Battery Ground Resistance Measurer






Switchgear and control Floating Battery Systems are not intentionally grounded. The advantage of the design is that a single field ground will not cause the battery operated control system to malfunction. It takes at least two field grounds (or even more, if the ground resistances are very large) to cause a control system malfunction.

If the field resistances are very small, the malfunctions can consist of blown fuses and tripped circuit breakers, which can result in the complete loss of battery power to the control system. If the field resistances are larger, but still less than the system Critical Resistance, circuit elements such as relays, solenoid valves, solid-state inputs, etc. can fail to activate or deactivate.

But what is the system Critical Resistance in your application? You can review the vendors' specifications for your control elements and you can perform tests. Place a variable resistor in series with a 125 VDC relay coil. Set the battery charger to its equalize voltage. Slowly decrease the resistance until the relay pulls in. Remove the resistor and measure its resistance (e.g. 8K ohms). Reconnect, and slowly increase the resistance until the relay drops out. Remove the resistor and measure its resistance (e.g. 38.5K ohms). Do this with all the circuit element types (solenoid valves, solid-state inputs, etc.). The largest variable resistor setting of all the circuit elements is the system Critical Resistance. If, in our example, the relay had the largest resistor setting (38.5K ohms), the system Critical Resistance would probably be declared as 40K ohms. Any battery ground equal to or less than 40K ohms must immediately be found and removed.



The Battery Ground Resistance Measurer (BGRM) is an extremely lightweight and portable system designed for your field technicians. With it, they can quickly determine the battery positive post to ground resistance and battery negative post to ground resistance. If either of these resistances is equal to or less than that site’s Critical Resistance, the sources of the battery grounds must be immediately found and removed. If any of the resistances are starting to approach the system Critical Resistance, the site should be marked for more frequent monitoring, or scheduled for a ground search in the near future.


The BGRM consists of three parts: An accurate digital Voltmeter, An Interface Test Box, and a Calculation Device. You supply the accurate digital voltmeter. It must be capable of measuring your battery voltage, and it must be able to accommodate shrouded banana plugs, such as those supplied with Fluke DVM’s. It must have an input resistance of at least 10 Megaohms.


The standard Interface Test Box consists of a plastic case containing two 2 Amp, 500 VDC fuses and a 200K ohm, precision, metal film resistor. It has two shrouded banana plug leads for connecting your DVM. It has two battery clip terminated leads, each over six feet long. The battery clips are connected to the leads with stainless steel hardware. One battery clip is connected to ground; the other is connected to a battery post.


The Interface Test Box places an approximately 196K ohm battery ground on your system while it is used to measure the battery positive post to ground voltage and battery negative post to ground voltage. The battery clips allow your technician to walk away and perform other duties while the 196K ohm resistance quickly dissipates your system’s battery to ground capacitive charge. When the ground voltage stops changing, the technician records the value. The third voltage your technician measures is the battery voltage. There is no need to use the Interface Test Box for this measurement, but the internal fuses would provide additional protection for your DVM.


The Calculation Device can be a portable computer running the Excel spreadsheet file provided on a CD with the BGRM, or as an option, we can provide a pre-programmed calculator which tells the technician when to input each of the measured parameters, and provides error messages if there is a problem. An additional benefit of the pre-programmed calculator is that the technician does not need to wait for his/her portable computer to boot up.




In addition to calculating the two battery ground resistances, the Excel spreadsheet file can also predict the ground voltages you will measure if you enter known battery ground resistances. This is useful for determining alarm points on certain battery charger, switched resistor ground detectors that only display voltages.


Most battery chargers, if they have a ground detector, have a balanced resistor ground detector. They are unable to detect battery grounds, even those below the system Critical Resistance, if the field positive post to ground resistance and field negative post to ground resistance are nearly the same. The BGRM has no problem measuring balanced battery grounds.


If you have a battery charger with a balanced resistor ground detector, the battery ground resistance calculated by the BGRM contains the battery charger ground detector resistance in parallel with the field ground resistance. You only want to know the field ground resistance. Both the Excel spreadsheet file and the pre-programmed calculator can provide the field ground resistance after you input the BGRM calculated resistance and the known ground detector resistance.


The standard BGRM, with its 200K ohm precision resistor, is designed for systems with a system Critical Resistance of 40K ohms or less. If your system Critical Resistance is significantly greater than 40K ohms, please contact us for a custom design discussion. The standard BGRM is designed for battery systems not exceeding 280 VDC. The BGRM will not accurately calculate the ground resistance of a battery ground internal to a battery string (between the cells of a battery).


Low resistance battery grounds are caused by water entering junction boxes or switch/sensor terminations, wire splices soaking in water, critters who gnaw insulation off wires for nest building material, cracks in aged insulation, sharp objects abrading insulation, etc. The Battery Ground Resistance Measurer is a powerful tool. It allows you to easily determine the battery ground status of your control system, in ohms, as often as you like. It is lightweight, portable, and inexpensive, allowing you to equip all of your technicians with this handy device.

Standby Power System Consultants, Inc. (SPS) is the exclusive provider of the Battery Ground Resistance Measurer. We supply many battery system related services in addition to those discussed on this page.


Please contact us at:

(630) 769-1138

fax:(630) 769-1174


Standby Power System Consultants, Inc.

77 W. 61st Street

Westmont, IL 60559



Packages Offered:



Battery Ground Resistance Measurer

Includes Interface Test Box, Excel Spreadsheet File CD, Instructions, and Easy Clean Case.

BGRM with Pre-programmed Calculator

Includes Interface Test Box, Excel Spreadsheet File CD, Instructions, Multi-line Display Pre-programmed Calculator, and Insulating Case (to reduce calculator exposure to temperature extremes while traveling with the technician outdoors).

½ Day, On Site, BGRM Course

Limited to 8 students per class. Includes 8 Class Manuals, 1 Excel Spreadsheet File CD, and Professional Instruction in sources of battery grounds, determining and decreasing system Critical Resistance, theory of battery charger ground detectors, and theory, operation, limitations, and troubleshooting of the BGRM. Suitable for engineers and technicians. 4 Pre-programmed Calculators and 4 Interface Test Boxes are provided for the students to use during the class.

1Day, On Site, BGRM and Battery Ground-Fault Tracer Course

Limited to 8 students per class. Includes the ½ day BGRM course and a ½ day course on the Megger 246100B BGFT, used for tracking the source of battery grounds. Includes student participation on a battery ground simulator.


For current prices, please contact Standby Power System Consultants, Inc.