# how to calculate the temperature at the battery post?

Discussion in 'General Electronics Chat' started by Jas9, Nov 11, 2010.

1. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
hi,
i am trying to calculate the temperature of a battery negative post(where clamps are connected to the external circuit.) So how to find it?
My idea is that the temperature at the post will be equal to the ambient temperature of the air inside the hood. Is it a right prediction? If yes then this ambient air temperature will depend only on the heat dissipated by the engine???
thanks

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Something like a small thermocouple junction clamped under the connector should show the battery post temperature.

As to your predictions, only the experiment will support or disprove them.

3. ### iONic AAC Fanatic!

Nov 16, 2007
1,420
68
If your preditions are correct, and the temperature of the batteries (-) terminal IS the temp of the ambient air, then why not just sense the temp of the air? You can use a simple device such as the LM34(deg F) or LM35(deg C). But what are you trying to accomplish as your description is a bit vaigue.

Last edited: Nov 11, 2010
4. ### timrobbins Active Member

Aug 29, 2009
318
16
Jas9, what are you really wanting to measure???
- the power loss from the engine, or
- the air temp under the hood, or
- the battery internal temperature.

Oh yes, and why??

5. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
@all thanks a tonn for all your replies but i am doing through a simulation in matlab so i cant use any real temperature measuring sensors, what i have is readings of intake air temperature, coolant temperature and environment temperature....so with these readings how the task could be proceeded???
thanks

6. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
@timrobbins
I already developed a model to calculate internal temperature of a battery but now i want to calculate the temperature at the terminal of the battery that is battery post because it plays a very important role in calculating the internal temperature of the battery
Initially i had this reading of battery post temperatures which i used to calculate the internal battery temperature. But now i dont have this reading i.e. internal battery temperature and temperature at post

so in short i want to measure the temperature at the negative post of the battery where the clamps are connected
any help/ideas???
thanks

7. ### timrobbins Active Member

Aug 29, 2009
318
16
Jas9,

Is this for a battery in a car engine bay, or a battery in a more benign environment?

In an engine bay, the ambient air temperature, air flows, and convective and radiative heat flows are very fast changing and variable and dependant on the vehicle and operation.

Apr 20, 2004
15,815
282

9. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
A lead-acid battery is quite a large thermal mass. If it is being charged or discharged rapidly due to high current, it's internal temperature will rapidly increase due to the power dissipated in the battery. Increased temperature leads to increased chemical activity.

You might get close to the internal temperature by taking the ambient temperature, and adding the power dissipated due to charge/discharge currents, multiplying by the battery internal resistance and adding roughly 12% to 17%; as charging/discharging a lead-acid battery is only about 75% efficient.

The ambient temperature affects battery internal temp a lot less; as the thermal mass is large compared to the external dimensions, and the plastics used are not really good at conducting heat.

For monitoring battery internal temperature it's recommended that the temp sensor be connected to the battery positive terminal. I'm not quite certain why that is, except on top-post batteries, the positive terminal is somewhat larger than the negative batteries.

You could use an LM34, LM35, thermistor, or even an amplified diode drop to monitor the actual temp on a real battery.

10. ### timrobbins Active Member

Aug 29, 2009
318
16
Sgt, You're right, there is usually a little more lead associated with positive plate in lead-acid battery, due to different corrosion mechanisms on the pos versus neg plates, and the need to keep a balance in active material throughout the service life of the battery. But to a first-order, both terminals have equivalent terminal metallic cross-section and group bars down to the plates, and so either terminal is fine to get the 'best' estimate of an 'average' internal battery temperature.

Ciao, Tim

11. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
thanks a million for all the answers but as i said i want to calculate the temperature at the post that is the negative terminal not the battery internal temperature. I have already calculated the internal battery temperature. Now my concern is to calculate the temperature at the post of the battery that is where the clamps are connected to draw the current.
I cant use the internal temperature of the battery to calculate the post temperature because of some restrictions. The inputs that i have for this temperature estimation is the coolant temperature, engine speed, vehicle speed, intake air temperature, environment temperature. So my idea was that the terminal temperature would be related to the ambient temperature in the car hood and also the amount of heat released by the engine. But as i am not from mechanical background its difficult for me to calculate the amount of heat released by the engine.
Actually when the car is idle at that time the temperature of the battery terminal post is almost same as that of the intake air temperature. But as the engine starts the temperature of everything inside the hood will change.
I am trying to develop a simulation model in Matlab/simulink so i need help of you all to guide me in some way or other.
thanks a lot for all your postings

12. ### iONic AAC Fanatic!

Nov 16, 2007
1,420
68
Bottom line, I do not think there is much correlation at all between the batteries negative terminal and the ambient air. There are way too many variables to measure and consider just for that one assumed correlation. I'd say that the terminal is far more related to the internal battery temp then the ambient air. I know my battery is actually housed in a plastic compartment to protect from the elements such as dirt, moisture, and perhaps inadvertainly, the ambient temp around the engine.

Even a direct measurement of the ambient temperature somewhere under the hood is nearly as plagued with uncalculatable variables as the battery terminal temp and ambient temp correlation. The variables are ever changing depending on a multitude of conditions that it would make a mockery of science for someone to attempt(maybe slightly overstated).
It would be rocket science for sure. You need not only a mechanical and electrical backgroung, but a strong foundation in Physics to boot.

physicsforums

Last edited: Nov 13, 2010
13. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
yes you are right but lets see how it will be proceed anyways thanks again for your post

14. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
You may find that the quality of the connection between the terminal post and the cable clamp or screw connection will have a significant effect as the electrical loads change. It is somewhat difficult to obtain a really good lead-to-lead contact that does not degrade somewhat over time. A small resistance in the contact will result in a higher than expected temperature.

A second sensor recording ambient in the vicinity of the post might be a means to determine if the post reading seems reasonable, although ambient in the area under the hood might seem to be a very variable state.

15. ### retched AAC Fanatic!

Dec 5, 2009
5,201
312
Using a sealed container, and placing the temp-probe inside will give you an average ambient temperature without the sharp changes from wind, and or current changes.

The air in the container will take a few moments to change so it will stay at the average temperature.

A film canister, or a "ziploc" type container should do fine.

Place it on the battery at the negative terminal.

That will give you the average ambient temperature AT the negative terminal without including the internal temp FROM the negative terminal.

16. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
thanks again but the thing is i am restricted to use sensor so the only things that i have is the intake air temperature, vehicle speed, engine speed, environment temperature, exhaust gas temperature so i am trying to find out only from these available given quantities

17. ### iONic AAC Fanatic!

Nov 16, 2007
1,420
68
Honestly, I think the assumption you are making is flawed, especially to question/assumption two.

But the only available data you can rely on to make the calculation is:
1) coolant temperature
2) environment temperature
3) intake air temperature
4) vehicle speed
5) engine speed
6) exhaust gas temperature

Is the "environment temperature" a different variable or the one you are trying to solve for, also known as the ambient air temperature?

Didn't you mention that you also know the battery internal temperature?
What known vlues did you use to obtain this temperature?

iONic

18. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
well i am trying to calculate the post temperature without the internal battery temperature and also the environment temperature which i said is given is actually the temperature of the environment outside the hood so internal temperature that is inside hood is the ambient air temperature which i am thinking of calculating first

19. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
I vote an impossible task. The closest I would think would be engine coolant temp, but that would be only valid on a windless day at the stop light.

I second beenthere's suggestion of voltage drop across the connection contributing to heat generation.

How would you know if your even close, other than to measure it. I'm a visual kind of guy so the first thing I'd do is chart your various inputs against actual and look for correlations.

20. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
thanks for your reply but i have a readings of this post temperature to which i will compare with my calculated data. So that not the problem of knowing whether the calculated values of mine are right or wrong.
thanks