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Battery Temperature and Charging Voltage Compensation

A project log for ARDUINO MPPT SOLAR CHARGE CONTROLLER

An Arduino based Solar MPPT charge controller.

Debasish DuttaDebasish Dutta 09/16/2015 at 09:450 Comments

All chemical reactions are affected by temperature. Battery charging is also an electrochemical reaction, so it too is affected by temperature. As the battery gets warmer, the gassing increases. As the battery gets colder,it becomes more resistant to charging. So 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.

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.

Temperature Sensor :

To monitor the battery temperature a DS18B20 1-Wire digital temperature sensor from Maxim IC will be used. It can measures temperatures from -55°C to +125°C. Fahrenheit equivalent is -67°F to +257°F with ±0.5°C accuracy.

We choose a waterproofed version of the DS18B20 Temperature sensor. It is quite handy to measure something far away, or in wet conditions.

You can see it here

You can see the Data Sheet

Temperature Compensation Formula:

minus 0.018 volts per degree Celsius

Example :

1. Let the battery temperature is 0 degC and float voltage is 13.8V ( STC : at 25degC )

Compensation Required = ( 0 - 25) * ( - 0.018 ) = + 0.45V

Compensated Float Voltage = 13.8 + 0.45 = 14.25 V

2. Let the battery temperature is 35 degC and float voltage is 13.8V ( STC : at 25degC )

Compensation Required = ( 35 - 25) * ( - 0.018 ) = - 0.18V

Compensated Float Voltage = 13.8 - 0.18= 13.62 V

So cold batteries ( example-1) require a higher charge voltage in order to push current into the battery plates and electrolyte, and warmer batteries( example-2 ) 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.

Reference : http://solarprofessional.com

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