**A world first in Battery Management** - Battery Capacity at UPS with “Intermittent Charging”

03 May 2023

GENEREX BACS is the first manufacturer to supply a battery capacity (P_SoC - Periodic State of Charge) also for UPS and battery systems with "Intermittent Charging"!

BACS has been the key technology for active management of stationary batteries for 19 years. Balancing (or "Equalizing") ensures the stability of lead-acid batteries, NiCd or lithium (LTO/LiFePo) based cells and maintains the "health" of the cells - SOH (State-of-Health) - at the highest level with correct monitoring of the measured values / alarms.

Additionally, the interpretation and accuracy of the measured values of a battery system experience a massive improvement through use of balancing: Balancing keeps all cells/batteries closely within the manufacturer-defined "healthy" voltage window, allowing for a highly precise impedance measurement and thus a comparability of the impedance measured values from one cell to the next, as well as for the system as a whole.

Only with balancing do impedance measurements provide meaningful, comparable results!

This has resulted in BACS verifiably improving both the reliability and longevity of nearly every battery-based UPS concept. Our reference list of BACS users reads like a "Who's Who" of western industry. BACS is a "Game Changer" in the industry and the 1st choice of data center operators in the western world!

Since 2021 BACS has provided a percentage-based capacity indication (SoC - State of Charge) for each lead based battery, since 2022 also for NiCd batteries and also for lithium cells, type LTO.

The capacity indication determined by BACS is almost as good as the results of much more complex (and costly!) measuring methods like "current balancing". This is also made possible by the new, much more accurate current sensors of BACS type CSHxxx of the 5th generation. (For more information about our varying Current Sensor models, see our other article about Current Sensors featured within this Newsletter).

The problem with battery capacity:

The great difficulty in determining the "State-of-Health" to detect the unexpected failure of a battery leads to calculation problems of the "State-of-Charge". Suppose there is only one defective battery in a battery string and there is no discharge possibility like in an electric car. In that case, this defective battery will influence the total capacity massively, making every calculation uncertain. Especially with UPS systems there are almost never discharges with which one might check and calibrate the State of Charge; as such, with a UPS all batteries must always be considered as "Full" and  as "Healthy", meaning any calculation of a capacity is inherantly inaccurate.

Picture:  CSHxxxF Generation 5 – for “F” flexible installations. Also available as traditional DIN Rail version and as Ground Fault Sensor

Most UPS users are not aware of this inaccuracy, simply because discharges within UPS systems are too rare. Only in special cases, such as highly critical data centers or military facilities, are determined attempts made to determine the deficient or missing UPS capacity indication by means of regularly scheduled capacity tests. During such tests, the "State of Charge" is also measured and often a massive deviation is found without a clear indication of the cause. This is exactly where BACS provides the crucial clue: In the picture on the right you can see a BACS system at the “intermittent charging resting phase” where a battery in the string with less capacity is detected (orange) and the user is given the possibility to get the optimal capacity out of the equipment by replacement! Without capacity display, this problem would probably have been overlooked.

But even without a battery exchange, battery systems with BACS will always get "more" out of the batteries than identical battery systems without BACS in regular comparison tests. But so far this Capacity Increase was only true for UPS systems that operate "Float Charging"!

As soon as UPS systems use "Intermittent Charging", even BACS would not be able to determine the battery capacity, since in doing so the UPS automatically creates a condition in which balancing ceases functionality!


When the UPS finishes a given Boost Charge mode and switches to the “resting phase”, a 12V battery will drop from 13.60 volts down to its resting voltage, depending on the type of chemistry, from about 12.50-12.80 volts. This does not mean that the battery is "almost empty", it can be almost full OR almost empty. Everything in between is possible. The voltage of the battery has no informational value about the capacity during rest voltage!

Until now, BACS could not perform balancing when "Intermittent Charging" was active and therefore could not determine the battery capacity.

HOWEVER: New as of firmware 2.14, BACS can now also determine battery capacity of the "P-SoC" in the "special" situation created through Intermittent Charging! 

Many UPS manufacturers have been struggling with lead acid problems in their equipment for years. Although the UPS chargers are correctly set to the battery type, over time undercharging occurs due to inherent inter-cell voltage differences and consequent overcharging of individual cells/blocks, a creeping chain reaction that leads to battery failures.

Without BACS there would be no correction for the voltage difference, and as such half of the batteries will eventually be undercharged and the other half will be constantly overcharged. This could lead to the swelling of overcharged batteries. As a result, this causes considerable problems when replacing the batteries.

To avoid this effect, UPS manufacturers
have developed several different methods:

  • Some offer lithium based batteries instead of lead batteries:
    This is risky, expensive and not recommended for stationary systems. Lithium batteries are certainly smaller and provide more autonomy time than lead batteries of the same relative size, but without a BMS with balancing such batteries are not safe to operate. Thus, not only is the technology considerably more expensive than lead-acid batteries, but in the event of a fault it is also "fire-hazardous" and therefore broadly considered unsuitable for use in stationary UPS systems.
  • Other UPS manufacturers install BACS with balancing:
    An ideal solution, but only possible if "space" is available for the installation of the BACS module; otherwise a GENEREX "SMARTBATTERY" might be used.
  • Another "method" of the UPS manufacturers is to slightly lower the ideal charging voltage:
    If the battery manufacturer specifies 13.60 volts at 25°C, then some UPS manufacturers supply only 13.45 volts and thus avoid overcharging, but lose capacity and, in the long term, reliability.
  • A popular method used by UPS manufacturers is "Intermittent Charging":
    The charging voltage is completely switched off for weeks and only switched on again intermittently. This can be found e.g. with EATON models under the name "ABM". In the case of "intermittent charging", the otherwise commonly used trickle charge is switched off after a boost charge, until the UPS recognizes from the string voltage that the self-discharge has reached a limit after a few weeks. Then the charger is switched on again, and after the new strong charge there follows a longer pause in charging.

Use of „Intermittent Charging“ brings up 3 general problems:

  1. Due to the long “pause” between the boost charges, it might be the case that individual batteries become deeply discharged without notice. This can occur without being picked up by the UPS because other batteries can compensate the voltage loss, especially if many batteries are in one string, as is the case with modern UPSs with high efficiency. If this happens frequently, after 5-6 years at the latest there is a high probability of finding deeply discharged batteries or batteries with too many charge cycles in the string.
  2. The UPS can become "unreliable": after a long period of "intermittent charging", some batteries can reach the lower voltage limit and become sulfated. Sulfation within the battery prevents sufficient power from being supplied in the event of a power outage, since the voltage on badly sulfated batteries drops massively for a few seconds under load. If many batteries are affected, they may drop below the UPS shutdown voltage (Battery Low) threshold (about 10 volts) for a short time and the UPS will thus shut down almost immediately after the power failure. At the 2nd "attempt" the UPS works again perfectly because the subsequent charging phase has partially regenerated the batteries – however, the user now justly no longer trusts their UPS device.
  3. The third problem is that with batteries that are charged by intermittent charging it is not possible to measure and display the battery capacity in any meaningful way. Most UPS users have probably never noticed that UPS systems often do not have a battery capacity indicator. The reason is that especially with this charging method one is unable to tell how "fully" the battery is charged when it is at the rest voltage of about 12.5 volts. The battery might actually be between 40% and 100% state of charge at 12.5 volts, much too inaccurate for a display of capacity and thus the remaining runtime.

Even with BACS the capacity could – until now! – not be determined satisfactorily with this charging technique. With a battery at rest voltage, too little measurable current flows from the battery and the natural self-discharge is no longer compensated. BACS could not handle this problem and UPS systems with this charging principle had either to be changed to normal "Float Charging" (switch off ABM) or had to do without BACS Balancing and had then only pure battery monitoring - without real added value compared to a system without BACS. With FW 2.14 BACS has now an extra value again, even on ABM systems.  

A World First!
As of Firmware Version 2.14, BACS can provide a meaningful measurement value for the battery capacity even in "intermittent charging" mode!

The basic principles for this new measuring principle were developed in cooperation with the German Federal Ministry of Economics and Technology (BMWI). This was researched and tested in the "Home BMS" project and applied here for the first time in a product. GENEREX is a participant in this research project "Heim BMS" and the result of our research can now be seen here:

In the adjacent BACS screenshot you can see three pure lead batteries at 12.84 Volt. The batteries are in the "resting phase" and therefore in an "unknown state of charge" for BACS. There is no float charge voltage flowing and the amount of current drawn due to natural self-discharge is too low to measure.

BACS is missing important parameters for the determination of the state of charge.

One could assume that these 3 batteries are not full at 12.84 Volt. The new BACS Firmware 2.14 knows better! If you move the mouse over the column "Charge [%]", a blue field appears which shows the currently used measuring method.

Mode: Float" appears for "Float Charging". If "Intermittent Charging" was detected, "Mode: Idle" appears here, marked yellow.

BACS has recognized that "Intermittent Charging" is present and will now determine a 100% capacity despite only 12.84 Volt. With the same batteries under Float Charge Mode, 100% would need at least 13.5 Volts. In both cases BACS determines 100% as state of charge.

BACS correctly determines the full state of charge of 100% for both charge modes, despite massive voltage differences!

It becomes even clearer when the batteries have sunk even further after a few weeks of rest: (See the following BACS screen)

The voltage of the batteries has changed only a little in the 2 weeks charging break (from 12.84 Volt now to under 12.59 Volt). From the voltage one could assume that the batteries are almost empty - but BACS still shows a very good state of charge of over 90%.

=> BACS recognizes that with Intermittent Charging the capacity has changed only little even after 2 weeks charging break, and displays this correctly!

With BACS, GENEREX is proud to have the first BMS on the market that can determine the battery capacity "P-SoC" even with "Intermittent Charging"!