A low resistance conduction path throughout a standby battery circuit is essential for proper, reliable and safe operation. Connection resistance that's too high can, at a minimum, result in reduced run time. At the other end of the failure spectrum, overheated connections or worse, a battery fire, can result if the checks and balances don't add up. This month's blog addresses verifying inter-cell/inter-unit connections are adequate and up to the task when a discharge occurs.
Not a Level Playing Field
It's noteworthy that while tracking connection resistance is recommended, the batteries don't always cooperate. By that I mean the terminal construction and/or installation methods don't necessarily permit such measurement. One example is the poor VRLA battery stuffed into a cabinet. The best that can be expected here is for the maintenance technician to make sure the connections are clean and tightened to the proper torque value, in accordance with the battery maker instructions. There are other examples that prohibit this maintenance task that include large format VLA and VRLA systems installed on racks but I want to focus on the batteries where measurement is possible.
Test Equipment Requirements
In order to measure the very low resistance of interconnections, you need a micro-ohmmeter or one of the available internal ohmic test instruments that includes the capability to make the measurement directly. These days, many, if not most service companies and end users alike are opting for the latter of these two instruments to reduce cost. Below is an example of a micro-ohmmeter.
The results from making measurements are in the micro-ohm range (one-millionth of an ohm) on the ohmic scale, so a conventional Fluke-type multi-meter set to measure Ohms isn't going to cut it. There is no "typical" reading for connection resistance because there are numerous variables that affect determine the value.
Dimensions of the inter-cell/inter-unit connector
Number of parallel connectors, if any
Makeup of the connector, i.e., copper bus or cable
Connector plating material, i.e. lead or tin, etc.
Cell terminal dimensions
Terminal construction, i.e., copper inserted or solid lead alloy, 1" square, 2" square, chair, flag, etc.
Installer hardware preparation and assembly technique
Guidance from Industry Recommended Practices
IEEE 450, 1188 and 1106 recommended practices address verifying connection integrity and apply to VLA, VRLA and NiCad batteries respectively. If you're using any or all of these batteries in your plants, you should own a copy, in paper or PDF format. Most opt for the PDF. These are for-sale, copyrighted documents. They are available online at the TECHSTREET STORE. They are an indispensable source of guidance.
How High is Too High?
Generally, connection resistances should be measured and recorded upon completion of an installation. These readings become the baseline by which all future measurements are referenced. For a new battery, any reading for a given type of connection that exceeds 10% of the average for that connection or 5 micro-ohms, whichever is greater, should be investigated. Sometimes bolts are missed during the tightening phase or something did not get cleaned and prepared properly prior to assembly. If verifying correct torque did not lower the resistance to an acceptable value, disassembly and investigation is required.
Why So Picky?
The idea behind the tight resistance tolerance is to ensure the highest possible connection integrity and thus, the lowest power loss between cells/units. For a given geometry of connection, such as the many inter-cell connections on a large UPS battery system, the distribution of resistance values between cells can be observed to be very tight; as little as two to three micro-ohms. High resistance is a power robber and heat generator. As mentioned in the introductory paragraph, if connections are poor, connections will get exceedingly hot during discharge. This goes for the recharge cycle as well.
Down the Road
Connection resistance needs to be measured, recorded and trended over the life of the battery to ensure values remain within acceptable limits. A complete set of readings should be taken annually and compared with the baseline values on record from the date of installation. When they become too high, corrective action is required to get them back in line.
When you need the battery, you need the battery. You can't go the distance with too much resistance.
About the author
Rick Tressler, LLC is engaged in the business of developing and delivering high quality comprehensive technical training and educational seminars and related technical services for stationary battery users and service companies. With more than thirty years in the stationary battery industry, Rick possesses the experience and knowledge necessary as well as on-the-job the skills needed to deliver these services to the target audience. Training curricula focus on a mix of classroom and hands-on experience relating to the maintenance, testing, installation and operation of stationary batteries employed in a variety of applications. Customers include, but not limited to those in the electric utility, telecommunications, data centers as well as the oil and gas industries.
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