACS. This value is only important for lithium cells because of their special susceptibility to damage in deep discharge and overload situations. Lead and NiCd batteries do not suffer any permanent damage from these situations if they are only used for a short time. In addition, the use of lead and NiCds is typically focused on emergency power situations so that these areas that are dangerous for lithium are never reached anyway.

The interpretation of the measured values ​​of a battery system is significantly improved by balancing, and it is also a critical qualifier for any measurement having to do with impedance: Balancing keeps all cells/batteries closely within the “healthy” voltage window and thus allows a highly precise impedance measurement. Other BMS systems can also carry out impedance measurements without balancing - BUT: Without balancing, impedance measurements from other BMS systems cannot be compared because different voltages were measured per block/cell!
With BACS, the impedances are always measured at exactly the same voltage - and are therefore comparable for the first time! A battery with different impedance now clearly stands out from the crowd of others and clearly shows the user where the problem lies.

The result is that BACS can verifiably improve both the reliability and longevity of almost any battery-based UPS concept, including all types of battery chemistry on the market today!
Our reference list of BACS users now reads like the “Who’s Who” of multiple industries and market segments. BACS is a game changer and the first choice of datacenters and critical civil and military infrastructure in the western world!
Since 2021, BACS has provided a percentage capacity display (SoC - State of Charge) for every lead-based battery, and since 2022 also for NiCd batteries and lithium cells (type LTO), both for trickle charging and for intermittent charging processes.

The battery capacity says a lot about the condition of the cells!

E-car manufacturers have developed interesting solutions when it comes to the discharging behavior of electric cars: If there is a defective cell in the battery strings of an electric vehicle, you have to adapt the discharging behavior and the measuring method accordingly, otherwise the calculation of the total capacity is completely unusable “estimated” result. If several cells are defective, the entire battery string must be “disconnected” to reduce the risk of fire. For this reason, every electric car has more cells than are actually used for operation; some are “reserves” and are not used for driving.

In electric cars, the lithium systems must be oversized to avoid critical situations and to be able to replace the failed cells so that the SOC of the entire system can continue to deliver the minimum range.

This also applies to lithium cells in UPS systems!

Image: When discharged, BACS STATUS shows a battery in yellow with significantly less capacity than the other batteries in the string, with approximately the same impedance values.

The automotive industry uses lithium-based cells with extremely high energy density, which take the charging/discharging methods to a completely different level than is common in most of today's UPS applications. Especially in UPS systems, either the simplest charging technology and conventional lead or NiCd-based batteries are used, or the most complex charging technology and significantly oversized lithium batteries are used - and both technologies struggle with the problem that UPSs almost never discharge – limiting the opportunities to check and calibrate the state-of-charge. In a UPS, all batteries are therefore always considered “full” and “healthy” to be able to simulate a calculation of the autonomy time and thus capacity - which provides extremely inaccurate data if just one cell is not functioning properly, or the batteries start to age. Most UPS users are not even aware of this inaccuracy - a discharge in UPS systems occurs far too rarely to doubt a possibly fake measurement value for SOC or autonomy time.
Since each of these "simulated" SOC values of a UPS can only be checked and corrected with great effort and would have to be repeated regularly for a reliable "state-of-health" derivation and trend detection, such an effort is only required in a "capacity test" carried out regularly in highly critical datacenters or military facilities.

With the capacity display, BACS provides an automatable
and therefore a cost-effective solution for otherwise complex “manual” capacity tests.

In addition, unlike limited battery monitoring, BACS can “delay” the necessary replacement of batteries, for example to achieve a maintenance window.

  • Balancing automatically ensures that a weak cell can remain in the network until it can be replaced during a maintenance window. An impedance trend display shows how long this condition can probably be tolerated. BACS is the only system that tolerates a new battery with different impedances being installed in such an older system. BACS ensures that old and new batteries work together and saves the otherwise usual complete replacement of battery systems.
Image: BACS VIEWER shows the trend of the impedance of the battery No.106 for the period of 1 year as a dashed blue line. The info box shows a difference of 44.2% in “red” and thus signals that this battery has deteriorated significantly during this time. There is a need for action regarding the battery service. The “orange” warning threshold will be reached in 2 months at the latest and probably shortly afterwards the “red” alarm threshold will be reached.
  • If an electronic battery disconnector is used, then BACS - identical to lithium BMS systems - can automatically disconnect the entire string with the affected battery to avoid potential catastrophic consequential damage. (This function complies with US Firecode and requires the optional “GXRAUX” and an electronic battery isolator)

The SOH and SOC are the most important metrics for the operator of a battery monitoring system - if these data are missing, the actual meaning and purpose of such a product is missing.

SOH - State of Health – BACS delivers what “norms” promise

To determine the SOH (State of Health), each battery has to be recorded (multiple times) by the service technician when it is discharged using an individual measuring device. To determine the state of health, all battery cells are discharged when new - usually up to a defined limit (e.g. 10.5 volts - the shutdown voltage level of many UPS systems). This discharge results in, for example, 10 minutes in year 1 of commissioning, and thus defines the reference value for SoH at 100%.

These discharges are repeated every year with the same load and the same general conditions, and the results are compared with previous years. The “optimal point for battery replacement” ultimately depends heavily on the battery chemistry used and the usage pattern, but the main problem remains the enormous human and time investment required to determine this data.

Since BACS measures and monitors each individual battery anyway, this material- and personnel-intensive additional measurement could actually be eliminated. All BACS requires are discharges that either occur “on their own” (due to a power failure) or are triggered “manually” by the battery technician. Analysis tools such as the free BACS VIEWER software allow a quick and direct evaluation of the recorded discharge data and provide the SOC and SOH as a result and a direct comparison with the data from a previous discharge and thus by how much the battery life has decreased in the specified period of time.
The next step is to determine whether the cause of the loss of capacity is a single defective battery or whether all batteries have lost capacity due to simple aging?

Image: BACS VIEWER compares 2 discharges that are roughly similar and detects a significant loss of capacity.
Current discharge 1: Only 1.22 Ah per battery. Discharge 2 a year ago: 5.21 Ah per battery.
A capacity loss of 76% with identical load

The BACS VIEWER provides the answer: When discharging, the batteries that have collapsed prematurely are displayed.

Not all batteries necessarily have to be replaced. In this case, by comparing the impedances of the batteries that have collapsed with the other batteries in the system, it can be quickly determined whether the age limit has been reached - or whether replacing just a few batteries that have aged prematurely is sufficient. The battery replacement procedure can be individually adapted to the respective situation.

BACS as a battery management system offers customers freedom in decision-making that cannot be expected from a limited battery monitoring system: The active management of the batteries removes the immediate need to act in the case of weak cells/blocks and allows well-founded decisions to be made based on the measured values planned to be implemented.

Picture: BACS VIEWER shows the “Discharge 1” of all 120 batteries with a constant load over a period of 1 hour. The end of the discharge is represented by the vertical red dashed line. This discharge simply needs to be compared to a similar discharge 1 year ago or earlier to determine the capacity loss and culprit.

The aging of the batteries is most clearly visible in the impedance.

A battery, regardless of the type of chemistry, will show an increasing impedance after a long time in “standby”. The cause is not only aging or defects in the battery, but usually it is simply the influence of gravity on the differently dense molecules in the more or less liquid electrolyte. This becomes quickly apparent when you see the impedance fall immediately after a discharge, only to see it rise again to the previous value within a few days. If the impedance value of all batteries behaves the same and does not differ significantly from the measured values ​​of other batteries in this system - then “natural aging” prevails.

  • As long as all batteries behave “the same”, a “normal” aging process can be assumed and the end of their useful life (approx. 80% capacity) is reached at an approx. 30% increase in impedance over the years.
  • If one or more batteries show an increase in impedance of more than 30% over the years, it should be checked whether these batteries are noticeable when discharged, for example if they lose capacity faster than the others. In such a case, the battery must be replaced if it is not to endanger the capacity and thus the SoH of the entire system.
Image: BACS VIEWER shows the trend for the increasing impedance of battery number 107 with the dashed line. In less than 1 month the battery has a 3% higher impedance. This increase is significantly faster than all other batteries in the string - here yellow line - and with the absolute value of over 50mOhm, a clear indication that this battery has reached the end of its life.

In addition to voltage, impedance, capacity and their historical development, there is another parameter that limits the service life of a battery and thus influences the “SoH”: the energy throughput.

The Energy Throughput is also described in the new EU Battery Act 2024 as a parameter for determining the SoH.

The “energy throughput” is basically an indication of the accumulated discharges over the entire lifespan of a battery, i.e. the total amount of energy in Ah that has flowed through the battery. Behind this lies the desire to determine how many charge/discharges cycles a battery has gone through. Depending on the type of chemistry, the SoH decreases with the number of cycles and thus indicates when a battery needs to be replaced. As already shown above, the BACS VIEWER determines the energy throughput for each discharge. If these values are accumulated, the sum results in the energy throughput of these batteries in the system.
However, this value makes little sense for UPS systems because of the small number of discharges. A UPS battery very rarely comes close to its possible number of cycles, so it hardly makes sense to bother accumulating the values. For a UPS application, the impedance therefore has much greater information content than the energy throughput or the number of cycles.
BACS is a “real-time” monitoring and battery management system with the aim of monitoring operational safety and warning when limit values are reached. BACS allows trends to be determined and thus a comprehensive picture of the state of the current batteries compared with older data. In addition, BACS ensures automation of battery systems and is therefore suitable for managing extremely large numbers of batteries with minimal operating personnel. For this task, the energy throughput is not very meaningful.
But this changes when it comes to “battery storage systems” in which cycling operation is the “normal” operation. Since it is hardly possible to compare the measured impedances with constant discharges/loads and loads, the number of discharges - the energy throughput - is used as a measurement for the "SoH" and determines the battery service life. A BACS system or other network BMS is “overqualified” for determining the “Energy Throughput”. The SMARTLOGGER -or its “housing integrated” cousin product called a “SMARTBATTERY” -- is a “black box” similar to a flight recorder that records the most important battery data and determines the energy throughput for each battery extremely inexpensively.

Image: SMARTLOGGER permanently installed in a battery of a UPS system. This means that the measured values of voltage, temperature and discharges are recorded for up to 10 years and the energy throughput is determined. If this SMARTLOGGER has reached the number of discharges or energy throughput for this battery type, then this also applies to the other batteries in the same battery string. In addition, the SMARTLOGGER indicates if limit values for these batteries have been violated and damage may have occurred that could lead to a loss of warranty. => Every battery system with more than one battery in the string should have at least one SMARTLOGGER permanently installed, also in order to meet the EU requirements by 2024.