Monitoring SLA Battery Temperatures

The lug is not recommended for temp measurement of batty temp.

Time lag is too great. Lug Mass is too great. Heat loss to air is unpredictable.

Insert the temp element into each batty cell individually measure each cell and collect representative amount of data until data stabalizes. A thermistor is not recommended because of the influence of the cell voltage.

Insert a tc well into each batty cell. The tc well protects the tc and cell while allowing easy removal of tc. Do not use type K tc. Select tc type for temp range and accuracy reqmts.

I agree with you that inserting the sensors is the best way. For a very simply applicable sensor solution you can use solid state sensors like AD590 in a miniature version. They are supplied with an unregulated DC voltage and give 1µA/°K as a current source. You can put them easily in series to give the minimum temperature or in parallel to give the average temperature represented by the current flowing through a resistor which you can choose according to your voltage measurement range.
Regards Uwe

While I agree with Cornstoves that mounting the sensor on the battery lug is not ideal, in practical terms it is as good as any.

I install battery chargers which are temperature compensated. Invariably these chargers come with a battery temp sensor which the manufacturer recommends be placed on the negative post.

Some manufacturer provides a sensor that fastens to the case using double sided adhesive tape. The lug mount appears to be a better bet because the lead metal busbars connecting to the inside plates is fairly good at conducting heat compared to the plastic case. Depending on construction of the battery, there may be an air gap as well as considerable plastic comprising the case. This tends to slow down heat conductivity from the inside to the exterior compared to the electrical bus bar.

It has been my own observation that the end of the battery closest to the negative connection will become warmer sooner than the end connected to the positive cable. Of course once the battery heats up during fast charging, the temperature soon stabilizes and the whole battery reaches the same temp.

Two additional reasons for using the negative rather than the positive post. Another reason has to do with positive post corrosion in flooded cells associated with continual trickle charging. This is not as much of a problem in SLA as in flooded batteries. A third reason is that the DC electrical system is generally of the grounded negative polarity type. Any metal cased temp probe is therfore mounted to a ground potential lug instead of a lug at the maximum potential.

FWIW, most manufacturers use a wire cable lug as a protective and convenient mount for the temperature probe. The active sensor is epoxied or glued into this cable lug using a thermally conductive epoxy or glue. I have never seen an instance where the sensor is electrically connected to the lug. The potted method seems to do a good job of isolating the sensor and mounting lug.

Some manufacturers use a large flat heat sink which is adhesively mounted on the top or side of the battery case. Some kind of plastic cover protects the active electronic sensor from mechanical damage. Case construction often make this mounting style a problem of finding a good placement that picks up on the hottest point in a battery compared to the battery lug approach.

Hank Wheeler wrote:

I want to attach directly to the terminal (the negative terminal seems to be the best choice, thanks to elnav's comments). The terminals are spade terminals, so I need to find a low cost thermistor that can attach to the spade terminal. <snip>

REPLY: that configuration does pose a problem. Bolted connections provide a secure and usually adequate contact surface area. Spade connections by comparison have negligble contact area. In addition, spade connections like you illustrated are prone to over heating due to the charge current and the limited contact area.

This would skew your temperature sensing. For this particular application I would recommend using a temperatuer sensor hat is placed directly against the battery case.

Perhaps you can use the hold down strap to press the sensor against the case.