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Out of all the old time battery designs, lead-acid is the kind most widely still in use. Its energy density (watt-hours per kg) and low cost make them widespread.

As any kind of battery, it is based around an electrochemical reaction: an interaction between different chemical substances that, essentially, produces an excess of electrons on one side and a deficit on the other side. This difference ("potential") is voltage, and enables a flow of current as electrons circulate around the circuit to fill that deficit. As the difference neutralizes the available charge in the battery decreases. The key in rechargeable batteries is that this reaction is reversible, as applying a current into the battery (as opposed to drawing out from it) will restore the charge. Other electrochemical reactions may yield higher energy densities at the cost of not being rechargeable.

The voltage generated by each reaction is more or less fixed (it varies a little depending on percentage of charge). Lead-acid is 2 volts. For example, nickel based rechargeables are 1.2 or 1.4v, and lithium cells are 3.7v. Because of this, if you want a 12v battery you will need to place several of these reactions in series to add the voltages. Each of these is called a cell. As you can see in the pictures, a 12v lead-acid is composed of 6 cells. 12v, 6v, 8v and even single-cell 2v batteries are common.

Next I'll explain the ways in which lead-acid cells can be constructed, so you can identify what needs to be done to your particular battery.

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## Step 1: Identify Your Type of Battery

There are 3 main components to these batteries. Yes, it's lead and and acid. Specifically, a solution of sulfuric acid, lead plates and lead oxide plates. Lead plates are the negative. Lead oxide makes the positive, as the oxygen atoms bound to the lead "lack" electrons (electrons have negative charge), thus is "less negative"= positive. The sulfuric acid, dissolved in water, is called the electrolyte and carries electrons to and from these plates, and upon reaction with the lead, release electrons.

The amount, thickness and size of the plates can vary, as well as the way the electrolyte is held.

Starter and deep-cycle batteries

The different purposes of these batteries mean the size of the plates is different. A starter battery is what you commonly find in gas cars. Their main task is to deliver a large current for a short time in order to turn the motor that cranks the engine for starting. Their normal use doesn't discharge them too much -- just one big, short dip that is recharged rather quickly. The alternator in the car keeps the battery charged as it runs the lights, stereo, ECU and all other electronics.

Deep cycle batteries, on the other hand, are designed to handle slow but considerable discharges. They might not be able to provide so much of a "punch" on a whim (i.e. large current surges) but can be discharged much more before incurring damage. These are what you find on UPSs, solar power systems, emergency lights, and many electric vehicles like forklifts, golf carts, some delivery trucks, early and DIY electric cars, and kid's ride-on toys.

Flooded and sealed batteries

This distinction arises from the way the electrolyte is held in the cell. The plates need to be surrounded by the sulfuric acid solution so the reaction can occur. The simplest way to achieve this is to just immerse the plates in the liquid solution. There you go: flooded battery. Flooded batteries can be either starter (most car batteries) or deep cycle (forklift or golf cart batteries for example)

A big advantage is that since a little water is lost when charging (more on this later), you can charge more quickly as you can afford to lose more water, and just top it off every so often. A big disadvantage is that they can only be installed horizontally.

Sealed or "maintenance-free" batteries instead have a sheet of fiberglass in between the plates -- an absorbent glass mat or AGM which is also another name for these. The fiberglass soaks up the solution and keeps it in contact with both kinds of plates, while also preventing them from touching and shorting out in the event of damage to the battery. This means they also can be installed at an angle and be subject to more abuse before spilling or giving trouble.

Since the charging reaction releases hydrogen, lead-acid batteries need venting so they can let out the excess gas. Sealed batteries have valves to control the release, which leads to yet another name for sealed batts: VRLA for valve-regulated lead-acid

Another kind are gel cells, which have a thickener in the solution, therefore combining some benefits of both of the previous kinds. I have not come across these, but in principle can be restored in the same way, although may require some shaking. These are common in the starter kind as high performance car batteries.

## Step 2: How a Lead Acid Battery Dies

Now that we've gone over the way the batteries work and are constructed, it will be easier to explain the ways in which they can fail. These are the two main ways they become unable to hold a charge:

Sulfur problems

The chemically inclined will have noticed that as the sulfuric acid deposits the electron in the other side, the sulfur atom has to go somewhere, so it forms lead sulfate on top of the lead plate. This is in theory reversed upon recharging, but in reality does not occur for 100% of the sulfur. Crystals can form and either get stuck to the copper, reducing its active surface area (sulfation), or drop to the bottom carrying some of the lead with it leaving pits in the plate (pitting or corrosion) as well as reducing the amount of sulfuric acid available in the solution.

Some amount of sulfation is inevitable with charge and discharge cycles and is the main way in which a battery ages and becomes unusable. Improper charging and discharging (too fast or too deep) can lead prematurely to this.

Water problems

The sulfuric acid is only a small portion of the liquid inside the battery, around 25%. Therefore it needs to be dissolved in water so it reaches the entire area of the plates. As they have different boiling points, water can evaporate and separate out of the mixture, reducing its volume and effectively "drying out" the battery.

This is more common with batteries that are not frequently cycled and happens instead from environmental factors.

In either case the voltage across the battery terminals will be very low (only few mV). The resistance will also be very high, but don't use your multimeter's ohm mode to measure this! It rather means that it only allows a very small amount of current to circulate through it, like a large resistor would. You can see this putting your ammeter in series between the battery and charger, where you will only measure a small current (few milliamps).

The battery I'm using as an example had premature water loss. It was bought new 10 years ago, and never used. All of the water evaporated and therefore there was no way for electrons to get around.

If your battery has become sulfated, this method will probably not work very well. It could yield no results, or only limited ones. For one, battery capacity will likely be smaller. I have read that a high current can be used to force the lead sulfate crystals to dissolve the sulfur back into the solution and off of the plates, but I've never tried it. The currents involved are in the 100-200 A (yes, whole amperes!) range, so a welder is normally used (they give off low volts at very high amps)

## Step 3: Open 'Er Up

For the rest of the steps I'll be focusing on sealed batteries like the ones I am recovering myself

Flooded batteries are meant to be opened and will have an indication of where you can pry off the lids. They are meant to be refilled, too, so this should give good results if you see it's dried out.

On the other hand, sealed batteries were not meant to be opened. But we don't mind about that too much. You will probably notice slots around the lid. These are actually the vents where the excess hydrogen comes out. You can use these points to pry the lid off with a small flathead screwdriver. Although it might feel like it has clips, the lid is in fact glued in several spots.

Now you can see the 6 valves that compose the 6 cells of this battery. To see inside, let's take them off, but be careful:

• There could be some pressure inside, leading the valve to fly off when lifted. Pliers are recommended.
• There could also be some acid hanging around the valve, which by removing it could get sprayed on you. Gloves and/or goggles are suggested, as is keeping a shaker of sodium bicarbonate to neutralize any spills
• The valves are very important. Do not lose them!

## Step 4: Inspect

Light inside the valve holes and see into the cells

If it all looks very dry, great! Adding some water will give back life to your battery. At least a little. So read on.

Remember: if you can clearly see liquid, yet only get a few mV on the terminals, this method will not work for you. Your battery is probably sulfated.

Poke with your multimeter leads into adjacent cells and measure voltage and resistance. This is to look for shorts. Check voltage first, and you should get a few millivolts at most. If the measurement seems to be zero volts, or too close to it, measure resistance.
A very low value indicates that a cell has shorted out, that is, that opposite plates are touching. I wouldn't recommend to recover these, as the charging voltage will be lower (you're charging fewer cells) and a normal charger will damage the others. If you know what you're doing and can live with managing the voltage for your handicapped battery, by all means go ahead and give it another chance at life. If not, remember that these batteries are something like 95% recyclable.

## Step 5: Get the Right Water

Opposed to popular knowledge, pure H2O is actually not conductive. Tap water will conduct electricity because of impurities dissolved in it. Sodium and other minerals present in it form salts that can carry electrons.

Since the reaction in our battery depends on the sulfuric acid carrying the electrons, it's very important that no other charge-carrying molecules be present in the water we add.

Enter distilled water!

This water has had all impurities chemically separated. It can be found in many supermarkets. It's common for use in clothes irons since tap water contains calcium that can clog their small internal conduits.

Furthermore, injectable water has been handled in a sterile manner after distillation. It is not necessary, but since this is available in drugstores, for many (as it was for me) it can be easier to find, and just as cheap.

In a pinch, or in post-apocalyptic survival scenarios (how are you reading this?) rain water works well too, since it has been naturally distilled (it was evaporated into clouds).

## Step 6: Refill

Allow me to repeat: distilled water!
The bigger the battery the more water it holds, as cells are larger; my 12AH held about 30mL per cell (1oz?). It's good to use a graduated container or a syringe so the amount of water you put into each cell is equal.

With the help of a funnel or the syringe pour a moderate amount of water into the first cell, wait for the mat to absorb it (unless you have a flooded battery, which has no mat), and fill up to just below the top of the plates.

The level might change after a couple of charges as the mat absorbs the solution and some of the water separates (electrolyzes) away. Fill the rest of cells with the same amount.

Watch out for capillarity! A cell may appear full when a fat drop clings to the valve hole walls. A cotton swab or some tapping should leave the opening free again. All cells should take in more or less the same amount of water.

## Step 7: First New Charge

The first charge will be an "activation charge", where we're restarting the reaction. At this stage the current going into the battery will be very low. It will pick up speed and charge at normal speed by the 2nd or 3rd cycle.

It's important to make the first handful of charges with the lid and/or valves off so the excess solution that inevitably is now in your battery does not spill as much. This will come off as hydrogen so it's also important to have the area ventilated to avoid explosions!

To make the first charge, connect the battery to the charger with the ammeter in series. We're going to need to measure current for this. You can also always use an adjustable power supply. It has to have voltage control, while current limiting is useful but not necessary.

Check the battery label for a charge current limit. If your supply has current limiting I suggest setting it to about 80% of this.

If your battery has no stated limit, or the label has worn off, consider the limit to be some 40% of the rated capacity.

Set your voltage to 14.4 volts to begin. This is the standard charge voltage for a 12V. The initial current will be very small. If your power supply is capable, you can increase the voltage to accelerate the reaction. Many chargers with "recovery mode" do this. It is safe to go up to 60V for a 12V battery as long as you decrease the voltage as the battery starts accepting higher and higher current. The current limit on your supply will keep decreasing this voltage for you.

If you cannot go beyond 14.4v (for example if you're using a dedicated charger), just keep checking the current. It will increase only slowly at first, then faster and faster, up to a point where it starts dropping. Congratulations, this is normal charging!

The photos show this increase-then-decrease in current!

When the current gets to around 0.03 times the capacity of the battery, it has been charged to over 90-95%

## Step 8: Seal Back Up and First Few Uses

(Unless your battery is flooded, then just pop the lids back on)
As mentioned, water level might change. If you have time, charge and discharge the battery a few times (connect a lightbulb, motor or some other load that will discharge it quickly) to get the solution to a stable level.

Clean and dry the valves and valve posts. Put the valves back on and glue the lid back on, looking for the spots where it was glued and using a drop of cyanoacrylate glue on each. Put some weight on top for a while and let dry.

## Step 9: Keep an Eye on It

Your battery is ready but it was brought back from dead so, understandably, it may behave oddly.
Capacity might be reduced, depending on the cause and degree of damage. Mine seemed almost unaffected, others may only give 20% of their previous capacity. It is likely that they have excess water. This is okay. Just remember to let charge in a ventilated, flame free area, and that spills will occur occasionally. I keep the salt shaker with sodium bicarbonate nearby.

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