The Lead Acid Battery

It is easy to take the battery for granted until it fails so it is an advantage to know how a battery functions and how to maintain it in good working order.

The car battery consists of a number of cells coupled together to give either a 6-volt or 12 volt energy supply. The definition of an electric cell is a device for generating electricity by chemical action.

Each cell will have a terminal voltage of approximately 2 volts, dependent on its state of charge, as described later.  For a 6V system 3 cells are required and for a 12V system 6 cells are required.  To obtain the correct voltage these are coupled in series, that is one after the other.

The principle components of each cell are the positive + and negative – plates, the separators to keep the plates apart (which stops the cell being short circuited) and the electrolyte, the fluid in the cell.  The cells are housed in one box and coupled together with lead straps.  In some batteries the lead straps connecting the cells are external and you can see the cell series connections. + 2v - + 2v - + 2v -; giving 6 volts.  In other batteries the connections are internal and all that can be seen is the terminal pillars, positive and negative.

The positive and negative plates consist of two types of lead and with the plate separators sit in the electrolyte fluid.  The Negative plate is made from pure Lead, whilst the Positive plate is made of a compound of Lead, Lead Dioxide.  At the plates, the action of discharging the battery converts both plates into a new compound, Lead Sulphate, and the acid is converted into water, reducing the concentration of acid in the electrolyte.  Recharging the battery reverses this action, with the sulphate returning to the sulphuric acid and the plates convert back to the previous materials.  If a battery is allowed to discharge too far, this conversion becomes permanent, and the battery is said to be “sulphated up”.  This sulphation is also a problem if a battery is unused for a long time – hence the advice to occasionally trickle charge a battery that isn’t in use.

Now we do need to know a little about this fluid, as we need to look after it.

It is a mixture of pure distilled water and sulphuric acid roughly in the ratio of 10 parts water to one part acid. It’s still plenty strong enough to cause serious injury to people and paintwork, so treat it with care and ALWAYS wash off spills.

lead 1

The electrolyte level is important and must always be maintained just above the top of the plates.  If the electrolyte liquid level drops too low, the plates are exposed to air, lose capacity, and are damaged.  Topping up is done by adding pure distilled water to each cell.  There should never be a need to add sulphuric acid since it is not consumed even on overcharging.

The varying amount of water in the electrolyte changes its “specific gravity”.  (The specific gravity of the electrolyte is its weight compared with the same amount of pure water).

The table below shows the specific gravity at 20 deg C; it varies a bit with temperature. To measure it, you need a Hydrometer, which will include a correction chart for temperature. For the serious battery enthusiast, you can buy these at most motor factors.

lead 2

The Hydrometer is rather like a large pipette with a weighted float inside.  It is very easy to use: just squeeze the bulb to suck up enough acid to cause the float to rise a little.  Then read off the specific gravity from the scale (A) and correct the reading from the manufacturer’s temperature chart.  Then return the acid to the cell.  Take care not to drip acid on your paintwork!

Some batteries even have these built in, looking like a green spot in a window when the battery is charged.  Usually, it’s easier to estimate the charge using a cheap digital voltmeter.




The open circuit voltage is measured when the engine is off and no loads are connected.  It can be approximately related to the charge of the battery by:

Open Circuit
Voltage (12V)
Open Circuit
Voltage (6V)
Relative acid
Water = 1g/cm3
12.65 6.32 100% 1.265g/cm3
12.45 6.22 75% 1.225g/cm3
12.24 6.12 50% 1.190g/cm3
12.06 6.03 25% 1.555g/cm3
11.89 6.00 0 1.120g/cm3


The following is common for a six-cell car battery at room temperature:

  1. Quiescent (open-circuit) voltage at full charge: 12.6V or 6.3V

  2. Fully discharged: 11.8V or 5.9V

  3. Charge with 13.2 - 14.4V or 6.6V - 7.2V

  4. Gassing voltage: 14.4V or 7.2V

  5. Continuous maintenance or trickle charge with max. 13.2V or 6.6V

  6. After full charge the terminal voltage will drop quickly to 13.2V / 6.6V and then slowly to 12.6V / 6.3V

  7. For best results, open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate and enable a more accurate reading.

All voltages are at 20 °C, and must be adjusted -0.022V/°C (12V) or –0.011V/°C (6V) for temperature changes.



  1. Always keep the electrolyte level just above the plates.

  2. Keep the top of the battery clean and lightly grease the terminals with Vaseline.

  3. Batteries will self-discharge over time therefore if the car is unused for any long period then charge the battery from time to time.

  4. Never discharge the battery below 5.5V or 11.1V i.e. 1.85V per cell, or you will almost certainly ruin the plates.

  5. Now the big don’t.  DANGER; when a battery is being charged, it will emit gas.

The gasses it gives off are hydrogen and oxygen, a highly explosive mix.  No naked lights or sparks should be in the vicinity of the battery.  If the gasses ignite, the explosion will shatter the outer casing spraying acid and shrapnel.  So, NEVER connect or disconnect your charger when it is switched on.

When buying a new battery, purchase the largest battery that will fit into your car, that is, the one with the biggest ampere-hour capacity.  Ampere-hour capacity is a term to measure the electrical energy which can be expected from a fully charged battery.

The capacity that manufacturers print on a battery is usually the product of 20 hours multiplied by the maximum constant current that a new battery can supply for those 20 hours at 68 F° (20 C°), down to a predetermined terminal voltage per cell. Most Austin – sized batteries are around 50Ah, which means that you should expect to be able to draw 2½ Amps, (enough to run the ignition), for 20 hours before the battery risks being discharged too far.  (Discharging faster actually reduces the capacity a bit, hence the standard “20 hour” number.)  That’s why you should never worry about charging problems on a Club run – you will have plenty left if you don’t use the electric starter too much!


This article, written by Ian Leaver, originally appeared in CA7C Seven Focus in February 2011pp18-21.

NB:  Most 'Sevens' are fitted with a 56amp/hr battery, type 421, ie the poles are diagonally opposite.  The Big Seven is fitted with a 63amp/hr battery, also type 421.  Be careful to connect with the correct earth.


See also:

Battery Care and Maintenance

Failing battery - what's going on?

The Lead Acid Battery