Optimal location of temperature sensor on lithium battery cell? - Page 1
Forum section: electronics / general

author: MicroControllerGuy
author_rank: Newbie
timestamp: March 21, 2020, 03:14:59 pm
content: Hi,My internet searches come up empty on this topic, so please shed some light on this topic for me.I'm designing av BMS for my LiFePO4 battery pack. The pack consists of four 26650 size LiFePO4 cells.I will equip each of the four cells with its own temperature sensor, to monitor cell temperature during charging and use.I can think of three different locations for the cell's temperature sensor:- Half-way up on the cylindrical cell wall,- on the cell negative terminal, and- on the cell positive terminal.What is the optimal location on the LiFePO4 cell for the BMS' temperature sensors?

author: tunk
author_rank: Super Contributor
timestamp: March 21, 2020, 03:36:31 pm
content: I've opened a few laptop batteries and I seem to remember thatthat they're placed half-way up. If you assume that all heat isgenerated uniformly through the cell, then the cooling shouldbe a bit better at the ends since there's more surface area pervolume at the ends.

author: MicroControllerGuy
author_rank: Newbie
timestamp: March 21, 2020, 04:54:51 pm
content: OK, that makes sense, assuming uniform heat distribution.But is heat build-up in LiFePO4 uniform, and does the heat transfer easily to the side of the cell? If not, I can imagine the interior of the cell heating up quickly and exceeding the recommended +55 Celsius (typically) temperature limit well before the outside wall of the cell is anywhere near 55 Celsius.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: March 21, 2020, 08:59:32 pm
content: Granted that the cans are probably stainless which is a comparatively poor thermal conductor, but you might need (thermal) insulation between the cells so that you know which one is generating heat.MarkMLl

author: MicroControllerGuy
author_rank: Newbie
timestamp: March 22, 2020, 12:29:44 am
content: Thanks tunk and MarkMLI for valid points.Although the cells are packed tightly together, the area of the touching surfaces is very small, due to to the circular shape of the cell's cross-section. And each cell comes wrapped in some sort of semitransparent plastic (PVC?), so I think heat transfer from one cell to the next should be negligible, but I may we wrong.The best solution I can come up with is to press a thermistor against the side of the cell, half-way up. The legs of the thermistor will be soldered onto on a PCB, with both legs bent in a 45 degrees angle. The side of the thermistor's body (suspended in air above the PCB in a 45 degree angle) will press against the side of the cell when the cell is mounted to the PCB by spot welding of the battery tags.I could also press the thermistor against the negative terminal, but the risk of accidentally short-circuiting the thermistor (due to slight battery movement) is much higher here. Less margin for error, so not my favorite option.

author: digsys
author_rank: Unknown
timestamp: March 22, 2020, 02:41:34 am
content: Having deals with this in many EV battery packs, it isn't easy, and usually ends up just as a "backup"option. Even using a FLIR and deliberately running packs hot, it is evident that temperature sensing is very unpredictable and suffers terrible lag, for the reasons you point out. We usually put a few sensors right near the battery tab, as close as possible to where it exits the pack. That is at least the most reliable place that doesn't suffer from much lag / insulation. For more accurate temperature we run a complicated VI power curve, comparisons to all cells in the pack, 2-4 pack temperatures etc. Some of the older LiFePO4 packs took ages to show overheating, and usually already started bloating before it was picked up. They now have new generations with much less dense packaging.