How Do Lead Acid Batteries Work
How Do Lead Acid Batteries Work


How Do Lead Acid Batteries Work


Lead Acid batteries have changed little since the 1880's although improvements in materials and manufacturing methods continue to bring improvements in energy density, life and reliability. All lead acid batteries consist of flat lead plates immersed in a pool of electrolyte. Regular water addition is required for most types of lead acid batteries although low-maintenance types come with excess electrolyte calculated to compensate for water loss during a normal lifetime. Lead acid batteries used in the RV and Marine Industries usually consist of two 6-volt batteries in series, or a single 12-volt battery. These batteries are constructed of several single cells connected in series each cell produces approximately 2.1 volts. A six-volt battery has three single cells, which when fully charged produce an output voltage of 6.3 volts. A twelve-volt battery has six single cells in series producing a fully charged output voltage of 12.6 volts.

How Do Lead Acid Batteries Work

A battery cell consists of two lead plates a positive plate covered with a paste of lead dioxide and a negative made of sponge lead, with an insulating material (separator) in between. The plates are enclosed in a plastic battery case and then submersed in an electrolyte consisting of water and sulfuric acid (see figure # 1). Each cell is capable of storing 2.1 volts.

In order for lead acid cell to produce a voltage, it must first receive a (forming) charge voltage of at least 2.1-volts/cell from a charger. Lead acid batteries do not generate voltage on their own; they only store a charge from another source. This is the reason lead acid batteries are called storage batteries, because they only store a charge. The size of the battery plates and amount of electrolyte determines the amount of charge lead acid batteries can store. The size of this storage capacity is described as the amp hour (AH) rating of a battery. A typical 12-volt battery used in a RV or marine craft has a rating 125 AH, which means it can supply 10 amps of current for 12.5 hours or 20-amps of current for a period of 6.25 hours. Lead acid batteries can be connected in parallel to increase the total AH capacity.

In figure # 2 below, six single 2.1-volt cells have been connected in series to make the typical 12-volt battery, which when fully charged will produce a total voltage of 12.6-volts.

Lead Acid Batter Discharge Cycle

In figure # 3, above a fully charged battery is connected to a load (light bulb) and the chemical reaction between sulfuric acid and the lead plates produces the electricity to light the bulb. This chemical reaction also begins to coat both positive and negative plates with a substance called lead sulfate also known as sulfation (shown as a yellow build-up on plates). This build-up of lead sulfate is normal during a discharge cycle. As the battery continues to discharge, lead sulfate coats more and more of the plates and battery voltage begins to decrease from fully charged state of 12.6-volts (figure # 4).

In figure # 5 the battery is now fully discharged, the plates are almost completely covered with lead sulfate (sulfation) and voltage has dropped to 10.5-volts.

NOTE: Discharging a lead acid battery below 10.5 volts will severely damage it!

Lead sulfate (sulfation) now coats most of the battery plates. Lead sulfate is a soft material, which can is reconverted back into lead and sulfuric acid, provided the discharged battery is immediately connected to a battery charger. If a lead acid battery is not immediately recharged, the lead sulfate will begin to form hard crystals, which can not be reconverted by a standard fixed voltage (13.6 volts) battery converter/charger.

NOTE: Always recharge your RV or Marine battery as soon as possible to prevent loss of battery capacity due to the build-up of hard lead sulfate crystals!

Lead Acid Battery Recharge Cycle

The most important thing to understand about recharging lead acid batteries is that a converter/charger with a single fixed output voltage will not properly recharge or maintain your battery. Proper recharging and maintenance requires an intelligent charging system that can vary the charging voltage based on the state of charge and use of your RV or Marine battery. Progressive Dynamics has developed intelligent charging systems that solve battery problems and reduce battery maintenance.

The discharged battery shown in figure # 6 on the next page is connected to a converter/charger with its output voltage set at 13.6-volts. In order to recharge a 12-volt lead acid battery with a fully charged terminal voltage of 12.6-volts, the charger voltage must be set at a higher voltage. Most converter/chargers on the market are set at approximately 13.6-volts. During the battery recharge cycle lead sulfate (sulfation) begins to reconvert to lead and sulfuric acid.

During the recharging process as electricity flows through the water portion of the electrolyte and water, (H2O) is converted into its original elements, hydrogen and oxygen. These gasses are very flammable and the reason your RV or Marine batteries must be vented outside. Gassing causes water loss and therefore lead acid batteries need to have water added periodically. Sealed lead acid batteries contain most of these gasses allowing them to recombine into the electrolyte. If the battery is overcharged pressure from these gasses will cause relief caps to open and vent, resulting in some water loss. Most sealed batteries have extra electrolyte added during the manufacturing process to compensate for some water loss.

The battery shown in figure # 7 above has been fully recharged using a fixed charging voltage of 13.6-volts. Notice that some lead sulfate (sulfation) still remains on the plates. This build-up will continue after each recharging cycle and gradually the battery will begin to loose capacity to store a full charge and eventually must be replaced. Lead sulfate build up is reduced if battery is given an Equalizing Charge once every 10 discharge cycles or at least once a month. An Equalizing Charge increases charging voltage to 14.4 volts or higher for a short period. This higher voltage causes gassing that equalizes (re-mixes) the electrolyte solution.

Since most RV and Marine craft owners seldom remember to perform this function, Progressive Dynamics has developed the microprocessor controlled Charge Wizard. The Charge Wizard will automatically provide an Equalizing Charge every 21 hours for a period of 15 minutes, when the battery is fully charged and not in use. Our 2000 Series of Marine Battery Chargers have the Charge Wizard feature built-in.

One disadvantage of recharging a lead acid battery at a fixed voltage of 13.6-volts is the recharge time is very long. A typical 125-AH RV or Marine battery will take approximately 80 hours to recharge at 13.6 volts. Increasing the charge voltage to 14.4-volts will reduce battery recharge time for a 125-AH battery to 3-4 hours. Once a battery reaches 90% of full charge, the voltage must be reduced from 14.4-volts to 13.6-volts to reduce gassing and water loss. The optional Charge Wizard automatically senses when a battery has a very low state of charge and automatically selects its BOOST MODE of operation. BOOST MODE increases the voltage of a PD9100 Series converter/charger to 14.4 volts. When the battery reaches the 90% charge level, the Charge Wizard automatically reduces the charge voltage down to 13.6 volts to complete the charge. Again, this is a standard feature on our Marine Chargers.

Another disadvantage of recharging a lead acid battery at a fixed voltage of 13.6-volts is that once it is fully charged, 13.6 volts will cause considerable gassing and water loss. To prevent this from occurring the charging voltage must be reduced to 13.2-volts. The Charge Wizard will automatically select its STORAGE MODE of operation (13.2-volts) once the battery reaches full charge and remains unused for a period of 30 hours. This feature is standard on all of Progressive Dynamics Marine Battery Chargers.

At a charging voltage of 13.2 volts, the converter/charger will maintain a full charge, reduce gassing and water loss. However, this lower voltage does not provide enough gassing to prevent a battery condition called Battery Stratification. Battery Stratification is caused by the fact that the electrolyte in the battery is a mixture of water and acid and, like all mixtures, one component, the acid, is heavier than water. Therefore, acid will begin to settle and concentrate at the bottom of the battery (see figure #8).

This higher concentration of acid at the bottom of the battery causes additional build-up of lead sulfate (sulfation), which reduces battery storage capacity and battery life. In order to prevent Battery Stratification, an Equalization Charge (increasing charging voltage to 14.4-volts) must be applied periodically. The Charge Wizard automatically selects its EQUALIZATION MODE (14.4 volts) every 21 hours for a period of 15 minutes. This Equalizing Charge feature is standard on our Marine chargers.

As you have learned, in order to properly charge and maintain a lead acid battery you must use an intelligent charging system. Progressive Dynamics, Inteli-Power 9100 Series RV converters with a Charge Wizard installed, or one of our Inteli-Power Marine Battery Chargers will provide the intelligent charging system your battery needs for a long life, with low maintenance.

Dry cells

Batteries are split between dry cell and wet cell. The old original dry cells were the kind with a carbon rod down the middle, and they had little power and shelf life. Current non-rechargeable dry cells have an excellent shelf life and generally provide 1.5VDC. Rechargeable dry cells have a shelf life of less than a year before needing to be charged again.

NiCads have given rise to the idea of batteries having a memory. Failing to fully discharge a NiCad before recharging it eventually led to a loss of battery life. Companies started selling chargers that "conditioned" NiCads by setting up a discharge cycle before charging them.

None of that applies to other batteries, such as NiMH and lithium batteries. There is no need to "condition" these batteries, and it is better to keep them on a float charge and fully charged than it is to cycle them by discharge then recharge. Cycling a battery makes it wear out faster.
Wet cells

Wet cells are the batteries we have in our cars - an acid and lead plates that generate electricity. Most car batteries are not sealed. They will spill or leak acid if not kept upright. Sealed lead-acid batteries will not leak, although they may discharge gasses if not charged properly. Sealed lead-acid batteries come in two forms: Gel cells and AGM (absorbed glass mat).

Gel cells are somewhat more difficult to manufacture (getting the gel just right is the problem); instead of liquid acid, the acid is in a gel form which remains between the lead plates. Gel cells are said to work better than AGM batteries in conditions where they have long use and frequent cycling. Golf carts and wheelchairs are uses where gel cells work better than AGMs. The batteries are more deeply discharged and are used frequently, then recharged.

AGMs seem more efficient at uses where they are deeply discharged but only infrequently, being kept on a floating charge. Uninterruptible power supplies is one such use. The battery in a UPS is kept fully charged while the power is on, but when the power goes off, the computers, monitors, printers, and such pull a lot of current from the battery until they can be shut down properly.

The differences between gel cells and AGMs may make no difference to non-critical users who need a big capacity, portable battery for occasional use. This may include ham radio operators who need a big battery for a day or two in the field for an event or for campers who want a battery on camping trips to run lights or to recharge AAs.

Using a charged battery discharges it. Wet cells and dry cells "self discharge" over time. Wet cells can maintain a workable charge for about a year after the last full charge. When cold, wet cells loose power and cannot deliver their rated charge. Instead of waiting a year for the battery to run down, either recharge your wet cell every six months or keep it fully charged all the time by putting a floating charge on it.

When the manufacturer gives a rated capacity, it is based on a draw of 1/20 of the rated capacity. This is sometimes expressed by the formula 1/20 x C, where C is the rated capacity. If the battery has a manual, it may have a chart showing different capacities based on different draws. If the discharge rate is greater than 1/20 x C, the battery will not last for its rated capacity. For example, a 10 amp-hour battery discharged at 1/4 x C will not last for the expected time.

"Full" discharge for a 12V lead-acid battery is 10V. When fully charged, a 12V battery will measure over 13.2V at the terminal. When the battery shows 10V at the terminal with no load, the manufacturer deems the battery fully discharged. Frequent full discharges damage the battery, causing the lead plates to sulfate. If you have a 7 amp-hour battery and drain it at the recommended 1/20 x C until the battery is at or less than 10V, you will damage the battery, and it will no longer charge and deliver its rated capacity.

Going back to the electric cart, if you go out for a weekend of shopping and dining and use the cart for a couple of days without recharging, even if you drain the preferred 1/20 x C from a 7 amp-hour battery for 24 hours (12 hours at the mall both days), you have drained (.35 amps X 24 hours =) 8.4 amps from the battery - it will be dead before you finish your second day, and the battery will be over discharged. On the other hand, if you have a UPS and the power fails, you have a job to finish, so you run the computer, monitor, and printer for 30 minutes to print, save, and shut down, but your gear draws 7 amps, you have drawn only (7 amps X .5 hour = ) 3.5 amps from a 7 amp-hour battery. It will recover.

The rule of thumb is to divide the capacity by 10 (C/10) to recharge a battery. With the 7 amp-hour battery, we want about 700 milliamps to recharge it. A hundred or so milliamps either way will do no damage. Smaller amperages will take longer to charge and may not ever fully charge a big enough battery. Larger amperages, though, may damage the battery. The higher current will heat the solution (liquid or gel), causing it to give off gasses. Sealed batteries have one-way valves to vent the gas, but remember the gas is hydrogen, which is flammable. Do not charge batteries in an enclosed box. With non-sealed lead-acid batteries, the liquid will evaporate when overheated, exposing the lead plates and allowing them to sulfate.

This rule of thumb means that it can take over half a day to fully charge a battery; 15 hours is not uncommon.

If your recharger is too big for the battery (providing, for example, 2 amps for a 7 amp-hour battery) and the charger provides a switch from charge to maintaining a float charge, the battery may never provide enough resistance to the charger for it to sense that the battery is fully charged and to switch from charge to float. This means you will still be charging the battery as long as the charger is connected, and you will overcharge the battery.
Battery life

Battery life depends on many factors, and I'll cover only cycles. Discharging a battery, then recharging it is a cycle. A sealed lead-acid battery which is fully discharged (as defined above) has a life of about 200 cycles. If a battery is subject to "shallow" discharges (30% or 40% down from the voltage read when fully charged), then the life may be 800 or more cycles. It is better to keep a sealed lead-acid battery on a floating charge than to allow it to fully discharge by sitting a year between uses. The next best option is to fully recharge the battery monthly.

People who use their batteries infrequently and then use them to full discharge (and beyond) will damage their batteries and shorten the battery's life substantially from any advertised life. I have in mind people who go camping once a year for a week. They don't recharge their batteries until a day or two before they leave, drain the batteries dead during the week, then bring the batteries home and put them away discharged until next year. If the batteries are not critical to safety or health, this is not a problem when they fail early and often.

You can estimate your battery's current condition by fully charging it, measuring the voltage (it should be over 13.2V), and putting a known draw on the battery while keeping the volt meter connected. Let's say that you have an automobile headlamp that you know draws 4 amps. If you have a 12 amp-hour battery and fully charge it, connect the lamp and volt meter to the battery, turn the lamp on, and note the time. When you notice the light fading, start watching the meter and note the time the voltmeter reads 10.5V. Let's say it took 2 hours. For 12 amp-hour battery, the battery's rated capacity at a draw of four amps is 3 hours, so your battery's capacity is now 2/3 its rated capacity. Its capacity will continue to degrade until you find the battery useless and recycle it.

Answers to Common Questions about Batteries

Do lead acid batteries discharge when not in use?

All batteries, regardless of their chemistry, will self-discharge. The rate of self-discharge for lead acid batteries depends on the storage or operating temperature. At a temperature of 80 degrees F. a lead acid battery will self-discharge at a rate of approximately 4% a week. A battery with a 125-amp hour rating would self-discharge at a rate of approximately five amps per week. Keeping this in mind if a 125 AH battery is stored for four months (16 weeks) winter without being charged, it will loose 80 amps of its 125-amp capacity. It will also have severe sulfation, which causes additional loss of capacity. Keep your batteries charged while not in use!

Do lead acid batteries develop a memory?

Lead acid batteries do not develop any type of memory.

Do I need to completely discharge my lead acid battery before recharging it?

No, in fact you should never discharge your lead acid battery below 80% of its rated capacity. Discharging it below this point or 10.5 volts can damage it.

When do I need to perform an equalization charge?

Equalizing should be performed when a battery is first purchased (called a freshening charge) and on a regular basis (every 10 discharge cycles or at least once a month). Reduced performance can also be an indicator that an equalizing charge is needed.

What is an equalizing charge?

An equalizing charge for a 12 volt battery requires that it be charged with a voltage of at least 14.4 volts for a period of at least one hour once a month, or every 10 discharge cycles. An equalizing charge prevents battery stratification and reduces sulfation, the leading cause of battery failure.

When should I add water to my batteries?

How often you use and recharge your batteries will determine the frequency of watering. Also, using batteries in a hot climate will require more frequent watering. It is best to check your battery water level frequently and add distilled water when needed. Never add tap water to your battery. Tap water contains minerals that will reduce battery capacity and increase their self-discharge rate.

Warning - A brand new battery may have a low electrolyte level. Charge the battery first and then add water if needed. Adding water to a battery before charging may result in overflow of the electrolyte.

What is the proper electrolyte level?

Battery electrolyte levels should be just below the bottom of the vent well, about - inch above the tops of the separators. Never let the electrolyte level to drop below the top of the plates.

Do I ever need to add acid to my battery?

Under normal operating conditions, you never need to add acid. Only distilled or deionized water should be added to achieve the recommended electrolyte levels.

Can my batteries freeze?

If your battery is partially discharged, the electrolyte in a lead acid battery may freeze. At a 40% state of charge, electrolyte will freeze if the temperature drops to approximately -16 degrees F. When a battery is fully charged the electrolyte will not freeze until the temperature drops to approximately -92 degrees F.

What are the most common mistakes made by owners of lead acid batteries?

Undercharging - Generally caused by not allowing the charger to restore the battery to full charge after use. Continuously operating a battery in a partial state of charge, or storing the battery in the discharged state results in the formation of lead sulfate (sulfation) on the plates. Sulfation reduces the performance of the battery and may cause premature battery failure.
Overcharging - Continuous-charging causes accelerated corrosion of the positive plates, excessive water consumption and in some cases, damaging temperatures within the battery. Lead acid batteries should be charged after each discharge of more the 50% of its rated capacity and during or after prolonged storage of 30 days or more.
Under-watering - In lead acid batteries water is lost during the charging process. If the electrolyte level drops below the tops of the plates, irreparable damage may occur. Check your battery water level frequently.
Over-watering - Excessive watering of a battery results in additional dilution of the electrolyte, resulting in reduced battery performance. Add water to your battery after it has been fully charged, never when the battery is partially discharged.

Can I reduce the need to add water to my battery by lowering the charging voltage to 13 volts or less?

Lowering the charging voltage will reduce the need to add water, but this will cause a condition known as battery stratification. Battery stratification is caused when the sulfuric acid in the electrolyte mixture separates from the water and begins to concentrate at the bottom of the battery.

This increased concentration of acid increases the formation of lead sulfate (sulfation). To prevent stratification, your battery should receive a periodic equalizing charge (increasing the charging voltage to 14.4 volts or above).

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