Understanding and Optimizing Battery Temperature Compensation
Inside this Article
Most PV designers and installers who work with battery based systems know that temperature compensation is important. Without it, battery banks are not optimally charged, resulting in poor performance and decreased operational life. However, many installers are not familiar with some of the technical subtleties related to temperature compensation, appropriate charger setpoints and the importance of verifying temperature compensated voltages.
Battery Temperature and Charging Voltages
Battery charging voltages should be corrected based on battery temperature. This adjustment is referred to as temperature compensation, a charging feature that helps ensure that a battery is neither undercharged nor overcharged regardless of battery temperature. All chemical reactions are affected by temperature. Battery charging is an electrochemical reaction, so it too is affected by temperature. Specifically, cold batteries require a higher charge voltage in order to push current into the battery plates and electrolyte, and warmer batteries require a lower charge voltage to eliminate potential damage to valve regulated lead acid (VRLA) cells and reduce unnecessary gassing if flooded cells are used.
Using normal target voltages to charge a battery that is colder than approximately 25ºC (77ºF) will result in an undercharged battery, which will deliver lower performance, reduced life and a higher life cycle cost. Applying normal target voltages to a battery that is hotter than 25ºC may result in an overcharged battery. This condition could lead to the drying out of VRLA battery cells. With flooded cells, the result will be excessive outgassing, increased battery maintenance in the form of more frequent watering and reduced battery life due to thermal stress. In fact, some battery manufacturers and charger manufacturers recommend not charging a battery that is 50ºC (122ºF) or hotter.
In the past, it was common to manually increase a charger’s target voltages to charge a cold battery during the winter or to manually decrease the target voltages to charge a warm battery during the summer. Some early chargers included switches to adjust target voltages over coarse temperature ranges. However, target voltages in many chargers, such as automotive chargers and those built into generators, usually cannot be adjusted. Today, the prevalent solution featured in most photovoltaic charge controllers and inverter/ chargers is automatic temperature compensation. A remote battery temperature sensor (RTS) is attached to one battery in the bank, and the charger uses electrical data from the RTS to determine battery temperature and automatically adjust the target charge voltages accordingly.
Since charging efficiency is less than 100%, batteries typically warm up as they are charged. Fluctuations in ambient temperature also cause battery temperature to change. Automatic temperature compensation allows chargers to dynamically adjust target voltages during the charging process as battery temperature changes and ensures optimal charging. For example, the target absorption and float voltages for a set of cold batteries on a cool morning might be relatively high. However, in response to changing RTS data, the charger will automatically decrease the target voltages as the batteries and ambient environment warm up.
Temperature Compensation Formulas
The most widely used temperature compensation formula is:
-0.005 V per ºC per 2 V cell.
An equivalent variation is:
-0.028 V per 10ºF per 2 V cell.
Some batteries, such as those manufactured by Concorde, have more complicated temperature compensation formulas. The formulas listed here work well enough over the temperature range that batteries are subjected to in most photovoltaic applications. Because each formula includes a negative multiplier, the slope is negative— as battery temperature decreases, charging voltage is increased and vice versa. Since each formula takes battery voltage into consideration (more 2 V cells in series equals higher voltage), the slope is steeper for a 24 Vdc nominal battery bank than for a 12 Vdc nominal bank, and steeper still for a 48 Vdc nominal battery bank.
To understand and apply these formulas, you need to know three parameters specific to a given system or equipment: charger reference temperature, battery temperature and nominal battery voltage. The typical reference temperature for most chargers is 25ºC. If the RTS value indicates the battery is below 25ºC, the charger increases the target voltages, subject of course, to system capability. If the RTS value indicates the battery is above 25ºC, the charger decreases the target voltages. Why 25ºC? Batteries generally exhibit their optimal combination of energy storage (amp-hours: AH) and life cycles at about this temperature.
Nominal battery voltage is determined by how many cells make up a given battery bank. Virtually all lead acid battery cells are 2 Vdc nominal. Therefore, individual batteries or strings of batteries will have the following nominal voltages based on the number of cells:
6 cells = 12 Vdc
12 cells = 24 Vdc
24 cells = 48 Vdc
Using the above formula, the correct temperature compensation for a charger with a 25ºC reference temperature and a 48 V, 24-cell battery bank operating at 10ºC (-15ºC from the charger’s 25ºC reference) is:
-0.005 V x -15 x 24 = 1.8 V
If a manufacturer’s recommended absorption voltage for a nominal 48 V battery bank at 25ºC is 59 V, then the temperature compensated absorption voltage for that battery bank at 10ºC would be:
59 V + 1.8 V = 60.8 V
Temperature compensation applies to float mode target voltage as well. For example, if a manufacturer’s recommended float voltage for a nominal 48 V battery bank at 25ºC is 54.8 V, then the temperature compensated target float voltage for that battery bank at 10ºC would be:
54.8 V + 1.8 V = 56.6 V
The application of temperature compensation to a charger’s equalization (EQ) mode varies among charger and battery manufacturers. Check if your battery manufacturer recommends EQ at all, since most VRLA battery manufacturers do not recommend regular EQ cycles. If EQ temperature compensation is recommended, and if the charger includes a temperature compensated EQ mode, system specific settings may be required, which are beyond the scope of this article.