Proper Float Voltage Selection

This month's blog addresses the topic of float voltage (float charge) selection for stationary batteries. In addition to setting this important operating parameter's actual value, I'll also discuss the best way to measure it and why. There are misunderstandings on both topics. I'll address this as it relates to lead-acid batteries employed in standby (full float) service such as UPS, telecommunications, switch gear and control, etc.

First, let's officially define float voltage, aka, float charge. According to IEEE 1881-2016 (IEEE Standard Glossary of Stationary Battery Terminology) float charge is defined as "A constant-voltage charge applied to a battery to maintain it in a fully charged condition, while minimizing degradation or water consumption." This operating parameter is important to proper operation and goes a long way to helping the battery meet its expected service life in a given application. It is too often not given the attention it deserves.

Second, there is no "universal" float charge voltage that can be used for all batteries. Grid alloy, specific gravity rating and operating temperature are key factors that determine the voltage to be used. Therefore, it is important that the user knows the make and model of their battery has access to a copy of the operating and maintenance instructions.

I'll use a popular battery to serve as an example; a vented lead-acid (VLA) type, employing calcium grids with a rated specific gravity of 1.215 at 77 degrees, F. For the example battery I'll use the C&D DJ series. It is employed in a range of applications including utility and switch gear. The reference document is C&D's RS-1476 (12-800) Standby Battery Vented Cell Installation and Operating Instructions, which can be found on their website.

The DJ battery is covered in Table 5, with an excerpt illustrated below. Since this battery is used in switch gear and control applications, the second line for 1.215 specific gravity applies. An acceptable range of 2.20-2.25 VPC (volts per cell) is recommended for an operating temperature of 77 degrees, F. Temperatures other than this require compensation of the float voltage using a slope of 2.8 millivolts per cell per degree F. Float voltage is increased for lower temperatures and decreased for higher temperatures.

It is noteworthy that battery manufacturers live in the world of volts per cell (VPC) as the number of cells for a battery vary widely across numerous industries ranging from a few to hundreds of cells that make up a battery system. The manufacturer has no way of knowing how many cells a user has, so it is up to you to make the calculation. A user with a 60 cell bank of DJ cells would apply a float charge of 132 to 135 volts when operating at 77 degrees, F. Simply multiply the VPC required by the number of cells in the battery to arrive at this figure.

Now armed with knowing what the float voltage should be for your battery, the next question is, where and how should it be measured? Battery float charging equipment is generally equipped with an output volt meter. They can be terribly inaccurate and should not be trusted unless calibrated periodically. Additionally, there can be voltage drop between the charger and the connected battery when the battery is in recharge mode, drawing current which results in voltage drop after an outage. Otherwise, there is no voltage drop between the two when the battery is fully charged. That said, inaccuracy can still be a concern. Therefore, float voltage should be measured directly at the battery main positive and negative terminals with a calibrated digital volt meter (DVM).

From a maintenance standpoint, float voltage should be measured and recorded on a routine basis; typically once per month, to verify it is correct. If it is not, adjust the charger to bring it into proper specification. Chargers are typically well regulated and require infrequent adjustment. However, failure and drift do occur on occasion which is why periodic checks are highly recommended.

Your battery is designed to be float charged over a fairly narrow range. Using the correct voltage maintains plate polarization within design parameters which in turn, along with proper maintenance and testing, helps extend the overall useful life. Over/under floating a battery, especially for months or years at a time is a sure-fire way to cut down life and reliability, resulting in premature replacement, unnecessary expense and potential loss of emergency power.

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