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When Should I Add Extra Sulfuric Acid to my Battery?

When you see your battery acid level begin to drop, you may wonder when and if it is appropriate to add acid, or just water.

We recommend that unless a battery was tipped over and all the acid spilled out, that you add only distilled water. Think of it like this: If you were to fill a pot up with salt water, and put it on your stove to boil, what happens? All the water will boil off, and you will be left with only salt. To re-create the original salt water mixture, you simply add straight water, because all the original salt is still in the pan, waiting to be re-suspended in the water. Batteries are the same way. All the chemical needed to make sulfuric acid is still in the battery, it is just the water that is gone. If you add more acid, you will be changing the chemical makeup of the battery which can lead to an incorrect sulfuric content.

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23 people commented, Fonzie Fox, Tech, Matt, TECH, and 19 others
This article is rated 5.0 out of 5
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MattI purchased a used forklift with a large 36V battery, 18 cells. It had clearly been overfilled and boiled over repeatedly, as was evident by loads of corrosion on, around, and underneath the battery. Battery would only charge to ~70%, which dropped quickly and lasted only a few hours of use. I have a background in chemistry, so felt comfortable trying to revive it by adding acid. I removed the battery, cleaned up all the corrosion, washed everything thoroughly, and neutralized with baking soda solution. I Measured specific gravity of each cell to find they were on average ~25% sulfuric acid, far short of the factory spec of 33.93%. There was not visible evidence of sulfation as far as I could tell, so my assumption was that most/all of the loss was from repeated boil overs due to over filling. I added 96% sulfuric acid in varying amounts to each cell, based on specific gravity. Being cautious, I added about 1/3 of what I calculated was needed to bring up to factory levels. Ran a few battery cycles and saw significant improvement, so I repeated this process again. Cells are now all ~32% sulfuric acid, battery charges to 100%, and is good for many days of use. Should I continue with my final addition to get that last ~2% increase to factory spec, or quit now, as the battery is working quite well? My concern is the possibility that there was some sulfation contributing to the low SG, and I risk that breaking free, going back into solution, and having too high of a concentration.
Reply  •  Rated article 5  •  December 27, 2024 at 9:54 am
BatteryStuff TechMatt, kudos on getting the batteries revived. You have done more than we would have ever recommended. I would count your blessing the batteries are working well and stop while you are ahead.
Reply  •  January 31, 2025 at 10:41 am
CassidyThank you for the information
Reply  •  Rated article 5  •  August 25, 2024 at 4:11 am
Clinton SharpI have a very large, 2 year old (or less) battery that sat for about 6-8 months. It would only charge to 5v so I checked cells and ended up adding about 1/2 gallon of distilled water to get the levels right. It charges to 6v now. Is there anything else I could add to help it? Most expensive battery I've ever bought so I'm crossing fingers. Also, should i take it out and shake/ mix it? Thanks
Reply  •  Rated article 5  •  August 8, 2024 at 3:28 pm
TECHUnfortunately, once the plates get exposed the paste covering the plates will dry. This acts as a barrier that prevents chemical reaction and often makes the battery unrecoverable.
Reply  •  September 27, 2024 at 11:19 am
Liebardt TjiuonguaI put distilled water in my 722 Willard battery now it is not charging.must I reduce the water level.
Reply  •  Rated article 5  •  April 5, 2024 at 6:55 am
BatteryStuff TechIf the battery doesn't charge after adding water, it might be sulfated, or if the cells were exposed to air, the paste may have dried, creating a hardened barrier that inhibits charging. I would suggest using a hydrometer to see where your batteries sit.
Reply  •  April 8, 2024 at 9:09 am
PioI added battery acid to my battery when the battery water levels had gone low am l in trouble now or we'll see the effects in a prolonged period of time
Reply  •  Rated article 5  •  December 7, 2023 at 9:46 pm
TECHAdding more acid will increase the sulfuric acid concentration. Only time will tell if the amount you added has detrimental side effects. Just like the article says, the acid never leaves the battery, so only distilled water should be added.
Reply  •  Rated article 5  •  January 31, 2024 at 12:57 pm
Alinanye PatrickIs it right to refill drained cells with chloride exide?
Reply  •  Rated article 5  •  February 11, 2023 at 10:34 pm
BatteryStuff TechFor cells that have evaporated, we only recommend distilled water. If the battery tipped and the electrolyte drained out, I would talk to the manufacturer about obtaining some premixed electrolyte.
Reply  •  Rated article 5  •  February 16, 2023 at 2:52 pm
EricWith an emptied battery ready to fill acid will i need battery water to top up or it's acid to the maximum mark?
Reply  •  Rated article 5  •  May 23, 2020 at 6:26 am
BatteryStuff TechWe would suggest following the manufacturers guidelines. Most manufacturers have acid packs all ready premixed.
Reply  •  Rated article 5  •  June 15, 2020 at 11:53 am
Always Choosing WronglyPlease help…my friend put salt and water in my less than 2 year old battery whi h continuously needed a jump start. Battery water seemed to boil and bubble whenmuxture was added along withcharger. Battery responded well i think to charger, although it is not fully charged because we had to remove it due to bad lightning storm. Battery turned super hot to the touch!!!is battery now totally destroyed now? Xar starts and runs but battery is HOT!!! Will battery blow up? Will battery now mess up cars computer? What can i do now????!!!
Reply  •  May 18, 2016 at 11:00 pm
Fonzie FoxSounds like your alternator is not keeping the battery charged.
Reply  •  April 7, 2025 at 11:53 pm
BatteryStuff TechIt is only recommend to put distilled water in a battery once the acid is originally added. The only other exception is adding specially made chemicals such as the Battery Equaliser, which is designed to recover batteries on a chemical level. We have heard of all sort of ways people try to recover batteries, and we some customers tell us their success stories, and other end up with a dead battery. I believe in your case the battery is most likely damaged, as the battery shouldn’t be hot to the touch while charging. If it is hot it is only speeding up the sulfation process, and could eventually comprise the case if left on the charger. I would suggest replacing the battery.
Reply  •  Rated article 5  •  May 19, 2016 at 1:27 pm
Ninja KevYou answered my question and I’m glad I checked with you first! Thanks alot!
Reply  •  Rated article 5  •  November 25, 2014 at 1:22 pm
JustinOf course you recommend it, I assume for liability purposes… However, can one empty, ensure all the cells are empty, and refill them with electrolyte’s? FYI, I bought this 5lb dirt bike battery ($65), sealed it… then the bike sat for a year. Today I noticed the batteries cells were completely dry/empty when I reopened it. Just buying a container of Electrolyte’s $8.
Thank you for your response. Disclaimer; I am responsible for any damage I may bring upon myself ;)
V/R Justin
Reply  •  October 5, 2014 at 5:46 pm
BatteryStuff TechJustin it sounds like the battery is an AGM battery that you originally introduced the Acid to, and then sealed. Once sealed it is never recommended to open the battery, but as you have you would notice that it appeared dry, and that is because the fiberglass matting absorbed the electrolyte. It would not be my recommendation to add electrolyte to a sealed battery as all the electrolyte would most likely not be absorbed by the fiberglass matting.
Reply  •  Rated article 5  •  November 18, 2014 at 7:23 am
DavidI know it’s 1.5 years later, but this may help someone else…
As I understand it, when filling an AGM battery (I think all ATV/motorbike batteries are AGM) you have to to add the electrolyte(acid) slowly, or more specifically, keep adding it for a long time as it will slowly absorb and soak down through the glass matting and after a while appear dry. Then you have to add more and wait. Repeat this process until you’re 100% certain that it has completely soaked in and saturated the glass matting. If you just top it up once and close it up, there will not be enough and it’ll shorten the life and reduce the strength of your battery.
Reply  •  May 18, 2016 at 5:46 am
BatteryStuff TechTheir are some suppliers out ther that will ship an AGM battery without an acid pack, which is hard on customers. They don’t always come with instructions as to how much acid to put in the battery. Unfortunately the only way to know how much acid to put into the battery is if the battery comes with instructions, or you have a pre-measured acid pack with the battery. I would contact your supplier if your battery didn’t come with a pre-measured pack just to be sure you add the correct amount.
Reply  •  Rated article 5  •  May 19, 2016 at 12:58 pm
BatteryStuff TechIt is impossible to do so accurately. We do not recommend adding anything other than distilled water.
Reply  •  May 16, 2014 at 2:18 pm
Vswhat to do if some qty. acidic water spilled out? How to regain the quantity of acid that spilled out?
Reply  •  May 15, 2014 at 5:57 am