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Battery testing FAQs


Regular battery testing will allow trending of the battery's health and an estimation of the remaining life of the battery. The IEEE and NERC recommends two types of tests:

An impedance test mesures the condition of the entire electrical path of the battery from one terminal plate to another. This reveals any weaknesses in cells and inter-cell connectors, and is often used to establish a baseline value for the battery.

A capacity or discharge test is the only way to get an accurate value of the actual capacity of the battery.

The following are some Frequently Asked Questions about battery testing. Feel free to browse our selection of battery testing equipment. If you can't find the answers you need here, please contact us and our application engineers will be happy to help.

Q. What is the definition of a "primary" or "secondary" battery?


Primary batteries or cells can produce current immediately on assembly. Secondary batteries are rechargeable batteries that must be charged before first use.

Q. What is the difference between "terminal connection resistance" and "inter-cell resistance"?


Battery terminal connection resistance is the resistance from the terminal to the strap. Battery inter-cell connection resistance is the resistance of the inter-cell connector or strap. If the measurement is taken from post to post, then both are taken at the same time. 

Q. How important is inter-cell connection resistance?


Experience shows that loose inter-cell connections that heat up and melt open are responsible for more battery failures than defective cells. This is particularly problematic with lead-acid batteries that are frequently cycled, as the negative terminal may cold flow, thereby loosening the connection.

Checking inter-cell connection resistance is, therefore, vitally important. Always be sure to follow the correct test sequence when working on multi-post batteries, and ensure that the instrument employs a method of testing that will provide valid results.

Q. What are the differences between the IEEE and NERC battery testing standards?


IEEE is the recommended battery maintenance procedure that will provide maximum reliability. It is not a mandated procedure. NERC, on the other hand, is the minimum required maintenance by law. 

For example, NERC recommends performing either an ohmic OR discharge test every 18 months. IEEE recommends performing both tests annually.

Q. Why are the NERC testing intervals for VRLA batteries shorter than VLA batteries?


In general, VRLA batteries have shorter life spans than VLA batteries. VLA batteries have large volumes of electrolyte that can be topped off by the user. VRLA batteries have minimal amounts of electrolyte in them and they cannot be topped off.

The recombination effect that recombines hydrogen and oxygen back into the electrolyte, is not 100% efficient in VRLA batteries.  Electrolyte is lost over time in VRLA batteries. This is why dry out is the common failure mode in VRLA batteries. This also can make VRLA batteries more prone to potentials for thermal runaway, which lead to catastrophic failures. This is why they need to be tested more frequently as they can fail in a shorter period of time.  

Q. How often should battery impedance testing be performed?


The best frequency for impedance testing depends on the battery type, the site conditions and previous maintenance practices. The IEEE 11888 standard for VRLA batteries recommends, for example, that a baseline impedance measurement is made six months after the battery has been put into service, and that further impedance measurements are made at quarterly intervals there-after. For NiCd and flooded lead-acid batteries, Megger recommends impedance testing at six-month intervals. Impedance measurements should also be made immediately before carrying out every capacity test.

Q. When should baseline internal ohmic values be established?


NERC recommends waiting 6 months after installation to establish the baseline values. In actuality this will depend on the type of battery used.

All lead acid batteries have a certain amount of gas and excess water in them when they are manufactured. This will lead to higher ohmic readings and lower capacity (this is why new batteries are called "green batteries"). Batteries need to gas this excess before they reach full capacity and reflect a lower ohmic value. This process is known as formation. The baseline value should not be taken until formation is complete. 

In VLA batteries this happens relatively quickly and they should be at 100% capacity a week or two on float. VRLA AGM batteries can take 3 to 6 months before they complete formation. This is why NERC recommends waiting 6 months. VRLA GEL cells can take 1 to 2 years to complete formation. Despite this long formation, a baseline should still be taken around 6 months because it is likely that the ohmic value in GEL cells may drop over the first 2 years.

Q. How can I predict when I need to replace a cell?


Even though the correlation between battery capacity and impedance is not mathematically perfect, increase in impedance is an excellent indicator of battery health. Megger has found that, for flooded lead-acid batteries, a 20% increase in impedance generally indicates that the battery capacity has declined to about 80% of its initial value. For VRLA batteries, the corresponding impedance increase is closer to 50%. When these figures are reached, it is usual to consider that cell replacement is necessary.

Q. Is there a way to determine 80% capacity with the impedance results?


No, although a battery's internal ohmic value does correlate with capacity, it is not a direct correlation. All that can be said is that as the ohmic value rises, capacity drops. No ohmic test on earth will tell an operator if a battery is good or bad. It will indicate when there is a problem. Troubleshooting is then required to determine if the problem is a strap connection a charger issue or a bad cell.

Q. Is it possible to carry out a discharge test on a battery while it is still online?


Yes, provided that the right test equipment is used. Megger, for example, has testers that automatically sense and regulate the discharge current even when the batteries are connected to their normal load. Most users choose to make an 80% discharge test if the battery is to remain on line, thereby ensuring that there is still some back-up capability remaining at the end of the test.

Q. Are there guidelines for acceptable variation of impedance of cells in a string?


The recommended starting values are as follows. These may change as you find better values for your particular battery in your particular application.

Battery Type

Recommended values

VLA batteries

50% from baseline

VRLA batteries

30% from baseline

Inter-cell connections

20% from baseline


Q. When should I stop replacing cells and simply replace the whole battery?


In short strings of less than 40 cells, it is advisable to replace the entire battery when between three and five cells have been changed. For longer strings, replacement is advised when more than 10% of the cells have been changed.

Q. Will capacity testing damage my battery?


Battery systems are designed to provide back-up supplies during power outages. Since a discharge test is nothing other than a simulated power outage, there is no risk of battery damage. Batteries can normally be deep discharged (that is, discharged to the manufacturer’s end-of-discharge voltage) between 100 and 1,000 times, depending on the type of battery. Using a few of these discharge cycles for capacity testing has a negligible effect on overall battery life. Nevertheless, there is no reason to carry out discharge testing more frequently than recommended by the relevant standards.

Q. Are there any issues with performing internal ohmic testing on a monthly basis?


No, ohmic testing does not harm a battery. 

Q. Do plates grow in thickness, length or both?


Plates will expand in all directions. The expansion occurs as the plates sulfate and sulfate crystals form. The positive and negative plates are sandwiched between insulators so they will tend to grow more in length. However a battery jar can bulge from either the top of the sides, pending on the nature of the plate growth.

Note: VRLA batteries may have a slight bulge to them because they are under pressure.   

Q. Can an equalization charge reverse plate sulfation?


Yes, this is the only effect that equalization does overcome. The longer a plate remains sulfated, the denser the sulfate crystalline structure will become. The higher voltage of the equalization charge will breakdown this structure provided it is not in phase 3 sulfation where it has become too dense to breakdown.

Note: ALWAYS follow the battery manufacturer’s recommendations when performing any equalization procedure. Batteries can be damaged if left in a high state of charge for too long. 

Q. Do different types of charging techniques e.g. pulse charging, extend the life of lead acid batteries?


This is dependent on the type of battery and how the pulse charging is performed. The belief here is that pulse charging reduces the heat the battery experiences, thus increasing the life span. The problem with this philosophy is that lead acid batteries have a relatively high rate of self-discharge. This means they may readily fall below their open circuit voltage. When this occurs the plates, will start to sulfate.

When the recharge is re-applied, the battery has to draw more current to reverse the sulfation. This will generate more heat than if the battery was just left on charge. In addition, if using VRLA batteries, this continuous recharging causes a recombination effect to occur which uses up the electrons that reside in the double layer on the negative plate. Over time this can lead to negative plate depolarization.

Therefore this is not a recommended practice for lead acid batteries. This can be done for NiCD batteries, since NiCD batteries are constant voltage devices and utilize an alkaline electrolyte. How much it extends the life of a NiCD is still subjective.     

Q. In dealing with thermal runaway, is there any evidence that VRLA or Flooded are more prone to failing shorted or failing open depending on the battery type?


Flooded batteries are more prone to fail in a shorted mode due to plate shedding. As the material falls off the plate, it builds up on the bottom of the battery jar. If this reaches the plate, the battery will short.

VRLA batteries are more prone to fail open. This is because shedding does not occur in them. The white lead sulfate is held in place by the AGM separators and never build up on the base of the jar. In VRLA batteries, the process of recombination during recharge slowly dries out the battery. The electrolyte is trapped in the AGM separator. As these dry out, the separator tends to pull away from the plate, which leads to a virtual open state. The impedance will be too high to pass any notable current.

Thermal runaway will occur in a VRLA battery due to excessive dry out. If the rate of dry out becomes too high, then the dry out can become a self-sustaining reaction drawing more and more current and generating more and more heat, until a catastrophic failure occurs. This is why using a charger with temperature compensation is recommended. 

Q. What are the most common issues with substation batteries?


The lack of maintenance. Corrosion on terminals lead to hot spots and poorly charged batteries, which in turn leads to plate sulfation. The natural failure mode in a VLA battery is positive plate corrosion. In hot environments this reaction will accelerate. High charger levels cause accelerated positive plate corrosion, while low charger levels lead to plate sulfation. 

Q. Does NERC provide any guidelines for test equipment calibration?


NERC does not have any information concerning test equipment calibration. However, keep in mind that ohmic testing is based upon repeatability and not accuracy. Three different units from three different manufacturers will report three different values. This is because they all operate at different frequencies, different currents and utilize different algorithms.

It is much more important for your instrument to get the same reading each time, than for it to match a reading from a different instrument. This is why it is important to keep your equipment calibrated according to the manufacturer's recommendations.

Also be cautious of units that test with low currents on large VLA cells. This measurement may not be repeatable. The ideal unit is one that injects high current into the battery. This allows for testing of large VLA batteries and provides repeatable measurements without the need to wait for a recharge.

Q. When should I measure float voltage?


In reality, float voltage measurements are of limited value. They can be used to confirm that the charger is working, but they give no information at all about the batteries state of health. Measuring the float voltage of a cell will also show whether or not it is fully charged. However, it is important to remember that just because a cell is fully charged, it does not mean that it will deliver full capacity. It is by no means unusual for a battery that is close to failure to have a float voltage that is within acceptable limits.

A low float voltage may indicate that there is a short in the cell. In a lead-acid battery, this should be suspected if the float voltage is 2.06 V or less, assuming that the charger is set for 2.17 V per cell.

In other cases, a cell may float at a considerably higher voltage than average. This may be because the high float voltage cell is compensating for another weaker cell that is floating low. It is also possible for one cell to float high to compensate for several cells that are floating a little low, because the total of all the cell float voltages must always equal the charger setting.

Q. What is ripple current?


In utilities applications, ripple current is generated by the AC current from the battery charger. In other applications like UPS or telecommunications, some ripple may come from the load.