Float Current Monitoring: a complete overview

The life of any lead-acid battery is not infinite due to the natural degradation of some electrochemistry properties in time. The way the battery is operated and cared for will have a major impact on its lifecycle. In this article, we will cover all the basics of float current monitoring and why is it so important in this day and age.

  1. Introduction
  2. VRLA batteries life expectancy
  3. VRLA batteries failure mechanisms
  4. What is the typical Float Current value for my batteries?
  5. How to determine the nominal Float Current value?
  6. How to determine high and low Float Current limits?
  7. 6 Float Current behaviors to monitor
  8. The best tool available to measure Float Current

1. Introduction

Stationary Lead-Acid batteries are used around the world for standby backup power of mission critical UPS/Data Centers, Telecommunication, Power Utility, Switchgear and more recently, ESS applications (Energy Storage Systems).  The VRLA (Valve Regulated Lead-Acid) technology has come to dominate the market as oppose to the VLA (Vented Lead-Acid) technology due to its unique advantages:

  • Minimal to free-maintenance requirements and spill-proof construction design
  • Space requirement and power density
  • Extended shelf life with low self discharge characteristics


Although VRLA batteries have unique features, they also have a few factors that users need to manage to maximise life expectancy, optimize battery performance and ensure that the service to the end-user is maintained when power outage occurs. 

VRLA batteries are offered in two types, the AGM (Absorbed Glass Matt) and the Gelled electrolyte design.  Their life expectancy is defined by the type itself, AGM or Gel, the quality of materials, construction design and manufacturing process.  However, based on the following factors, their SoH (State of Health) can be significantly reduced and can cause premature failures:

  • The selection process and the way they are warehoused, installed and commissioned
  • The environment they are operating in (ambient temperatures, float voltages, ripple currents)
  • The applications they are used for (cycling, depth of discharge, high discharge rate)
  • The way they are managed and cared for (proper management techniques, charging, equalization)

2. VRLA batteries life expectancy

In stationary applications such as 48Vdc telecom, 125Vdc electrical substation and UPS systems, VRLA batteries are continuously charged to counteract the self discharge phenomenon, guaranteeing a full state of charge and ensuring that the backup power will perform as specified when needed.

We would expect that a VRLA battery have an infinite life, especially when they spend their life on continuous charge and being rarely used (cycled), but it’s not the case. The design life of a battery is generally stated by the manufacturer as 5, 7, 10 or 15 years, but other durations also exist.

VRLA batteries will decline in capacity (AH) and fail due to internal cell chemical reactions resulting in grid corrosion and drying of the electrolytes.  Battery manufacturers recognize that accelerated grid corrosion and loss of electrolyte moisture are directly related to abnormal float charging current intensity.  

3. VRLA batteries failure mechanisms

Before looking at the list of “mechanisms” that leads to VRLA battery failures, it is important to select the ideal battery for your application and care for it, thus making certain you have:

  • An enterprise wide process in place to select the proper batteries for your various applications
  • Making certain the batteries are well maintained and cared for by qualified technical staff, trained on the testing tools they use and knowledgeable of the rigorous guidelines of your scheduled preventive and corrective maintenance practices


An optimal Battery preventive maintenance program is required by law for electrical utilities in North America where it keeps track of monitored battery parameters as recommended by PRC-005 standards and IEEE’s 1188 recommended practices.  As an example, IEEE’s 1188 recommends that monthly inspection should include cell/jar terminal float voltage, charger output current and voltage, ambient temperature and Float current per string.  With VRLA batteries, float current becomes an important operating parameter to monitor and trend over time as it provides early indications of various battery failures.

Here is a summary of the factors that accelerates VRLA battery aging or failure mechanisms:

  • Elevated ambient temperature and float voltage will greatly impact the battery life cycle and can lead to thermal runaway events. It is recognised that for every 10oC increase in temperature, the float current doubles at a constant float voltage.  Likewise for float voltage, every 0.05 Volts per cell increase, the float current typically doubles. Any excess of float current leads to overcharging which accelerates grid corrosion and the gassing rate thus reducing greatly the life expectancy.
  • Grid corrosion: This is the natural cause of VRLA battery failures. As the lead of the grid oxidate over time due to battery chemistry, the plates will grow and visible jar bulging may occur.  Excessive cycling, elevated ambient temperature and overcharging conditions can accelerate grid corrosion. Float charging current and ohmic value (Resistance/impedance) will increase as current path across the grids becomes more resistive.
  • Dry out or loss-of-compression: The one-way valve of VRLA batteries releases gas when the internal pressure exceeds rated psig limit, higher pressure is mostly due to overheating a consequence of excessive discharge rate.  Overcharging produces excessive gassing from the internal heat build-up, it bends the grid and accelerate active material shedding.  The safety valve frequently operates releasing toxic gas which in the end, the gas cannot recombine and leads to dry-out a potentially thermal runaway.
  • Improper charging – overcharging is any excessive charge that results in damage to a cell or battery. It can be the result of human error (i.e., setting the wrong parameters on the charger), or charger failure.  In UPS applications, charging voltage varies depending upon the stage of charging.  For example, the initial charging following a discharge is at a higher voltage (referred to as “bulk charge”) than at standby (referred to as “float charge”).  Overcharging can dramatically shorten the life of a battery and, in worst case, can lead to thermal runaway.
  • Improper charging – undercharging VRLA batteries or setting float voltage too low will lead to sulphate forming on lead plates. Another cause is not providing enough current during bulk charge cycle.  Sulphation will lead to higher float current and a lower capacity due to higher internal resistance growth. Undercharging can occur if the battery is not connected to the UPS for an extended period of time or if a battery cell fails because of an open circuit and goes undetected, it will results in sulfation of the remaining cells. This occurs when sulphate crystal forms on the negative plates which reduces the charging rate, battery capacity and life.
  • Internal Shorts: Can be triggered by various causes, sometimes due to dendrites of lead forming threads of thin metal caused by low specific gravity acid, damaged separators due to overheating cells. Internal short can also occur due to excessive plate swelling that end up touching adjacent plate.
  • Thermal Runaway: Thermal runaway is a condition which may lead to a dangerous outcome such as a battery electrical fire or an explosion evidently damaging nearby equipment and potentially injuring personnel. When the above-mentioned condition occurs and the battery internal temperature cannot be dissipated in the air,  a build up will occur between the float charging current and internal temperature.  The build up never stops unless the triggered condition is de-activated or when the battery is removed from the charging system. During the process, extensive gassing can produce dangerous level of concentration of hydrogen gas. However, a thermal runaway events can be detected and prevented in time by continuously monitoring each battery string float current. The leading factors of thermal runaway conditions can be:
    • Elevated ambient temperature
    • Elevated float voltages – Over-charging
    • Shorted cells
    • Increased current

4. What is the typical Float Current value for my batteries?

First of all, Float Current or Float Charging Current is the current required to keep the batteries at a full state of charge. The float current compensates for the self discharge process when a constant float voltage is applied on the battery.

Battery chemistry, battery design, quality of material, manufacturing process and battery capacity (AH) will ultimately influence the rate of self discharge of any lead acid batteries. Thus, the typical float current value will differ from one model to another. 

Unfortunately, battery manufacturers do not usually publish float current “reference values” on their specification sheets. It can be obtained by the interpretation of Tafel curves (Float Current/Average Cell Voltage) at a specific temperature, but it is a complex and tedious process.  Therefore, by simply asking your battery vendor for it, they will be able to provide a nominal/expected value for the battery assets used in your network

5. How to determine the nominal Float Current value?

A set of guidelines or “rule of thumb” have been developed to calculate the typical float charging current for VRLA batteries. A mean value with tolerances is given due to manufacturing tolerances and other factors. But again, if specific data is available from the battery manufacturer, you should use that data. The guidelines have been developed by Kyle D. Floyd formerly Covenant Service Company and Eric Boisvert of Multitel Inc. One guideline is applicable to AGM type VRLA batteries and another for Gel type. For a transcript of these guidelines, click here to request your copy. The guidelines provide a specific multiplicator based on a depicted ambient temperatures range (10oC/50oF to 35oC/95oF), depicted float voltage range (2.25Vpc to 2.35Vpc) and 8-hour discharge rate (1.75Vpc @ 25 oC).

As an example, for an AGM type battery rated 80Ah at an 8-hour rate to 1.75 Vpc, the following estimated float current at 2.30 Vpc and 25°C can be expected.  A tolerance of +/- 33% applies to the selected multiplier.

Typical float current = 1.6mA/AH x 80AH = 128mA +/- 42mA

6. How to determine high and low Float Current limits?

If batteries are operating in a non-environmentally controlled location, it is recommended to turn on the temperature compensation function, a feature available on most rectifiers and chargers nowadays. This function will have the effect of maintaining the float current around its nominal value and will reduce the rate of internal heat generation inside the battery.

With the battery charger temperature compensation feature “ON” with or without presence of air conditioning system, one can use 3x times the float charging current at 25°C with float voltage setting at 25°C. 

If the battery charger temperature compensation feature is “OFF” due to the presence of air conditioning system, then the multiplier should be 6x times the float charging current at 25°C and minimum recommended float voltage.

The above mention limits are recommended limits for early detection of thermal runaway.  It is up to the user to determine their limits based on the application, business risks, and technician dispatch frequencies (site visits).

7. 6 Float Current behaviors to monitor

Another interesting fact is that float charging current is part of the Ohm’s law triangle (V = I x R) where cell float voltage, internal resistance and float current are directly related.  Keep that in mind when assessing the condition of the battery based on float current and other battery parameters:  

Float Charging Current behaviors Range (mA) Possible problem root cause and implication
Steadily higher float current than usual
2x times higher than the nominal float current value
A higher float current value than normal can be an indication of a float voltage set too high, operating at higher temperatures than usual, presence of a ground fault on a floating battery system or it may indicate failing cells within the battery string. The internal battery problem could be dry outs, shorted cells or others.
Steadily lower float current than usual
1.5x times lower than the nominal float current value
A lower float current can be indicative of a problem with the rectifier, float voltage set to low or charger system, a loss of integrity with battery connection’s not to be confused with a lack of float current which will indicate an open circuit (See below).
Non-presence of float current or excessively low
DC float current between 0 to 10mA
An absence of float current will indicate an open circuit between the charging system and the battery string. Sometimes due to intrusive maintenance practises on UPS batteries and substation battery systems (125Vdc), battery disconnect operation or a rectifier/charger malfunction.
Significant upsurge of the float current
3x to 6x times higher the nominal float current value within a 2-to-16-week period.
An upward surge of float current over a few weeks will be indicative of a thermal runaway event in progress. Thermal runaway events occur in rare cases when the root cause condition is not investigated in a timely fashion.
Trending float current over time for battery aging
Pairing float current trending with internal ohmic measurements
As a battery ages, the cell deteriorates and with time causes the battery internal resistance to increase. One can expect the float current to ramp up in time as more current is required to move through the increasing battery internal resistance. An increase in float current can be a reliable indicator of battery faults and should be taken in consideration along with “internal ohmic measurements” which is the most accurate parameter to calculate the battery SoH (State Of Health)
Replaced SG (Specific Gravity) for SoC (State of Charge) determination
Float current is the current flowing into a fully charged battery.
Since the early 2000, float current measurements have been accepted for determining SoC (State of Charge) of VLA and VRLA batteries (see IEEE-450). As hygrometers are not easy to use and testing is time consuming, SG measurements can be replaced by float current measurements. Float current has an advantage in that it provides an indicator of the entire battery string, while specific gravity is measured on a cell-by-cell basis.

8. The best tool available to measure Float Current

Accurate, repeatable, reliable and non-intrusive float current measurement is not as easy at it seems with standard current measurement techniques (DVM with Ammeter probe, 50mV shunts or Hall effect DC current transducer). For optimal results, the following specifications are suggested:

  • 0.05% accuracy F.S. with 1mA resolution
  • 0 to 5A measurement range with polarity detection
  • Sustain up to 2000A discharge/recharge currents with no hysteresis build up


Measuring the DC component of float charging current can be an easy task with Multitel’s FCCP, the probe uses a patented measurement technique based on a split-core transducer coupled with a unique digital filtering.

The large ferrite core opening enables safe and non-intrusive measurement on small to large current carrying conductors without opening the battery circuit.  The embedded software recognises polarity of discharge & recharge current when normal battery rundown occurs, it provides an auto-calibration procedure to eliminate the earth magnetic field and offers temperature compensated thresholds along with time delays. Smart high or low float current alarms are provided using Form-C relay contacts.  An analog output signal enables trending of the float current value overtime.  The probe can be paired to a Battery Monitoring System to provide a comprehensive assessment of battery SoH when making battery replacement decisions. Telecom and power utility battery communities are using the FCCP to meet some of the following recommendations and standards: IEEE-1881, NERC PRC-005-6, NERC TPL-001-5, IFC 602.8, NFPA 1. Article 52.

The FCCP is design to be used in non-environmentally controlled telecom cabinets on side of the road up to Large UPS batteries systems in Data Center facilities.

Multitel’s FCCP (Float Charging Current Probe) features:


Capable of measurement between 0-5A with increment of 1 mA

Smart Alarms

Will not generate alarms when “Battery Discharge/Recharge” event occurs


Linear analog output signal to easily interface battery monitors or data loggers


A split-core transducer enables easy installation without opening the battery circuit

Immune to noise and temperature changes

Unique measurement technique and digital filtering technique eliminates AC ripples to provide a pure DC component

State of Charge (SoC)

DC float current is the parameter of choice to determine the battery SoC


Protection of small to medium battery backup power at your customer location against thermal runaway

Flexible Input Power

The controller is adapted to telecom, railway and power utility configuration

Thermal Runaway Prevention

Float Current is the parameter of choice when it comes to thermal runaway prevention

Open Circuit Detection

Help Electrical Utility meet the NERC’s TLP-001-5 standard for single charger station DC supply battery

Timeliness measurements

Benefit from continuous DC float current measurements

100% Compatible

Dry “C” contact enables compliancy to any remote telemetry device in the network

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