Lead-Acid Batteries were invented in1859 by a French physicist.  They are the oldest type of rechargeable battery.  They are the most popular starting (car) and marine battery worldwide because of their inexpensive cost compared to their electrical output.

Lead-Acid Batteries come in many voltage and cell configurations but this article will mostly be concerned with 12 volt, 6 cell, negative ground, starting and deep cycle batteries.  We will also address 6 volt, 3 cell batteries but in a more limited way.


Battery Basics

A 12 volt Lead-Acid battery is actually six 2.11 volt cells connected internally in series POSITIVE (+) terminal of the first cell to the NEGATIVE (-) terminal of the second cell and so on).  Six 2.11 volt cells connected in series create a 12.66 volt battery.

Each cell consists of lead plates (the NEGATIVE (-) plates) and lead oxide plates (the POSITIVE (+) plates) covered with a 35% sulfuric acid and 65% water solution called electrolyte.

The electrolyte acid being in contact with the lead and lead oxide plates causes a chemical reaction that releases electrons to produce electricity.

A lead-acid battery is always either discharging or charging.  If a battery is just sitting, it is actually discharging.  As the battery discharges, the sulfuric acid of the electrolyte reacts with the materials of the plates, changing the plate surface to lead sulfate.

A battery recharges when a current is given to the battery.  When the battery is recharged, the chemical reaction is reversed, the lead sulfate reforms into lead dioxide, lead and sulfuric acid electrolyte. With the plates restored to their original condition, the battery is recharged.

The potential energy of the battery is based on how many free floating sulfuric acid molecules are available in the electrolyte and how much surface there is on the plates for the sulfuric acid molecules to attach to.  We’ll address battery-life do’s and don’ts latter.


Battery Safety

A chemical reaction occurs that creates hydrogen gas when a battery is recharged.  You should recharge your lead-acid batteries in a well ventilated area that does not have an i
gnition source nearby.

Sulfuric acid is a corrosive acid. If it comes in contact with the skin, it can cause burns.  If it splashes in your eyes, it can cause blindness.  If your skin comes in contact with the acid electrolytes, flush the area with water.  If your eyes come in contact with the acid electrolytes, flush the area with eye wash, saline wash or water and get medical help.


If metal objects come in contact with acid electrolyte, the acid can be neutralized with a water and baking soda solution.


Battery Types

We are going to address two main battery types.  Starting or Car Batteries and Marine or Deep-Cycle Batteries.  Starting batteries are designed to produce large surges of energy or cranking amps. This is done by increasing the number of plates in the batteries but to maintain battery size, the plates are thinner.

Marine or Deep-Cycle batteries are made to produce slow, longer-term energy and have thicker plates.  Plate thickness has a lot to do with how deeply the battery can be discharged and then recharged.  Thin plates cannot take very many deep discharges where thick plates handle deep discharges and recharges much better.

Some battery experts don’t consider a marine battery as a true deep-cycle battery.  If fact they consider Golf Cart Batteries and Marine Batteries as “lite” deep-cycle batteries.  To these experts batteries like forklift batteries qualify as a true deep-cycle battery because they can stand thousands of recharge cycles.  For our purposes I’ll use the terms marine and deep-cycle interchangeably.

There are three sub-types of lead-acid batteries.  The most popular is the Flooded or Wet-Cell.  Flooded batteries can be Serviceable (removable cell caps) or Maintenance Free (sealed cells).  I like the serviceable batteries so I can check electrolyte levels and check the charge of each cell.

The other two types of batteries are Gel-Cell and AGM (Absorbed Glass Mat).  These two types cost twice as much as Flooded batteries but they do have some advantages.  They are sealed batteries so the safety concerns with hydrogen gas and sulfuric acid are greatly diminished.  AGM batteries are replacing Gel-Cell batteries.  AGM battery retain a charge longer but need a charger that provides a specific charging rate.


Battery Life

A lead-acid battery has a natural life based on the materials that are used to make it.  Based on the best of circumstances a starting battery normally has a 5-year life and a marine battery has a 4 year life.

As a battery discharges and recharges, the leads plates are broken down.  Even with the best of care, there comes a time when the lead plates can’t take or give back molecules from the sulfuric acid electrolyte because the plates are just worn out.

It is estimated that over 50% of batteries die because of poor maintenance and neglect before they reach the end of their natural life.


Battery Maintenance

Proper battery maintenance can help prolong the life of your battery.  Battery maintenance should be done at least every 3 months and more often during hot summer months.

1. Check battery terminals for corrosion or rust.  Remove any corrosion or rust with a brass wire brush (steel wire brush may damage the terminals).  If the corrosion is heavy, you can neutralize it with a solution of 2 – 3 tablespoons of baking soda in a pint of water. Be sure to place a pan or wash tub under the car to catch the run-off.          Tighten loose terminal clamps or lugs.  Applying a thin layer of wheel or silicone grease to the terminals, lugs, clamps and exposed cables.

2.  If you have a serviceable battery, make sure there is no debris or corrosion on the top of the battery that can fall into the cells and contaminate them.

Remove the fill or vent caps. Check the electrolyte level in each cell.  The electrolyte solution should cover the battery plates but should not be higher than the bottom of the refill well (electrolyte solution should not be in the actual fill tube or well).  Add only Distilled Water to any cell with a low electrolyte level. (Graphic Courtesy of Exide)




3.  Using a Hydrometer, measure the State of Charge (SOC) of each cell.  A hydrometer measures the Specific Gravity (sulfuric acid concentration) of battery electrolyte solution and displays a charge percentage.

Product DetailsE-Z Red S101 Battery Hydrometer Less than $10 shipped.

Measuring the SOC of each cell can tell you if you have a weak cell or if your battery is recharging fully.  It is possible that the cells will have different charge readings.  If cells have different readings you should Equalize your Battery.  A controlled 5% to 10% overcharge will equalize and balance the voltage and specific gravity in each cell.  Four-Stage battery chargers do this step automatically as part of charging.  More on chargers later.


4.  Using a “Smart” three or four stage charger, give your battery a top-off charge.


Battery Charging

Proper battery charging has more to do with prolonging battery life than any other thing except proper electrolyte level.  Improper battery charging can destroy a battery in just a few months.

Batteries that are not fully charged are losing battery-life.  The more uncharged a battery is, the more battery-life it is losing.  The deeper a battery is discharged before recharging, the more battery-life it is losing.

There are many types of battery chargers on the market but only one type is appropriate for everyday regular Joes.  That is the “Smart” charger.  A charger, with a micro-processor, that reads the battery’s condition and then applies the proper charge.  Smart chargers come in Three-Stage and Four-Stage chargers.  Gel-Cell and AGM batteries normally use Three-Stage chargers.  Float or Wet-Cell batteries use Three-Stage or Four-Stage chargers.


Charging Stages

Bulk Stage – The charger provides as much current and amperage as the battery will accept (to the maximum output capability of the charger) until the battery is 80% charged.

Absorption Stage – The charger provides a constant 14.4 volts (could be between 14.1 and 14.8 depending on the type of battery) with decreasing amps (current) until the battery is 98% charged.

Float Stage –  The charger provides a controlled voltage of 13.4 volts (could be between 13.0 and 13.8 depending on the type of battery) at only 1 or 2 amps of current.  If you have a battery that is not used often or that you put in storage for the winter, you can leave a smart charger attached to it and it will float charge your battery as needed.

Equalization Stage –  The charger provides a regulated 5% to 10% overcharge to balance the voltage and sulfuric acid molecule concentration in each cell.  This stage can correct Stratification and some cases of Sulfation.


Stratification and Sulfation

Batteries can develop a couple of conditions that affect the battery output and if left untreated can drastically reduce battery-life.  The first is Stratification where acid concentration is greater at the bottom of the cell than at the top of the cell.

The second is Sulfation.  As a battery discharges, lead and sulfur combine into sulfate crystals.  When a battery recharges, the sulfate crystals return to lead and sulfur molecules.  Sulfation occurs when the sulfate crystals do not return to lead and sulfur molecules.  Sulfation is caused by continually undercharging a battery (short trips in the car), by leaving a battery discharged, or by a low electrolyte level that exposes the top of the plates.

The longer the sulfate crystals remain in crystal form, the harder they become and the harder it becomes for them to return to lead and sulfur molecules.  Some sulfation can be reversed if the crystals are not too old and not too hard.

” This creation of hard crystals is commonly called permanent or hard sulfation. When it is present, the battery shows a higher voltage than its true voltage; thus, fooling the voltage regulator into thinking that the battery is fully charged. This causes the charger to prematurely lower its output voltage or current, leaving the battery undercharged.

Sulfation accounts for approximately 85% of the lead-acid battery failures that are not used at least once per week. The longer sulfation occurs, the larger and harder the lead sulfate crystals become.” (


Battery Chargers

Here are my current favorites battery chargers.  The Battery Tenders are slow charging with a maximum of 1.25 amperes.  They are best at maintaining a charge a battery after it is charged.  You can use them to charge a battery but it could take a day or more depending on the battery state of charge and capacity.

Battery Tender 021-0123 Battery Tender Junior 12V Battery Charger

“Smart” Three-Stage charger for all lead-acid, flooded, AGM and gel cell batteries. Spark proof and reverse polarity protected.  12′ output cord.  I use these to float charge batteries that are stored or not used often.  The maximum output is .75 amps.  Currently $23 shipped.


Battery Tender 021-0128 Battery Tender Plus 12V Battery Charger

“Smart” Three-Stage charger for all lead-acid, flooded and AGM batteries. Spark proof and reverse polarity protected.  The maximum output is 1.25 amps.  If you are recharging gel-cells, get the gel-cell model to maximize performance.  Currently $45 shipped.


Schumacher SSC-1000A SpeedCharge 2/6/10 Amp Battery Charger and Maintainer “Smart” Three-Stage charger for all lead-acid batteries. Charge rates include: 10 Amperes rapid charge, 6 Amperes fast charge, and 2 Amperes trickle charge  Currently $49 shipped.



Schumacher SE-1555A Automatic Elite Wheel 12 Volt Battery Charger with Engine Start  “Smart” Four-Stage charger for all lead-acid, flooded and AGM batteries. Spark proof and reverse polarity protected.  Charge rates include: 150 Amperes engine start, 55 Amperes rapid charge, 20 Amperes fast charge, and 2 Amperes trickle charge  Currently $99 shipped.





Depth of Discharge

Depth of Discharge is an important concept to help maximize battery life.  The depth to which a battery is discharged affects the life of the battery.  Very deep discharges will shorten battery life.

To maximize battery life, car batteries should not be discharged past 75% (80% is better), and deep-cycle batteries should not be discharged past 50% (60% is better).

You can use a hydrometer to measure the specific gravity of the electrolyte solution and determine the SOC (state of charge).  Here is a useful chart.

State of Charge

Specific Gravity





































There is a little difference between car batteries and deep-cycle batteries because of the number and thickness of the battery plates but the difference is not enough to worry about.

There is an easier way to check the state-of-charge.  You can attach a battery monitor to your battery or battery bank.  I have one attached to my battery bank and it makes checking the battery bank’s SOE very easy.



Equus 3721 Battery Monitor   Displays the condition of the battery and charging system.

For 12-volt negative ground systems.  Easily plugs into a cigarette lighter socket.  Head pivots for easy viewing.  Currently $15 shipped.


You will need to a cigarette lighter socket adapter.

Roadpro 12v Battery Clip-On and Cigarette Lighter Adapter  Run 12 volt appliances by connecting directly to car battery terminals.

Currently $5 shipped.


Battery Do’s

  • Inspect and service your battery every three months, more often in the summer.
  • Make sure plates are covered by electrolyte solution.
  • Only use Distilled Water when adding liquid to electrolyte solution.
  • Keep your battery as fully charged as possible.
  • Use a “Smart” Three-Stage charger.
  • Reduce your average depth of charge.
  • Perform a top-off charge at least every three months.
  • Keep a float charger on batteries in storage or batteries not used regularly.
  • Make sure batteries and the battery bank are fully charged before adding a battery to a battery bank.
  • Use #0 AWG cable or thicker to connect batteries in a battery bank.
  • Connect a 300 amp fuse to the Positive (+) lead between the battery bank and the inverter or other appliance.


Battery Don’ts

  • Don’t add more acid to the electrolyte solution.
  • Don’t use tap-water to add fluid to the electrolyte solution.
  • Don’t use an unregulated, high output battery charger to charge your battery.
  • Don’t overcharge your battery.
  • Don’t discharge deep-cycle batteries more than 50%.
  • Don’t discharge car batteries more than 30%.
  • Don’t let your battery sit unused or uncharged with a float charger.
  • Don’t mix different size, type, and age of batteries in a battery bank.


I have just scratched the surface of information for Lead-Acid Batteries.  Thankfully there is a great FAQ that has tons of great information.  If you need additional information or want to explore subjects that I haven’t touch on here (battery testing, battery jumping, battery installation),  I highly recommend Bill Darden’s Battery FAQ.  CAR AND DEEP CYCLE BATTERY FAQ by Bill Darden



Ed Rogers
Copyright “Keep It Simple” 2013. All Rights Reserved.

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