How do i tell when battery is charged. It reads 7.4 % right now on battery charger. My 2o11 chevy cobalt got the keys locked inside and drained battery after i got the keys out of the car . I tried to jump it off of the little post under the hood and then the key got stuck. After finding out through google that the battery was in the trunk i managed to get the trunk open finally and got to battery. un hookedd it from car and put charger on it. it reads 7.4. what is it supposed to read?


Anything below 10 volts is usually classified as a bad battery... sorry !

Every cells holds roughly 2.12 volts for a total of 12.72 volts on a full charge... The minimum a battery can hold is 0% which is a total of 10.50 ( 10.5 ) volts. That means each cell would be roughly 1.75 volts

Did you try to jump start the battery that had < 80% voltage... If so, It would of resulted in a blow cell or 2 depending on how many times you tried to jump start it... is that why It's on the charger now with 7 volts...

Take caution: You can pry off the battery caps and stick a long metal probe on the bottom of the battery and test each cell, dont forget your chemical gloves and glasses and respirator...

Also, dont pour or touch anything inside of the battery, or pour any purified or distilled water into the battery... not only will your H20:H2SO4 concentration ( 62:38 ) be accurate anymore, but sulfuric acid is heavier than water, that means the water will float on top of the sulfuric acid creating acid stratification

Plus: when sulfuric acid reacts with water, it's going to spit back at you... and it's not just the liquid that harmful... the sulfuric acid vapours can severely damage your nerves and cause deterioration and or death.

I hope you didnt and don't fill it with water and then try to run your car, now its reading 7 volts, because acid stratification is another reason your battery cells blow out

The best solution to a dead battery is putting it on a charger... If It's been sitting for a long while then shake the battery with gloves and eye protection... you need to get that concentration back up... even though battery acid has water and acid mixed... over time more acid can form on the bottom of the battery and more vapours can rise causing causing sulfation on the terminals, which is a white crystal like structure that is also extremely harmful to your skin and health... yet another reason batteries die early...

Do you have sulfate on your terminals, did you just clean the terminals and go on thinking the battery and alternator were ok... if you have sulfate on your terminals, clean your battery and then shake your battery up to improve the concentration again.

  • Do you add water to acid or acid to water?
    – Solar Mike
    Jul 4 '19 at 5:40
  • Acid to water, or together simultaneously... that way the H2S04 will penetrate the water... with the water on top, the water can never penetrate the acid unless its stired which most people dont do according to YouTube :/
    – hello moto
    Jul 4 '19 at 5:47
  • 1
    I thought you should make it clear in your answer from a safety perspective... I know, as I used to fill brand new dry batteries for new cars and I mixed gallons & gallons...
    – Solar Mike
    Jul 4 '19 at 5:49

Are you sure that it reads 7.4% Or could it be 7.4V...

If volts then you may need to replace the battery, they tend not to like being discharged that low...

Fully charged it should be around 12.5 to 12.8 V...

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.