The lead-acid accumulator was invented in the 1860's by Raymond Gaston Plante, and was the first electric accumulator or rechargeable battery, and is still one of the most used accumulator types since it offers high power at an affordable price. The positive electrode consists of lead dioxide while the negative is made from lead. The electrolyte is a strong acid, usually sulphuric acid. The first lead-acid batteries used an open glass container, but modern batteries uses a sealed and/or vented plastic container. Each cell have a nominal voltage of between 1.9 and 2.1V.

In cyclic charging a voltage of between 2.4v to 2.5v per cell is common, while in stand-by applications, like in UPSes and alarm systems, a voltage of 2.25v/cell is used.

The lead-acid accumulator is simple and cheap to manufacture, but due to the lead it is heavy and a can pose an environmental hazard. The battery might also vent hydrogen if overcharged(thus posing an explosion hazard), so good ventilation is important where a non-sealed battery is used. The most common usages for the lead-acid accumulator is in cars, boats and emergency power supplies.

Warning: The advice in this article is of a general nature and is targeted towards car starter batteries. If you have manufacturer's recommendations, follow them and ignore the specifics below, even though the general principle still applies. Batteries can be dangerous!

but as kidas points out: "...these things are *easy* for anyone to fit, so long as they have some vaseline ..." and that's for the battery terminals :-)

A traditional design for a secondary, or rechargeable battery, designed in the 1880s and not changed significantly since. In fact, about 60% of all batteries sold worldwide are lead-acid batteries.

Some of the reasons why this type of battery has become the workhorse of cordless power is that it is:


A lead-acid battery gives off electric current as the positive electrode's lead dioxide (PbO2) is converted to lead sulfate (PbSO4) at the same time as the negative electrode's pure lead (Pb) is also converted to lead sulfate (PbSO4). To make this possible you also need an electrolyte around the electrodes, and that is diluted sulfuric acid (H2SO4), normally around 30%, which provides the sulfate ion that is needed for the recharge.

When the battery is recharged by applying electrical current back into the battery, the process is reversed and the sulfate ions goes back into the electrolyte. Rest products are heat, hydrogen (H) and oxygen (O2).

The full reaction can be described as Pb + PbO2 + 2H+ 2HSO4 => 2PbSO4 + 2H2O + 2e- .

Each such battery cell can give a voltage of up to 2 Volts. That is why you see six little screw tops on a normal 12 V car battery.

Basic types

There are two main types of lead-acid battery design. Flooded and sealed-lead.

The flooded, or wet-cell, version is the original design from the late 19th century. It is very simple. The electrodes sit in a container with electrolyte. The top of the container is ventilated to let the explosive hydrogen out, and some way of filling it up with distilled water to compensate for electrolyte loss. This is the traditional car and truck battery.

Sealed-lead batteries, or Absorbed Glass Mat (AGM), first arrived in the early 1970s. The main goal was to create a cleaner and safer version of the battery. That this helped making the battery (almost) maintenance free made it popular in a lot of different applications, like computer UPS, rechargeable hand tools, security alarms, etc. Inside a sealed-lead battery the electrodes are wound around each other in a spiral pattern with a fibrous glass mat as separator. This means that the battery needs much less electrolyte, since it is either combined with the electrodes or absorbed by the separator. In addition it also allows gasses to move around freely inside the cell, which means that they are given the chance to recombine with the opposite electrode. Of course you also need a re-sealing security valve in case of excessive overcharge.


Before we go any further I would like to take a moment and look at the two different applications that lead-acid batteries are used for. One is high power for short time, like the start battery in your car, and the other one for lower power but prolonged time, like the battery in your golf cart or the ones you yse in your RV. Because the are very different in design on the inside it's important to remember that they are not interchangable. A start battery is not built for being seriously discharged, or deep cycled. If you do that a couple of times you will need to replace it. Similarily a deep cycle battery cannot deliver the electrical current required to turn a start engine. OK, there are dual purpose batteries that are supposed to be able to do both, but it's a compromise, and as such not really good at anything. Well, not bad either, but...

Choosing a battery

So we now know what kind of battery type we need. We could be cheap and go for the wet-cell or aim for the more expensive AGM. The advantage of the latter is longer life and less maintenance. On the other hand it's more intricate in its need for charging pattern. Anyway, on top of that we know if we want a starter battery or a deep cycle one. So we go to the store to make a choice.

Once in the store we are met with more choices. Like how many AH do you want, what CA to you need and how much RC is necessary. It's not that difficult.

AH stands for Ampere Hours and tells you how much energy the battery can store, or how many plates each cell has. A 60 AH battery can deliver 1 A for 60 hours or 5 A for 12 hours, etc. A higher AH makes a better deep cycle battery.

CA means Cranking Amps and is a measurment of how many Amperes the battery can deliver a 0° C for 30 seconds without dropping below 7.2 V. High CA is good in a starter battery.

RC stands for Reserve Capacity. It's a measurment for how long the battery can deliver 25 A until the voltage drops below 10.5 V. This is measured at 25° C. The higher the RC the better.

On top of that you can bet your ass that they will try to sell you the oldest battery in store. Batteries degrade by sitting on the shelf because it allows oxidation to occur, so try to see if you can decipher the manufaturer's dating code. So bottom line is buy a fresh battery with as high RC or AH rating you can fit into your appliance.


Once the battery is installed there are a lot you can do to give it a long life. We don't want to have to buy new batteries all the time, do we?


Before we worry too much about the battery's life, let's consider our own. Batteries are full of acid and generate highly explosive hydrogen gas when charged. That is not a good combination. So make sure that you are in a well ventilated area, that there is no open flame in the vincinity, that you're wearing protective glasses, and finally that your clothes are either made of polyester, or that you don't care if you get a hole or two in them.

On top of that we are talking about high electric currents, so make sure that you take off metal jewelry and disconnect the ground cable from the battery before doing ay electrical work.


Start by disconnecting the cables from the posts and clean the battery with a mixture of baking soda and water to get rid of acid residues and oil and grime. If you have a wet cell battery, open the cell lids and make sure that there is electrolyte above the lead electrodes and top up with distilled water so that you have about 5-7 millimeters' margin to the top of the filling tubes. Don't overfill, since this can cause acid to leak out when the electrolyte expands, like during charging.

Most problems with batteries are due to bad connections, so make sure that your connections are tight and free from oxidation. The best way is to clean off any old dirt using a metal brush or a piece of sand paper. Then you put a small amount of silicon sealer at the base of the post, put a felt battery washer over it and cover the washer with high temperature grease or petroleum jelly. Put the cable on and tighten, and then cover the exposed parts of the connection with the grease.


To test a battery you can either check the voltage or load capability (all types), or you can measure the specific gravity of the electrolyte. The simplest is to measure using a volt meter. On a fully charged 12 V car battery you should have 12.7 to 12.9 V. If your battery is fully charged, but you only get 10.5 V one of your cells has a short circuit and you need a new battery.

Thanks to the chemical reactions inside the battery the specific gravity of the electrolyte changes with charge. This is the best way of noticing if your battery needs replacing, but can only be done on wet cell batteries. Anyway, first of all you make sure that your battery is fully charged. Then you need to remove the surface charge. On a car that can easily be done by turning on the full beam for about 3 minutes and then wait a few minutes. Of course you have to have the engine off, otherwise it will still be charging. After that you can measure the specific gravity in the individual cells using a temperature compensating hydrometer. And, yes, you can buy them at an auto parts store. Here are the SG:s

Charge level      Specific gravity     Voltage in a 12V battery
   100%              1.265                12.7
    75%              1.225                12.4
    50%              1.190                12.2
    25%              1.155                12.0
   Discharged        1.120                11.9

At 75% the electrodes starts suffering from sulfation which decreases the capacity and ability to receive and retain charge. That is why it's important to always keep a lead-acid battery fully charged.


In a car the battery is normally charged by the alternator. This works fine if the battery is almost fully charged, like in normal daily use. However, if the battery has been discharged, say by lights left on, the alternator has a tendency to overcharge the battery. If this happens only a few times the capacity of the battery will be seriously reduced. The only way to charge the battery safely is to use a computerized charger, usually called smart charger. A smart charger does the charging in three steps. The first step is the bulk charging, which brings the battery up to 80% of capacity using a current up to half of the AH capacity of the battery. Once it hits 80%, or 14.4V, it start the absorbtion stage keeping the voltage constant reducing the current as necessary. Once the battery has reached 98% the float step kick in. Here the voltage is reduced to 13.4 V and the current limited to 1 A. This ensures that the battery doesn't start boiling. This kind of charger is necessary for lot of AGM batteries, although some might require other types. It will at any rate give the battery a much better life.

Do:s and Don't:s

There are some things that you should do with your batteries:

Along the same lines there are some thing you should never do:

  • Don't add new sulphuric acid.
  • Don't charge with cheap (unregulated) chargers.
  • Don't disconnect the cables with the engine running.
  • Don't use any other water than distilled water.
  • Don't let a battery get hot when charging.
  • Never let the electrolyte come in contact with salt.
  • Don't forget to take safety precautions.


As neither sulphuric acid nor lead are exactly good for the environment you have to be careful when you dispose of old lead-acid batteries. Leave it for recycling. Normally you can leave your old battery where you buy your new one. If that should not be the case, contact your local municipality to check where you can safely dispose of old batteries. On a positive note it seems as if most people take this seriously. About 93% of all battery lead is recycled, making it the most highly recycled consumer product. Help in making sure we go towards 100%!


  • Battery Care Tutorial -
  • Battery Council International -
  • Battery Resources & Information Links -
  • Hawker Energy -
  • Lead Acid Battery Theory -

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