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I'm laying out a PCB for a NiMH single AA cell charger following the AVR450 Application Note - Battery Charger for SLA, NiCd, NiMH and Li-Ion Batteries but I'm not sure where to place the NTC thermistor that will read the cell temperature throughout the charging process.

I plan to lay out the cell holder like in the picture below. The holders are the Keystone model 92. The red arrows represent positions where I think I can place the thermistor.

Battery holder and potential places to

I wanted to place the thermistor so that it makes best thermal contact with the battery cell, but the AVR450 application note doesn't mention NTC placement at all. Looking at the board shown on the AN, the NTCs are not present either. They are external to the board as the design allows the use of different battery packs and chemistries through the use of external battery holders. I've also searched the Net but couldn't find anything relevant.

My understanding is that these tin coated holders I'm using present high thermal conductivity, and thus, will be at a similar temperature as the cell. But I don't know whether the cell temperature is uniformly distributed throughout the cell when it's charging.

So, my questions are:

  1. Where should I place the thermistor? Should I place it in contact with the negative (position A) or positive (position B) battery holder terminals? Which one? Or should I try to locate the thermistor near the middle of the cell (position C)?

  2. What should I do to ensure good thermal contact between the battery and the thermistor? Should I apply thermal paste or something like that?

  3. Is it ok to use the thermistor like the one in the picture below? Or is there a more appropriate thermistor package for my application?

Thermistor I plan to use

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    \$\begingroup\$ Smaller thermal masses are more sensitive to temperature changes. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 19 '14 at 0:22
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I guess that the obvious answer is that it doesn't really matter where the thermistor is placed as long as it is in close contact with the battery being charged, because the cell temperature will be uniform during charge.

I did search the net quite a lot for an answer, but could not find anything about thermistor placement in battery chargers at all. I could only find placement of thermistor in circuit (the schematics). The only reference I found about its physical placement was this: Build a USB Powered AA NiMH and NiCd Battery Charger.

Picture of the battery charger

The completed charger with one cell in place. The 2-cell holder was made by cutting the outer positions off of a 4-cell holder. Notice how the thermistor is installed so as to make physical contact with the cells being charged. A small heatsink keeps Q1 cool.

The red marking is where I understand the thermistor was placed, which corresponds to Position C in my picture.

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