Questions on some things on MB document.

1992 300D 2.5 turbo

1. MB Maintenance Booklet, page 9.

Battery - add distilled or tab water


---> Why add water to battery?



2. Owner's Manual, page 83.

Important!

When installing new wheels the mounting bolts must be retightened after approx. 60 - 300 miles. Observe tightening torque of 80 ft. lb (110 Nm).

----> I talk to a tech in a dealer about this retightening, but he did not know it.

The water replenishes the level the acid should be at, try to use only distilled here. Be very careful in doing so as well, the sulfuric acid is known for eating holes through clothing and such.

Very true, furthermore did you know you aren't just supposed to go around in a circle, you are always supposed to go to the one across from it next. (almost like drawing a star) I can't imagine him not knowing this, it was probably more so because they don't do it. The vast majority of shops out there take their impact wrench and turn it until it stops, whether you got 100ft/lbs or 400ft/lbs of torque on it.

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

You need to add water every now and then because with the charge/discharge cycles on your battery and also with time water evaporates from the battery, which causes higher than desired density of the acid . Acid does not evaporate. That is why you need to bring water to correct level (up to the marking).

Distilled water needs to be used as tap water contains traces of elements that react with the acid.

More likely is that the water is hydrolized into hydrogen and oxygen that escape as gasses. THat's why every once in a while a battery explodes.

Newer batteries have calcium in the plates to minimize the hydrolization, but still a small amount of water sometimes needs to be replaced.

Retighten the wheel lugs after driving on them for a while? I knew that but I thought they were to be checked after 400 to 500 miles. I have found some to be loose after 500 miles so the theory is valid.

P E H

Dear Mr. Haiges:

The proper terms are "electrolyzed" (not "hydrolyzed") and "electrolysis" (not "hydrolyzation").

At Cathode(-):
Pb + SO4(-2) = PbSO4 + 2e(-)

At Anode(+):
PbO2 + SO4(-2) + 4H(+) + 2e(-) = PbSO4 + 2H20


Or the overall reaction can be simplified as:

PbO2 + Pb + 2H2SO4 <----> 2PbSO4 + 2H2O


Water H20 is needed for the ionization of sulfuric acid H2SO4 while water itself is also partly electrolyzed (aka. decomposed) by electric current into H2 and O2 (which in liquid form are the fuel stored in the multi-stage boosters of NASA shuttles!, very explosive stuff).

I believe that the water loss in auto lead-acid batteries is mostly due to evaporation (through vents) rather than to electrolysis.

As you can see from the simplified reaction, the chemical reactions occurring in lead-acid batteries are completely REVERSIBLE, which means that lead-acid batteries will theoretically last forever, as long as you replenish them with enough DISTILLED water and electrical charge (prolonged discharge will cause complete and irreversible sulfation on the lead plates' surface). However, in real life, minerals and chemicals in not-so-distilled water (causing corrosion), sulfation due to discharge, unchecked loss of water, extreme vibrations etc... all are factors reducing the working lifespan of auto lead-acid batteries.

Eric

Dear Jassper:

Every liquid does evaporate, including any acids (not to mention that molten metals such as extremely hot iron in liquid form would also evaporate if you keep subjecting it to higher temperature).

OK, let's refresh our physics knowledge:

Remember that the statistically average kinetic energy of a molecule or atom is E(K) = (3/2)*(k*T) where k is the Boltzmann constant=1.3806 × 10(-23) m2 kg s-2 K-1, and T is the absolute temperature in Kelvin degree temperature:

T(in Kelvin) = T(in Celsius) + 273.15
= (5/9) * [T(in Fahrenheit) - 32] + 273.15


The higher the absolute temperature T(in Kelvin) is, the higher the kinetic energy. When T reaches some high enough threshold, the molecules or atoms acquire so much kinetic energy that they will be able to break away from their surrounding siblings and jump out in vapor form.

Eric

Eric,

You are correct, I used the wrong word for the right idea. If it was just water evaporating from the battery, there would be no explosive gasses present.

If your evaporation theory is correct, what did battery manufactures change to make the newer batteries require much less water than batteries of 30 years ago?

P E H

Well if you mean the concentration of sulphuric acid I would think it'd be to resist freezing even more. If you're referring to the overall quantity of acid, the newer batteries are more efficient (dense) than the ones 30years ago and need less acid.

I believe the battery has to have a short in the first place in order to facilitate electrolysis. I used to know the number, but I'm guessing with about 1.7volts it can take place.

I don't understand why all batteries aren't maintenance free though, what is the perk of being able to put water in with the cost of letting it out in the first place? The Mercedes battery is the only I've seen you can add without it having vents to let the water out.

I've found a not very safe way to shock back 99% sulfated batteries. If it'll take just milliamps of current I can eventually work it back, but it won't have the capacity it did new. My dad has a problem with shelf life on batteries so I tried this out before and it has worked almost every time!

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

"Every liquid does evaporate, including any acids (not to mention that molten metals such as extremely hot iron in liquid form would also evaporate if you keep subjecting it to higher temperature).
"

Dear Eric,

of course anything can evaporate. I was rather referring to 'normal use' temperatures, where, in a battery (accumulator) you only lose water and don't have to refill acid because acid, under normal use, does not evaporate. My advice to a question thus was to refill distilled water only and no acid.

On the other hand, should it be a custom made vehicle used in exploration of active volcanoes, or simply a vehicle that happened to fall in to an active volcano, you would be correct and everything would melt, including a battery. And driver.

Also, according to quantum theory, it is no use to have a battery in car anyway as the atoms of the battery will travel through the atoms of the car so in the end you'll end up having the battery on the ground of your garage. And your car will travel thru the garage's walls. Only if you give it time.