# Battery Thermistor Wires

have the battery. just fully charged it. what is the small set of blue wires for?

I’ve moved this comment to its own post, since it wasn’t super related to the topic it was in.

They’re for the thermistor in the battery, in case you want to monitor its temperature (e.g. while charging and/or operating). That isn’t a plug and play functionality with our battery charger or vehicle electronics, but it’s there in case someone wants to use it, so that the battery doesn’t need to be disassembled to add a temperature probe later.

If you have access to a multimeter you can measure the resistance between the two wires and use it with the conversion equation:

\begin{align} \frac{1}{T_{Kelvin}} &= \frac{1}{T_0} + \frac{1}{\beta}\ln\left(\frac{R}{R_0}\right)\\ \rightarrow T_{Kelvin} &= \frac{1}{\frac{1}{T_0} + \frac{1}{\beta}\ln\left(\frac{R}{R_0}\right)} \end{align}

Given the technical details of R_0 = 10000\Omega\ \pm 1\%, \beta=3435 K, and T_0 = 298.15 K that ends up as

\begin{align} T_{Kelvin} &= \frac{1}{\frac{1}{298.15} + \frac{1}{3435}\ln\left(\frac{R}{10000}\right)}\\ \rightarrow T_{Celsius} &= \frac{1}{\frac{1}{298.15} + \frac{1}{3435}\ln\left(\frac{R}{10000}\right)} - 273.15 \end{align}

As a sanity check, an input (measured) resistance of 10000\Omega results in the expected output of 25℃, and since an NTC thermistor’s resistance should be inversely proportional to temperature we can check that a measured resistance of 11000\Omega is ~22.55℃, which is indeed below 25.

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