DC Supply! Monthly Electrical Ezine

CBC Design (tm) - April 2002 Issue. ISSN 1475-3464
Email: cbc_design@btconnect.com

"...Maintaining a reliable DC supply."



- Editorial

- Sulphation in Lead Acid Cells. (Article)
- Protecting Sealed Lead Acid Batteries. (Article)
- Competition - Win FREE battery pack!.
- Readers Questions

- Subscriber Ads



Well, spring is just around the corner and some of us will be looking forward to warm

days and a more relaxed way of life. The batteries many of us stored away for the

winter, on a temperature controlled constant potential battery charger hopefully, are

ready to be re-installed in our boats and vehicles.


Unfortunately, there will be a few batteries here and there that have been on charge

all winter but cannot deliver any current. They are victims of a phenomenon known as

sulphation, largely caused by in appropriate charging potential or deep discharge

and an often deadly condition, this is the subject of our first article.


Ok so your batteries died. Could it have been avoided, possibly but it is difficult to be

sure unless the history of the cells is well known. The best thing to do is replace the cells,

look after them for as long as possible, charging them correctly, and apply appropriate protection. Have a look at our second article and learn how to avoid problems next time



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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Editor: Alan Fidler.

Alan is the owner and manager of CBC Design, a leading battery management company

based in the UK. He has worked in the industry for over eighteen years and has designed charging equipment and battery monitors for some of the world largest companies.

ARTICLE: Sulphation in Lead Acid Cells. Author: Alan Fidler.


Those of us who work with batteries on a daily basis have undoubtedly come across

sulphated cells at one time or another. The symptoms are always the same. The

battery reaches the float voltage on a charge cycle very quickly drawing hardly any

current to do so. On discharge, the battery has no capacity and the voltage drops

very quickly in seconds or minutes rather than hours.


Of course the problem may not manifest itself until a load is applied for the first time

since the battery was put into storage or discharged the last time. The charger may

have maintained the float voltage but the battery still won't work because the cells

have been left in a fairly inactive state and sulphation has begun.


So what is sulphation and how can it be prevented?


The electrolyte inside a lead acid battery is usually in the form of a liquid sulphuric

acid. When batteries are left in a discharged condition or are simply not used, the

acid is absorbed by the lead plates and a substance called lead sulphate is

formed. The sulphate forms an insulating layer on the plates which increases the

internal resistance of the battery and prevents it from drawing or delivering current.

It does not kill the battery completely though if it is discovered early enough.


A typical example of a sulphated battery is a vehicle type which struggles to turn the

car over in winter but recharges quickly once the engine starts. The battery generally

behaves for the rest of the day only to repeat the performance the following morning.


Unfortunately, sulphation is the biggest cause of premature cell failure in most

industrial and automotive applications. There is a school of thought that suggests

that a periodic exposure to a higher potential of 2.45 volts per cell can help to

prevent sulphation or break it down where it has arisen. however we already

know that a high charge rate can cause internal heating of the battery which buckles

the plates and if this happens, the battery is permanently damaged.


The higher charging voltage certainly helps in some cases, but it should only be used

where cells are displaying the symptoms already. We would NEVER recommend applying

a higher than normal charging voltage to a healthy battery. If you recharge the battery

in the correct way for the application in which it is used, sulphation will not be a problem

to begin with.


We have visited websites during our research into this subject which state that

vehicle batteries benefit from a charge rate of 14.5 volts instead of 14.1. We agree

wholeheartedly since the application is cyclic in nature but there are other applications, 

typically stand-by, where this potential would damage the cells permanently. As always,

our advice is to check the battery manufacturers data and follow their advice first.


In reality, the actual charge voltage depends upon the application in which the cells

are used. Traction and automotive batteries require a different charging voltage

to batteries used in standby applications. Often, an installation for standby use has

no higher charging voltage or boost level at all but sulphation never occurs because the batteries are discharged every month or so, religiously, then immediately recharged.

This sort of regime prevents sulphation even though the charging voltage never exceeds

2.25 volts per cell at 20oC.


One thing is certain. There is a point when a battery become so severely sulphated

that it cannot be recovered, ever. Batteries have a finite life and will eventually fail

anyway dispite your best efforts. We managed to squeeze 4.5 years out of a battery

designed with a three year life in a vehicle in the UK which is about as good as it gets.


Most battery manufacturers recommend constant potential charging methods with

an equalising charging voltage on cycled cells to get the best performance from their

batteries. Furthermore, they are very keen that users should use temperature compensated chargers in applications where the ambient temperature varies. This is by far the best way to prevent sulphation in the first place. Charging the batteries correctly at all times as well as making them do some work occasionally will do more to prevent failure than anything else.


One interesting point is that cells which are partially sulphated are actually useful in

the testing of a typical constant potential battery charger. The float voltage can be

set with the minimum of charging current whilst a suitable load allows the current

limit to be set. Healthy batteries take time to charge up from a partial self discharge

and this can be somewhat frustrating so, sulphation can be beneficial to some anyway!




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ARTICLE: Protecting Sealed Lead-Acid Batteries. Author: Alan Fidler. 


Unlike Nicad batteries which, as far as abuse is concerned are more forgiving, lead acid batteries demand particular conditions to work at their best. There is a distinct voltage band within which a lead acid cell will operate. Push the cells above or below it for any length of time and they will be permanently damaged.


Lead acid batteries need to be protected then, particularly from deep discharge. If a typical

battery is allowed to fall much below 1.55 volts per cell and is left in this state for some

time, it will never recover, no matter how long it is left on charge. It has been damaged

permanently and must be taken out of service. A dreadful waste I am sure you'll agree.


The first problem, Overcharge, stems from a charger failure or incorrect float voltage setting.

Over charging a battery does two things, it causes the battery to gas and pushes the

internal temperature of the cells above recommended levels. This usually results in buckled plates, lost electrolyte and in the case of most lead acid batteries, a damaged pack.


Clearly the battery must be protected if you want total peace of mind. If the application

in which the battery is employed isn't a critical one, it may just be inconvenient when the

battery fails but in some applications, failure could lead to loss of life.


There is though a third condition which we covered in our first article, that of sulphation

which can also cause problems and it must be prevented for as long as possible or detected

at an early stage if reliability of the cells and the installation as a whole is important.


So to summarise, we have three conditions that must be avoided if we want the best from

our batteries, high voltage, low voltage and cell sulphation. Ignoring any one of these three

has disastrous consequences.


A high voltage alarm is by far the best way to protect the cells from over charge. It can be configured to energise a siren or led and disconnect the charging supply to prevent battery

damage. We also recommend fitting a fuse in series with the charger output positive lead

to protect the battery in the event of a charger rectifier diode short. Sadly, diodes have a

tendency to go short circuit when failing rather than blowing open circuit and this results

in unrestricted current flow and a high charging voltage.


Deep discharge is prevented by using a Low Volts Disconnect device. A typical LVD includes

a low volts monitor and a heavy duty relay or contactor which disconnects the load when the

battery reaches a pre-determined minimum. A siren or led can be energised by the alarm to

warn the user that the battery is approaching its minimum limit.


Sulphation can be avoided by charging the battery correctly and discharging the cells on a

monthly basis to keep them active. Wet lead acid cells must be topped up with

distilled water. Maintenance free cells require nothing other than the above to keep them

in tip-top condition.


That's really all there is to it. Protect the batteries from these three conditions and you will

enjoy many years of service from them. Neglect any one of the above and you will find your

self having to purchase new cells prematurely.


Remember: Look after your batteries and your batteries will look after you!




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Questions from Roger Burrows! 


Question 1.

What happens if a Sealed Lead Acid Battery is overcharged?


Sealed lead-acid batteries are usually fitted with special vents

which release some of the pressure that build up in the battery

through heat. The battery is permanently damaged of course. 

Question 2.

How can a battery be protected from overcharge?.


By fitting a fuse in series with the charger output in case of

failure and by using a High Volts Monitor or Alarm to detect the

high charge condition so action can be taken before the battery

is damaged.




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