Thursday, Mar 27, 2014 at 15:42
Hi
In my Troopy with a temperature compensated alternator that starts at 13.9v and after the motor is hot reduces to 13.6 volts.
Toyota engineers have selected these voltages to best suit the charging rate and life expectancy from batteries under high bonnet temperatures in vehicles such as mine.
The following is an extract from Solar Professional’s website – similar information is available from other battery professionals.
“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.”
“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”
“Temperature Compensation Formulas
The most widely used temperature compensation formula is:
-0.005 V per ºC per 2 V cell”
End of Solar Professional quote
If you don’t like doing the calculations go to battery websites such as Power Stream and read directly from their tables.
My diesel has an under bonnet temperature of around 80 degrees C.
Once up to temperature I have measured my batteries electrolyte above 60 degrees C.
Examples using a nominal charge voltage of 14.5 v at 25 deg C
At 65 deg C is .005 x 40 deg difference x 6 cells = - 1.2 volt
Temperature compensated charge rate 13.3 volt
At 55 deg C is 005 x 30 deg difference x 6 cells = .9 volt
Temperature compensated charge rate 13.6 volt.
I believe Toyota have their voltages set about right for the temperatures under my bonnet.
For a second opinion of under bonnet temperatures go to the Century website and look at their Under Bonnet Temperature Logging of Falcon and Commodore vehicles in
Melbourne and
Sydney.
These ranged from 73 to 85 degrees C under the bonnet with electrolytes in the range 55 to 72 degrees
For those that install voltage increasing diodes they will obtain a faster charge but have a shorter life out of their batteries - I will stick with the original Toyota voltages.
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