Cell float voltage measurements – what’s good vs. bad?

Welcome to the February blog. This month’s edition addresses a common question I receive in training classes. A student asks, “I just took a set of cell float voltage readings on my UPS battery. What’s a good reading and what’s a bad reading? We could use some help on this.” Not only is this a common question but is a very good one. Somewhere in the maintenance process, one must review the readings and make that determination.

A recommended source of good information, in most cases, is the battery manufacturer’s installation, operating and maintenance manual, also called the IOM. Table 9.1, clipped from Enersys’ IOM for its vented lead-acid batteries provides initial guidance.

The example I’ll use is for a high gravity (1.250), lead calcium cell frequently deployed in UPS and other applications. First, let’s review the recommended float voltage on a per-cell basis; the world in which the battery manufacturers live. Users frequently comment, “Wait a minute, I have a 60-cell battery. Why don’t they just tell me what the battery string voltage should be?” There’s a good reason for that. The battery maker has no idea how many cells may be installed. They live in a volts-per-cell (VPC) world as batteries have a wide-ranging cell count based on the connected system requirements. Users must calculate VPC values to battery voltage.

Figure 1

Figure 1

First, you need to look at the recommended float range. For the example 60 cell battery, that’s 2.21-2.30 VPC, or 132.6-138.0V. With this established, you need to determine whether the battery is being float charged within the recommended range. Float voltage should be measured at the battery main (+) and (-) terminals using a calibrated digital volt meter. Don’t use charger panel meters as they can be inaccurate. Use caution, being certain to use appropriate PPE and possess the necessary training. The battery is 77°F., therefore, temperature correction of cell voltage is not required.

Armed with this information, you can now review the individual cell float voltage readings. What do you see? Cell voltages normally are very close to one another when correctly float charged and operated within the manufacturer’s recommended temperature range across the system. That’s 5°F, per IEEE 450-2010.

The published minimum critical cell voltage is 2.14V according to Table 9.2, in Figure 2 below. Generally, cell voltages above 2.30V are not concerning. With more low cell voltages occurring in the battery, higher ones will show up. The low cell voltages are the potentially troublesome ones. From the maintenance perspective, cell voltages along with other data, should be tracked and trended over time. This is essential. Cell voltages usually do not drop from normal to critical overnight. They should be tracked and will demonstrate a decrease over time in many cases when an internal problem develops.

Figure 2

Per the manufacturer, an equalize charge may be indicated to correct a particularly low cell. This may not prove successful in all situations. A single-cell charge may be required to focus higher voltage and current to recover it. If this is also unsuccessful, cell replacement will likely be required.

Rick Tressler provides battery training and education services to technicians, electricians, installers, and maintainers and others working with stationary battery systems. For information on classes, seminars and technical support, check out the website at www.ricktressler.com or contact Rick at rick@ricktressler.com, 614.632.7521.

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