Larry,
The reading with the BTS disconnected should be 25C, this would allow non-battery temperature compensated charging. It is also my understanding that the BTS is optional but recommended.
Also, I believe that are two types of BTS, one that is connected to the negative terminal of the battery and one that is installed on, or in close proximity to the battery bank. Ours is connected to the battery.
This is from a Magnum manual.
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If the BTS is installed, the charge voltage settings will increase if the temperature around the BTS is below 77°F (25° C), and will decrease if the temperature around the BTS is higher than 77° F (25° C).
The inverter/charger will shut down if the Battery Temperature Sensor (BTS) has reached a temperature greater than 54°C/129°F.
Remove the BTS from the inverter BTS port. If the BTS reading goes to 25°C/77°F, replace the BTS. If the reading does not go to 25°C/77°F, then inspect the BTS port for cleanliness and/or have the inverter/charger serviced.
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