weird battery load test

[friendly_jacek]

I have almost 3 year old JC battery in RAV4 that I considered dying because voltage would drop to as low as 12.2V after sitting unused for a couple of days. Mind you, I checked for abnormal parasitic drain and there is none.

Today, I decided to put some load on it and turned ignition and lights on without starting the engine. The voltage under load went down to 11.74V and stabilized there, the lights were bright and everything worked fine. After a few minutes, I turned it off and voltage slowly returned to 12.4V, higher than before the test!

I couldn't believe it and repeated the load cycle again. Again, the voltage recovered to even higher 12.43V.

The battery tests weak for its size (10.3V) on my small, 50A load tester.

What would cause voltage to go down with sitting and improve with load?

Thanks!

 

[oilboy123]

Batteries can make a lot of weird things happen when they are not in good shape. They can make you chase your tail if you let them......

 

[hemitom]

To put it in simple terms when you turn your lights on or an other type of load to a cold battery you activate the acid or bring it to life so to speak. so on a battery with voltage like yours it will give a higher voltage reading . I hope this makes sense to you.

 

[friendly_jacek]

Originally Posted By: hemitom
To put it in simple terms


I would prefer complex terms, please.

 

[hemitom]

The molecular structure of the acid changes and moves more rapidly generating more voltage. just like boiling water. mind you on a near dead battery it makes no difference.

 

[JHZR2]

If it warmed up, impedance would drop and the voltage could rise.

 

[George7941]

Originally Posted By: JHZR2
If it warmed up, impedance would drop and the voltage could rise.


The internal resistance will drop when warmed up but that should have no effect on OCV.

 

[friendly_jacek]

But, I was talking about voltage under no load, so impedance plays no role here.

 

[George7941]

Some things I don't have an explanation for. I have a perfectly functioning battery in my truck and its fully charged OCV is only 12.4v.

 

[Silk]

That's normal, all batteries do that....if they are good ones. In my Lada Samara owners manual it said in extreme cold conditions to turn the headlights on for at least 30 seconds before starting the engine - for the exact reason you have discovered by accident.

 

[kr_bitog]

Measurement of battery voltage is also dependent on temperature, the standard measurement is done on 20C, so after the battery used or charged, the temperature may varies too due the temperature changed.

 

[Tegger]

Originally Posted By: friendly_jacek
Today, I decided to put some load on it and turned ignition and lights on without starting the engine. The voltage under load went down to 11.74V and stabilized there, the lights were bright and everything worked fine. After a few minutes, I turned it off and voltage slowly returned to 12.4V, higher than before the test!

I couldn't believe it and repeated the load cycle again. Again, the voltage recovered to even higher 12.43V.

You're doing your battery test all wrong.

Here's the proper way:
Key OUT, engine OFF,
1) let the car sit for several hours, so that the engine will cool to the same temperature as its surroundings;
2) turn headlights ON HIGH BEAM for five minutes;
3) turn headlights OFF, then wait ten more minutes;
4) using a VOM across the battery's terminals, read the battery's voltage;
5) observe the air temperature where your car has been sitting.

Now click this link:
http://jgdarden.com/batteryfaq/SoC.xls
Clicking the link will download Bill Darden's State-of-Charge Excel file (SoC.XLS), which contains a voltage/temperature table.

In the table, find the correct voltage for the air temperature you observed. If the two match, your battery is just fine.

More excellent battery information at Darden's outstanding BatteryFaq, here: http://www.batteryfaq.org

 

[friendly_jacek]

Tegger,
Actually the test I did was very close to what you posted above.

1. Car was cold (ambient temp at 70F) and not driven for over 24 hrs.
2. Ignition and headlights were on for several minutes without engine started (I cannot turn headlights with ignition off).
3. Then I waited a couple of minutes.
4. Measured open circuit voltage.
5. The ambient temp was unchanged.

Using the table from your link, SoC was 75% after the load exercise (12.44V @70F).

However, it was only 50% before (12.23V @70F). That's the part I don't understand.

Battery heating internally should be negligible, even if we assume 30F jump, that would only produce 0.01V difference between 70 and 100F according to the table.

 

[friendly_jacek]

Originally Posted By: Silk
That's normal, all batteries do that....if they are good ones. In my Lada Samara owners manual it said in extreme cold conditions to turn the headlights on for at least 30 seconds before starting the engine - for the exact reason you have discovered by accident.


That is true for extreme cold conditions alright, but not for me today when we had a 70F/21C day.

 

[artificialist]

Southeast US? At 3 years the battery should be replaced already!

 

[Tegger]

Originally Posted By: friendly_jacek
2. Ignition and headlights were on for several minutes without engine started (I cannot turn headlights with ignition off).
3. Then I waited a couple of minutes.

Is that five and ten minutes? No.

Try again, this time being consistent from one test to the other. You cannot introduce variables into a test and then expect a consistent result.

 

[wrcsixeight]

Perhaps the load of the headlights allows de stratification of the electrolyte.

I've noticed some physical agitation of the batteries while charging seems to allow my 2 deep cycles to accept higher amps and perform slightly better the next discharge cycle.

 

[Silk]

Originally Posted By: friendly_jacek
That is true for extreme cold conditions alright, but not for me today when we had a 70F/21C day.


It doesn't matter about the temp, the effect still happens anyway, it's just a useful thing to do in extreme cold.

 

[mechtech2]

Originally Posted By: Silk
That's normal, all batteries do that....if they are good ones. In my Lada Samara owners manual it said in extreme cold conditions to turn the headlights on for at least 30 seconds before starting the engine - for the exact reason you have discovered by accident.


This may help some start some cold winter morning!

BUT! Try normal means first!

 

[Propflux01]

My poor mans test:

1. Go out to cold engine.
2. Turn on ignition (engine not running), hi beam, radio, rear defogger.
3. Wait one minute.
4. VOM across battery terminals. Should not be less than 11.5v with everything on as in step 2.
5. If less than 11.5, replace battery.

May not be scientific, but has worked for me.