VOLTCRAFT BT-4 Mode D'emploi page 34

If "Err" appears in the digital display when performing the load
test, the battery tester is defective or the battery voltage is too
low.
After having performed the load test five times in a row, the
digital display will display "OL-L" if pressing the LOAD button
one more time. In such a case, the terminal clamps must be
disconnected from the battery and then be reconnected, as the
battery tester will not work otherwise.
If the temperature of the battery tester is too high, the digital
display will display the internal temperature of the battery tester
in ° C. The battery tester will not be ready for use in such a
case. Once the battery tester has cooled down, it will be ready
for use again automatically.
Effect of cold temperatures
Due to the chemical composition of car batteries, test results for a
cold battery will be lower than those for a warm battery. If the internal
temperature of the car battery is too low (below 4.4 ° C), such effect must
be compensated.
For example:
The cold start current of the car battery is 800 CCA and the internal
temperature is approx. 1.6 ° C. That accounts for approx. 70% of the
cold start current. 800 CCA x 70% = 560 CCA. See picture below. To
compensate for the temperature difference to the CCA value, refer to the
following picture that shows the approximate values.
Conduct the measurement based on this percentage value
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Internal temperature of the car battery ° C