Temperature-Compensated SLA Battery Charging Voltage

Quote of the Day

If the tiger ever stands still the elephant will crush him with his mighty tusks. But the tiger does not stand still. He lurks in the jungle by day and emerges by night. He will leap upon the back of the elephant, tearing huge chunks from his hide, and then he will leap back into the dark jungle. And slowly the elephant will bleed to death.

— Ho Chi Minh, who viewed the Vietnam War as a contest between a tiger and an elephant. In a nutshell, this quote explains how the strategy of Ho Chi Minh and General Giap defeated the US.

Introduction

Figure 1: Typical Temperature Compensated
Battery Charging Voltages for AGM and gel
Lead-Acid Batteries.

I have had a number of questions at work the last few days on the proper charging voltage for Absorbed Glass Mat (AGM) and gel Sealed Lead Acid (SLA) batteries. I regularly discuss charge profiles with UPS and charger vendors. I grabbed data at random from two good ones and plot the data in Figure 1. One of specifications I used was for gel SLAs and the other was for AGM SLAs.

Both vendors reduce their charging voltage by 3 mV/(cell·°C). The vendors do have a 20 mV difference in absolute level, which I view as a minor difference – both battery construction types should charge just fine using either voltage profile.

For the more equation-oriented among you, Equation 1 shows the formulas behind Figure 1.

Eq. 1

where

• V_{AGM Cell} is the cell voltage in V. Note that the difference in voltage between AGM and gel SLAs probably reflects the difference in vendor more than the requirements of the battery construction.
• V_{Gel Cell} is the cell voltage in V.
• T is battery temperature in °C.

To avoid thermal runaway, there is a temperature above which  charging should be stopped, but I have not seen a consensus on this value – it varies between battery vendors. I commonly see the statement that you should not charge a battery at temperatures above 50 °C , while you can discharge them at temperatures as high 60 °C. You need to consult with the specific battery vendors you are using for their recommendations.

Postscript

Figure 2 shows the charger voltage as a function of battery temperature (Source).

Figure 2: Another Vendors Approach to Temperature Compensation.