I have already done quite a bit of measurements of several NP-W126S batteries, either originals or 3rd party clones in my big test. However, I haven’t gone beyond the + and – terminals, while the batteries have 4 in total. It’s about time to correct this omission today!
A bit of theory
All Li-ion batteries obviously have at least two terminals: + and -. Some of them feature more than these two though, as is the case of NP-W126S, where also Ⓣ and Ⓢ are present. What are they for? Generally speaking, other terminals give more information about the battery, its temperature, charging state, capacity and possibly even other more detailed information. They can be provided as an analog value (usually measured as a resistance between the terminal and the ground (-) terminal) or as a digital value in case the battery employs a circuit called Battery Fuel Gauge, which is actually a small computer itself.
The provided values can be used by device to better handle the battery. Particularly during charging, it’s good to know battery temperature and in case there’s anything wrong and the temperature increases too much, charging can be terminated in order to prevent problems. And we are talking about some pretty serious problems in case of Li-ion batteries, including explosion or fire.
I haven’t found any documentation about NP-W126S terminals and so I tried to find out myself what are they used for.
The Ⓣ terminal
This one isn’t hard to decode, as the T usually stands for Thermistor, a special type of a resistor, which resistance changes with temperature. In case of NP-W126S it means that the resistance is about 10kΩ when the battery has roughly room temperature (20°C) and the resistance decreases as the temperature rises. It’s enough the hold the battery in hand for a while and the resistance drops to 9 or even 8kΩ.
Both the older original NP-W126 and the newer NP-W126S have this terminal with the same functionality. A bit disturbing fact is that none of the 3rd party batteries that I own has the thermistor properly implemented. They only have a fixed 10kΩ resistor, which means that the connected device doesn’t have any information about the internal battery temperature!
I’d like to know, whether there’s any 3rd party battery with a proper thermistor. If you own a battery that isn’t present in my test and have a multimeter to measure the resistance, please let me know your results in the comments below. Thanks!
The Ⓢ terminal
The S usually means Size, Status or System indicator, but what it does in case of Fujifilm batteries? I’m not sure. The resistance is 98kΩ for the older NP-W126 and all the 3rd party batteries, while the newer NP-W126S has 680kΩ. So, there’s definitely a way how to distinguish the ‘S’ battery version, but I’m not sure whether the terminal doesn’t also feature some digital interface provided by an internal Batter Fuel Gauge. To be investigated in the future.
Update: Patona Platinum doesn’t have any resistance on the Ⓢ terminal. Not sure whether it make any difference in various devices (e.g. X-T3).
Termianals usage in Chargers
The original Fujifilm BC-W126 charger supports all 4 terminals. As I tested, the Thermistor terminal is used — when it’s disconnected, the charger stops charging. On the contrary, all the other 3rd party chargers I know only provide + and – and so the battery temperature isn’t measured at all. So, if you want to play it safe, use the original charger.
X-T2 terminals usage
While older Fujifilm X system cameras, like X-T1, only have the + and – terminals, the newer X-T2 features all 4 terminals provided by the NP-W126S. So, obviously it could use them to measure battery temperature and shut off as soon as the battery starts overheating. Or, they possibly could provide more accurate battery state of charge. Or, after a firmware update, X-T2 could even reject non-original batteries (just a speculation, of course).
Are these contact actually used? I don’t know. The camera normally works even when I covered the two additional contacts by a tape. So, possibly they aren’t used yet, but are ready there for some future use.
What does it all mean?
So are actually the original batteries safer than the alternatives? Apparently they are safer when charged in the original charger. The fact that none 3rd party battery has a working temperature monitoring is quite surprising and makes one wonder, whether they are safe particularly for some heavy duty usage, like 4K video or a lot of continuous shooting. On the other hand, it isn’t clear, whether any Fujifilm camera actually does measure battery temperature in order to prevent the infamous battery swelling.
I’ll post updates in case I find anything new, so please subscribe (Facebook or RSS) in order to receive news. Until then, I can recommend the original NP-W126S in case you want to be really sure or some of the recommended brands from my battery test in order to save some money.