3 Car Battery Truths/Myths: Not Fully Charged, Overcharged and Epsom Salt

Most people know that a dead car battery may be irreversible, but fewer understand that even a partially discharged battery suffers irreversible sulfation damage if left in that state for a significant time. In this video, Dr. David Zhang explores three critical battery topics through actual experiments and explains why some commonly repeated advice is only partially correct.

Topic 1: Not Fully Charged

During periods when a car sits unused, such as during the pandemic, the battery gradually discharges. Sulfation begins forming on the lead plates almost immediately once the battery drops below full charge. The longer it sits partially discharged, the harder it becomes to revive. Even partial discharge over an extended period causes measurable capacity loss.

Topic 2: Overcharging Damage

Dr. Zhang learned this the hard way. Initially dismissing overcharging as not a big deal, he discovered that overcharging caused his battery voltage to drop to 8V, making it even harder to save than a sulfated battery at 11V. Overcharging causes water loss, plate warping, and can permanently damage a battery faster than sulfation.

Topic 3: The Epsom Salt Experiment

Some people swear Epsom salt revives batteries while others call it a myth. Dr. Zhang conducted an actual experiment showing that both perspectives may be correct under specific conditions. The effectiveness depends on the type of damage and the chemical state of the battery. He invites viewers with chemistry expertise to provide exact chemical equations explaining the mechanism.

For more car maintenance, see changing transmission fluid and Honda Accord brake fluid bleeding.

The takeaway: keep your battery fully charged, never overcharge it, and understand the chemistry before trying Epsom salt remedies.

Pandemic Cars and the Sulfation Problem

Welcome to the Genius Asian family. Today I’m going to show you how to recover a car battery and how you may destroy the battery if it is not treated properly.

During the pandemic you may have a car — or cars — you have not driven for quite some time. You go to start the car after a few months and find your car battery is dead. Whenever the battery is not fully charged you will have sulfation, and if not remedied you will have irreversible damage. The longer the battery is not fully charged, the harder it will be to revive it. Driving your car, or charging the car battery so that it is fully charged, is the only way to avoid sulfation.

I hooked up a smart battery charger to our car battery. This kind of charger may take days or weeks to fully charge the battery because it is mostly intended for maintenance recharge. When your battery is not completely discharged, the internal resistance is not very high. I can also use an ordinary charger because a completely discharged battery’s internal resistance is quite high.

Three Ways to Overcome High Internal Resistance

There are three ways to counteract high internal resistance. One: use a higher voltage. Two: use a lower voltage and wait a long time. Three: add chemicals to lower the resistance.

This charger has two settings — two amp or six amp. When you use the six amp setting the voltage is higher. If you use the higher voltage — six amps — you may hear a loud bubbling sound. Some battery cells will heat up and they will lose water. I found that after two days of charging at six amps the voltage dropped quite low after the charger was disconnected. It was overcharged.

At that point I used a hydrometer to test the specific gravity of the acid in the battery. One cell looked perfect — the indicator was in the green area. However, the specific gravity for another cell was too light; it was in the red area. In fact, two of the six cells in the battery were bad. Since the six cells are connected in series, the overall voltage cannot reach the 12 volts it needs. As a result of that overcharging, the battery had to be replaced.

To avoid overcharging, the charging period should be short and the battery should not be allowed to get too hot. You could use the two-amp setting to slow-charge the battery, which drastically reduces the chance of overcharging. The main problem is that it may take a very long time if sulfation in the battery cells is very bad.

Reading the Date Code and Long-Term Maintenance Tips

To know how old your car battery is, look at the label. For example, one label reads “200214D.” The digit 4 means 2014, and the letter D means April — the fourth month. This will save you the time and trouble of trying to revive a battery that is simply too old to last any longer.

To prolong the lifespan of a battery, either drive the car periodically or use a charger to charge it and avoid sulfation. Make sure you do not overcharge it. I usually use a smart charger for maintenance charging so that I do not end up overcharging it.

Another tip: add distilled water when the battery fluid drops below the indicated low mark. When the electrolyte level drops below the tops of the plates, oxygen in the air causes a chemical reaction that damages the exposed portion of the plates.

The Hidden Resistance of a Corroded Terminal

Take this example. You see oxidation on the terminal because it was exposed. I’ve cleaned it using sandpaper on the lead pole — it looks very clean and quite smooth. I connect the battery charger. The supply voltage is over 13 volts. Note that I first measure between the two clamps: over 13 volts. But when I measure the voltage between the two lead poles it is only 11.4 volts — it should be over 13 volts. If you keep charging the battery in that condition it will not work.

If I measure the resistance between two points on the surface of a single pole, it reads 1,500 ohms. That high resistance reduces the current to a few milliamps — too little to start the car, and it may take a thousand days to charge the battery. The lead pole looks smooth, but the color is very black. The black layer is so thick that sandpaper is no longer effective.

Using a screwdriver to scrape, you remove the black layer and expose the shining metal color. Measuring again: 24 ohms. Much better. After cleaning more: 0.1 ohm — perfect. Measuring the voltage between the two poles now shows over 13 volts. Now you can really charge the battery. Imagine what happens when the liquid drops and exposes the plates: that black layer increases internal resistance and degrades performance.

The Epsom Salt Experiment: Conductivity, Not Sulfation Reduction

Some people swear that Epsom salt works; others say it does not. Epsom salt does not particularly help to reduce sulfation. It is possible that Epsom salt reduces the internal resistance and enables a higher current during charging, thus leading to a false impression that it works to reduce sulfation.

Here is a simple experiment. I pour some distilled water into a bottle cap. To measure conductivity — the resistance — of the distilled water, I make a sensor: a rubber band attaches two electrodes on opposite sides of a squared stick (in this case a pen). I set the multimeter to the resistance setting and connect it to the two electrodes. When I dip the sensor in the distilled water it shows 100,000 ohms.

I then add some Epsom salt. The reading drops from 100,000 ohms to below 50,000 ohms immediately. If you pour that kind of liquid into the battery it will increase the ions in the battery, thus decreasing the internal resistance. That may explain why adding Epsom salt appears to work for some people — those people were not able to increase the voltage of the charger to overcome the resistance on their own.

My recommendation is to use high voltage briefly at the start, make sure the battery does not get very hot, and then switch to a low current to charge for a longer time. If you do not have a charger that can overcome the battery’s resistance, you may as well try Epsom salt if you happen to have it on hand — at least it cannot hurt.

I would prefer to increase the voltage of the charger rather than adding something not recommended by the manufacturer. However, if you can prove that Epsom salt can reduce sulfation, please leave your exact chemical equation in the comments below. If you think Epsom salt can have some reaction with the sediment at the bottom of the battery, please give your chemical equation as well.