HD100 Battery
 

I have a simple question for which I never managed to find an answer. The Battery which came with the camera has 3 terminals. 1 is the +ve, 1 is the -ve, and thats ok. But there is a 3rd terminal in between with a letter T. What is this terminal for?

The 'T' pin is the thermistor safety pin (Smart Battery Charger Interface).

The battery control electronics uses it to signal the camera whether it is okay to charge the battery or not. It cuts off the charge if the battery is too cold or hot. In layman's terms, it's basically a kind of electronic thermometer.

The same thing is found on mobile phone batteries as well. Hope this helps.

Thanks for your explanation, it was quite clear. So from what I understood the T pin is used only while the battery is charging. But the HD100 battery does not charge from the camera, but still on the camera side there is a pin to connect with the T pin of the battery. Why is that for? I want to understand this in detail because I am making a bracket myself which connects to the camera and then 2 batteries connect to this bracket. Is this case is there a need for me to take care of the T pin in the bracket? As for charging, the batteries would still charge independently on the charger supplied with the camera. Thanks for any help given.

I have no experience using a battery without the T-pin connector so I cannot confirm whether the internal circuitry will work or not. However, if i have to guess, I think it should, but it might affect the way the LCD display works.

To answer your first question, all I can venture is a guess. The camera must protect itself from faulty batteries and it probably monitors the internal temperature of the battery while in operation. Even though the voltage supplied might be within acceptable limits, the battery might be operating under a dangerous temperature. The charger, too, for that matter, will monitor the battery temperature while charging.

Hope this helps.

I'm guessing a little too :) but most batteries hold a charge differently at different temperatures.
The camera may well use a combination of actual voltage and battery temperature to come to a reading of the remaining capacity- and the charger likewise, when charging. At least- the inverter we run our house from does this- gives you the actual voltage (the simplest measure of remaining capacity), but also the temperature compensated voltage, which in very hot or cold weather can be quite different- and we use the later for calculating when the batteries are fully charged.
(But these are deep-cycle lead-acid batteries, and have a different chemistry to NiLi or NiCads)