Charged car battery to 15V. What H2SO4/H2O ratio to refill?

Joined
Apr 30, 2011
Messages
16
Being an idiot, I failed to verify that my chinese charger wasn't going overcharge my 12v car battery. Worse, I let it sit in float mode for a week. Well, the picture here shows why I'm here. https://www.quora.com/Is-it-true-that-all-Chinese-battery-chargers-both-overcharge-and-have-a-float-post-charge-maintenance-voltage-that-is-too-high/answer/Jason-Taylor-16 (The charger took the battery to 15 volts.) Needless to say, above about 14.4v a flooded lead acid battery will release gas. Now the battery's energy capacity seems low. I will need take it apart and refill it with water or sulfuric acid. But what I don't know is how much H2O vs. Hydrogen Sulfide was released, so I am not sure which ratio of which to add.

Now, conventional wisdom seems to be that you should not ever add any sulfuric acid to a car battery. If I knew this for sure to be true I would not be here. The problem with this idea is posts like this https://batteryuniversity.com/learn/article/health_concerns which includes this sentence: "Over-charging a lead acid battery can produce hydrogen sulfide."

Assuming I can somehow get the top off of the car battery, what's the ideal amount of water to add? What's the ideal amount of sulfuric acid to add? Is the conventional wisdom correct? How would the the exact model of the car battery come into play?
 
Too many unanswered questions here. Is the 15V shown with charger attached and turned on? (I think yes but not sure.) What is the voltage after 24 hrs off the charger? How are you judging reduced capacity of the battery? Is a sealed battery or old-style with six removable caps you can remove and see the fluid level? If not old-style, what reason do you have to believe to believe the fluid level is low? How old is the battery? Why are you charging it with a maintainer?

Only distilled or deionized water is ever added to a battery in ordinary circumstances. Modern sealed batteries do not lose fluid like the old ones and are never topped off. I have heard of draining, possibly accidental, and refilling with standard battery acid which I believe is 100%, undiluted H2SO4, sulphuric acid. Other than that, only water is ever added and there is no guideline known to me for adding back a mix, except don't.

Bottom line, if that battery's performance is compromised, buy a new one and don't leave the charger on forever. If you insist on opening a sealed battery, which was not forseen in its design, recognizse that is a hazardous operation and be very careful and wear appropriate protective gear.
 
Normally you would add only distilled water. If you add water up to the mark and let it sit long enough for the water to completely mix (might take a long time) or replace the caps and swish it around to mix it up, then you can use a hydrometer to measure the specific gravity of the electrolyte. A hydrometer will tell you whether you need to add acid or not. The specific gravity of the fluid will be maximum when the battery is fully charged. Adding excess acid will increase the likelihood of sulfation. Unless you smelled a strong sulfur smell, I would assume only water was lost. You can get a cheap hydrometer at most auto supply stores.

+1 on the safety precautions. A full hazmat suit is not a bad idea.
 
CORRECTION to my post above. Sulphuric acid as used in batteries is diluted with water to about 35%, reported numbers vary so apparently there is a range, possibly depending on various battery designs/specifications. Pure sulphuric is quite viscous.
 
Hydrogen sulphide = rotten eggs stink. You should habe been able to smell it if it was making it in quantity.

Agree that a hygrometer is the only way to tell if you need to add acid, and how much.
 
Punx0r said:
Hydrogen sulphide = rotten eggs stink. You should habe been able to smell it if it was making it in quantity.

Agree that a hygrometer is the only way to tell if you need to add acid, and how much.

You mean a hydrometer. Lol. Well, I do have one. And I was able to open the battery without trouble. The brand of the battery is hidden actually by a company that refurbished it. But, I never smelled rotten eggs, so I am assuming I just needed to add water. It's easier, and I think that at first you loose water, then once the concentration of sulfuric acid goes up it will start electrolysis of it. I decided to add 4 tablespoons of water to each cap. It's one of the largest passenger car batteries you can purchase. (E39 BMW.)

If I do measure the specific gravity I will give another update. Until then, thanks for all the suggestions and help everyone.
 
FLA is very robust

Voltage going a bit high won't hurt much, but yes a good charger does eventually drop to a low Float.

The usual problem is that happening before 100% SoC, as per endAmps spec.

As long as the plates are kept covered with electrolyte should be fine, keep them regularly topped up do not ignore that.

A rough capacity test with a CC load will reveal State of Health%.
 
Back
Top