I have a 12v car battery that I thought was good but when I put it on a charger, it goes for days and days and never finishes charging.

The charger meter shows the charge at 50% and the battery seems to be pulling a good bit of current since the cables are quite warm.

But even after a few days at 6Amp charge rate, the battery is still at 50%?

How can a battery just keep on pulling 6 amps for days on end?

I have disconnected the battery from the charger

 

I just went and felt the battery and it alarmed me.

The first cell felt room temp....but every cell progressively going toward to positive side got hotter and hotter. In fact, it's so hot at the cell on the positive end I'm worried. it's REALLY hot. So hot I'm now surprised it didn't explode or ignite.

The battery is now disconnected from the charger so that it can cool down.

Is this an indication it's internally shorted?

 

What is the open circuit voltage of the charger, and what is the voltage of the battery when cold and left to rest for a few hours? have you got access to each cell and measure their individual voltage? (I.e external link bridge bars)

 

If it has been drawing 6A for days, then it has almost certainly got one or more shorted cells.
I have no idea what the A/h capacity of your battery is but 2 days = 48hours (x 6A = 279 A/h)
This would have to be an enormous battery to draw 6A for even 2 days if the battery was in good condition.

A good battery will normally charge at a high current , which tapers off rapidly as the cell voltages rise. Once the battery is fully charged the water in the acid starts to break down into hydrogen and oxygen . In your case the 6A seems to be feeding a short in at least one cell and maybe more and is just generating heat. Time for a new battery i'm afraid

 

Ok,

If you're into making your vehicle batteries last longer, this could be one of the most helpful posts you'll read this year

So for anyone curious as to why this happens to car batteries (at least to SLA batts), there is a chemical process where, as a battery is fully charged, sulfuric acid is released (from the lead plates), back into solution in the battery. This concentrated sulfuric acid is heavier than water and so it sinks to the bottom of the battery.

If a SLA battery is left sitting for long, this concentrated Sulfuric acid eats away at the lead plates at the bottom of the battery.
Before long, you have a conductive paste at the bottom of the battery shorting the cells.

Normally, a battery that is in a car is jostled around a good bit every day and the charging process causes movement and mixing of the contents in the battery, thereby slowing the erosion of lead from the plates at the bottom. it's gonna happen eventually no matter what, but a lot slower.

So, want to make your car SLA (Sealed Lead Acid) battery last a LOT longer?

1). Keep it charged. Keeps the H2SO4 in liquid state and off the plates where it can solidify.
2). Never leave your battery sitting still for too long. If it's not in a vehicle that's being driven every day, tip it back and forth at least every few days to mix the heavy Sulfuric acid at the bottom with the more dilute mixture above. GREATLY extends service life.
3). A trickle charger can do this automatically. Get a 500mA trickle charger that keeps the battery topped up.....the tiny bubbles produced inside will also help keep the solution mixed. But even with a trickle charger, tip the battery at least once a week back and forth.
4). NEVER leave your battery sitting still and without a trickle charger for long. This is a sure way to shorten the life of the battery.

There's probably a way to FLUSH the conductive paste from a shorted cell and restore an SLA battery to service, albeit at a reduced capacity, but still quite usable....but I'll need to do more research on that. Will post back when I know for sure.

Hope this helps!

 

there is a chemical process where, as a battery is fully charged, sulfuric acid is released (from the lead plates), back into solution in the battery. This concentrated sulfuric acid is heavier than water and so it sinks to the bottom of the battery.
If a SLA battery is left sitting for long, this concentrated Sulfuric acid eats away at the lead plates at the bottom of the battery.
Before long, you have a conductive paste at the bottom of the battery shorting the cells.

I find that hard to believe .... where did you come by that information??? .... It's true that sulfuric acid is generated in charging , but no way it can become concentrated and "sink to the bottom " ... And "eat away at the lead plates"

 

I find that hard to believe .... where did you come by that information??? .... It's true that sulfuric acid is generated in charging , but no way it can become concentrated and "sink to the bottom " ... And "eat away at the lead plates"

Indeed. The sludge at the bottom, which can end up shorting a cell, is some of the paste from the plates dislodged by time and bubbles.

 

I just went and felt the battery and it alarmed me.

The first cell felt room temp....but every cell progressively going toward to positive side got hotter and hotter. In fact, it's so hot at the cell on the positive end I'm worried. it's REALLY hot. So hot I'm now surprised it didn't explode or ignite.

The battery is now disconnected from the charger so that it can cool down.

Is this an indication it's internally shorted?

You should thank God for stopping them on time. If they catch fire or explode it will be very bad for these big batteries.

Your batteries are worthless. You might be able to recover them with the help of some chemistry/physics expert but maybe you should not risk your life again and just throw them!

Theoretically its possible that the charger is faulty also, but I do not believe it in this case.

 

From Wiki...................

Lead and lead dioxide, the active materials on the battery's plates, react with sulfuric acid in the electrolyte to form lead sulfate. The lead sulfate first forms in a finely divided, amorphous state, and easily reverts to lead, lead dioxide and sulfuric acid when the battery recharges.

 

https://www.ausetute.com.au/pbbattery.html
In particular:
"Overcharging a battery electrolyzes water, producing hydrogen gas and oxygen gas.
The bubbles of gas degrade the surfaces of the electrodes causing the PbSO4(s) to fall off the electrodes.
This reduces the capacity of the cell."

 

There are six 2V cells in a 12V lead-acid battery. If a single cell is shorting, the battery will never reach full charge voltage of 13.8-14.5V.
Hence the charger never moves to the next stage, topping charge and float charge. The result is overcharging which is very dangerous.

 

Or when you have one of the "modern, smart chargers" and the battery is below the voltage the charger recognizes, it will show a high charge level on the meter, but it isn't putting voltage out of the charger. This is meant to signal the owner that there is a problem.

 

If they catch fire or explode it will be very bad for these big batteries.

Can you show examples of that ever happening?

 

Can you show examples of that ever happening?

Don't worry you will see it only once!

 

Don't worry you will see it only once!

I have seen it once, a battery exploding. The only result was the acid burned winter coat and bad spots on the cars fender paint. But this was from trying to jump a frozen car battery. Not from charging one with a dead cell.

 

I have a 12v car battery that I thought was good but when I put it on a charger, it goes for days and days and never finishes charging.

Take it to an auto parts store and have them test it. If they think they need to charge it first, the test will take an hour or two.

 

Many years ago, I disconnected a charger from a gassing car battery and accidentally shorted the + battery terminal to the chassis battery clamp.
The battery exploded with great force and blew the whole top off, showering me with acid and razor sharp pieces of jagged plastic. Luckily, my arm was across the front of my face and prevented the acid from going into my eyes. I still have the scars where two pieces of casing ripped through the sleeve of a leather jacket and impaled my lower forearm with another piece lodging into the top of my forehead!
A neighbour just happened to be watering his lawn and hosed me down as we ripped my clothing off. I went to hospital where they treated the acid burns. I was very lucky that none went into my eyes!
A very hard learnt lesson to treat batteries with great care. Hydrogen is a powerful explosive!!!!

 

For anyone who doubts the power of an exploding battery, here are the scars 55 years later. This was through the sleeve of a tough leather jacket!!!!

Take care folks.

 

Wow reckless, that's amazing your eyes were not harmed. Could have been so much worse. Seems your extensor tendons were extremely close to being cut from the photo.

It's good you posted that story since many may not be aware of the dangers of hydrogen gas. As a youngster nerd of 13 - 15, I had a rather large chemistry lab at home and played around with hydrogen gas. I would fill garbage bags with it and attach stuff to them and watch them drift away, amazing the other kids in the neighborhood. And then sometimes,....we'd light the bag and it was quite a boom but harmless because the bag was so easy to rip and the hydrogen had lots of room to expand easily. IIRC, I would put Zinc pellets in a flask and introduce H2SO4.

But batteries can indeed explode with great force. Considering the volatile gas inside and the amount of current involved, isn't it amazing battery explosions are so rare?

 

I find that hard to believe .... where did you come by that information??? .... It's true that sulfuric acid is generated in charging , but no way it can become concentrated and "sink to the bottom " ... And "eat away at the lead plates"

Sure!
It's technically called acid stratification.
https://batteryuniversity.com/learn/article/water_loss_acid_stratification_and_surface_charge

The specific gravity of Sulfuric acid is 1.85.
The specific gravity of water is 1

So even though Sulfuric acid is soluble in water, it tends to concentrate at the bottom if undisturbed for a period of time

At least, that's what I recall to the best of my aging gray matter.
A "Real" chemist will come along and set me straight.