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Battery desulphation.


Having lived aboard yachts for more than thirty years I have been through several sets of batteries. Some lasted well and some did not. But like most people I need batteries aboard as I want to use all the modern electronic tools that are available today.

We want our batteries to last for as long as possible and to fully understand how to make them last I have been experimenting and researching this subject for several years, which has included carrying out tests aboard my yacht.

What follows is a summary of my experiments and my conclusions. If you only want to see a simple list of the rules to be followed to make your batteries perform well and last well then to jump to the list at the end of this article. Click here

When away from shore power the batteries need to be capable of being charged quickly either by the use of a high power alternator controller or by a battery charger powered by a generator.

This means that the only batteries which are suitable are normal lead acid which can be topped up with distilled water. Sealed, AGM and Gel types are not suitable for fast charging as when fast charging gas is given off which must be replaced with distilled water.

We want our batteries to last as long as possible and experience has taught that:-

1/ For general yachting, use low cost lead acid batteries which can be topped up with water. So called leisure batteries are best.

2/ For long term cruising or living aboard, then 6 volt traction types are best. 

As the battery is used and discharges sulphation takes place. The lead from the plates combines with the electrolyte to make lead sulphate. When the battery is recharged this process is reversed. However, some of the lead sulphate crystallizes on the lead plates. The deeper the battery is discharged, the more the lead sulphate crystallizes, acting as an insulator and reducing the plate area in contact with the electrolyte. Over time this build up of lead sulphate crystals goes deeper and deeper into the lead plates and becomes more difficult to remove, eventually rendering the battery useless. 

This is especially relevant to yachts as it can be difficult to fully charge our batteries. It is generally recognised that despite believing that the battery is fully charged in fact most yachts batteries are rarely charged above 80%. This is true despite the charging system showing the battery floating at around 13.6 volts or having charged to 14.8 volts.

(The voltage to fully charge a battery is discussed later.) 

When batteries spend long periods of partial discharge, which despite the use of solar panels and wind generators, are normally only charged to around 80%, then this when a lot of sulphation takes place. 

There are other causes of battery failure such as a shorted cell or worn out plates but the biggest cause of failure is sulphation. 

Sulphation if treated in time is not irreversible. 

Desulphation is a process which breaks down these crystals and dissolves them back into the electrolyte. This will help keep new batteries working for longer and help restore sulphated batteries to some of their former power.

When lead sulphate crystals build up on the lead plates, it is not an easy task to remove them and thereby recondition the battery. As more and more crystallization occurs, the process of removing the crystals also becomes more difficult.

So, with battery sulphation being biggest cause of batteries failing aboard yachts and motor boats, I set out to find a method which would keep the batteries working on a yacht.

Here are the results of several years research and testing. 

Pulse Desulphator. 

First I tried a pulse desulphator. These are a device which take power from the battery to which it is attached and then releases the stored power as a high pulse / frequency of electricity several times a second.  This will apparently dislodge the sulphate and recombine it with the battery acid.

My tests with this over one winter initially seemed to work – until I found that it was blowing sensitive electronics. Initially I went to check the battery voltage with a multi-meter and received the strangest readings with the pulse desulphator attached. This started me checking other devices some of which were damaged. 

What I came to realise is that the battery needs to be disconnected from the boats supply while the pulse desulphator is connected. As it can take several months to completely desulphate the battery this is difficult, especially, on a long term or live aboard boat.

It also led me to believe that the desulphators sold as ‘fit and forget’ or those that are built into some battery chargers, are not powerful enough to really work on sulphated batteries, otherwise they will blow your electronics. On the other hand, the lower power pulse they appear to give, may work if fitted to a new set of unsulphated batteries but I have no experience of this.

(This apparently lower power pulse is for the same reason that nearly all chargers only go to 14.8 volts when claiming to disulphate when 15.5 is needed. This is covered later.)  

I also read a report from a leading battery manufacturer (Trojan) which stated that the active material in the positive plate is lead dioxide. This molecule is a relative of rust, it is a corrosion product. When you charge a lead acid battery, one of the things you are doing is the repair or reformation of the corrosion layer of the positive plate. If you don't properly charge the battery, the corrosion layer begins to break down in the acid and the voltage characteristic of the battery changes for the worse.

Pulse charging does not influence or improve the corrosion layer of the positive plate and therefore, does not properly improve the performance of the battery. It is only through the electrochemical process of corroding the positive electrode that you optimize the battery's performance. To properly corrode the positive electrode the battery voltage has to reach and then exceed the gassing potential of the battery. Also in a deep cycle battery, not gassing / bubbling the cells will result in stratified electrolyte, ineffective corrosion of the positive plate, reduced performance and shortened life.

It appears that to fully charge or equalise a lead acid battery and ensure that all the cells are working the same, the cell voltage must be taken to between 2.58 and 2.67 volts. The following table gives the generally recommended voltages for each type of battery. 

Battery type

6 volt

12 volt


48 volt

Equalizing for 5 to 8 hours





Absorption or daily recharge










It can be seen that to fully charge or equalise a battery it must occasionally be charged to a higher than normal voltage. A pulse type desulphator or combined pulse / charger will not do this.

(Some manufacturers and battery technicians also call the equalising charge a full charge.)

Therefore as the pulse desulphator damaged electronics, does not apply the full charge needed I have concluded that this method should not be used and I no longer use a pulse desulphator, so another method was needed. 


I purchased some EDTA which is an acid that can be added to the battery fluid and which will dissolve the sulphate on the plates. It is reported that some battery manufacturers include a small amount from new as it will keep working for a long time.

As with most things concerned with batteries there are a lot of conflicting views regarding how to achieve the wanted result.

The amount to be added to each cell is generally regarded to be about a rounded teaspoonful to each cell – this equals a total of 25gm to treat one average car sized battery with a weight of 15-17kgs. Thus if your battery weighs 30kgs then you'll need 50gm, battery weight 45kgs you'll need 75gm etc. etc.

EDTA does not readily dissolve in battery acid so I had to dissolve the EDTA in as small amount of distilled water as possible. Gently heating the solution assisted the process & also reduced the quantity of water required. I had to ensure that the electrolyte level did not exceed the cell’s capacity once the additional solution is added, so I drew a small quantity of electrolyte off. I mixed up enough for the whole battery and divided it between the cells.

The exact amount is not thought to be critical. 

It should be left for a few days to allow it to work and the battery should be agitated (shaken up) regularly to ensure that the acid is distributed throughout the cell. After a few days the battery should be given a good charge. ( In fact what the battery needs is a full charge which means 15.5 volts)

Some people recommend changing the battery acid after using the EDTA – some do not. I left the original acid with the EDTA in the batteries. 

On the five year old Trojans that I tried this on, it did seem to help and I think that it may well help more if your batteries are not as old as the ones I tested it on. 

However, this method still leaves the battery needing a full charge. Therefore a charger which will take the voltage to the equalising or full charge voltage is still needed. 

Desulphation Charge. 

Most good battery suppliers state in their technical information that to desulphate their batteries they will need charging at between 2.58 and 2.67 volts per cell for generally between 5 and 8 hours. Some say once a month and some say when the performance is seen to drop. Whichever, they will need treating regularly.

The problem is that nearly all the chargers available, despite claiming to desulphate, only raise the voltage to 14.8. This is not a truly desulphating voltage. I did ask around some of the charger manufacturers and only found one (which only works on USA voltage) that would really work. This is a Xantrex TC20. By pressing a button this charger will take the voltage to 15.5 for a timed 6 hours. (The battery being treated must be disconnected from the yachts electrical supply) 

The reason that charger manufacturers only make chargers that go to 14.8 volts is simply that if a higher voltage is supplied to most of the instruments and bulbs on board, they will be damaged, often beyond repair. The manufacturers do not want claims against them for this damage and also there is the danger of operators being hurt and yachts damaged by over charged batteries which will give off explosive gas and can even explode if badly overcharged. 

Despite knowing that there are dangers with an overcharge desulphate I started to research how to desulphate this way. As we have a perfectly good charging system I did not want to purchase a new charging system and looked for another method. I found that there are some bench power supplies that will do the job. The two that I found are:- 

1/ A VOLTCRAFT PPS-13610, 360W Dual Output Programmable DC Power Supply, Switched Mode, Bench, 1-18V, 0-20A – Available from 

2/ A Circuit specialists CSI3020X available from USA supplier 

Both will cost around $299 or £170. About the price of one decent battery. 

Voltcraft power supply

I am sure that there are others just as good but you will have to search for them. Both these units will work on 90 to 250 volts and variable hertz. So they will basically work anywhere in the world. They both have adjustable volts and current which is what we want.


Using the power supply to disulphate batteries.



 1/ This will only work on vented lead acid batteries. It will not work on Gel – sealed – AGM - low maintenance etc. It will be disastrous and extremely dangerous to do this to anything but vented lead acid batteries.

2/ When applying a high charge to a battery explosive gas will be given off. This needs to be able to vent away so I always leave the top of the battery box open.

3/ The temperature should be monitored. Trojan recommend that the maximum battery temperature while charging should not exceed 120 degrees Fahrenheit. The battery manufacturers maximum temperature for the battery should not be exceeded. (The good manufacturers will freely give all this info, some do not.)

The battery was disconnected from everything aboard by having the wires removed. The reason for this is that some equipment aboard may not be capable of handling the high volts and may be damaged.

The power supply was connected to the battery, the power turned on and the voltage adjusted. 

The volts were set at 15.5 and the amps set at a maximum of roughly 10. The amps proved to be limited by the volt setting, automatically adjusting as the resistance of the battery changed. 

In one technical paper it recommended that if trying rejuvenate badly sulphated batteries then the first attempts should not be for the full time and that the length of time should be slowly increased. Therefore for the first attempt I only charged each battery for 3 hours, intending to increase the length of time to probably 5 hours the next time and then to the full 8 hours after that.

Keeping a close watch on what was occurring I noticed that the amps required slowly increased over the 3 hours to about 8 amps. This happened on all four batteries treated. Two weeks later the battery PH value was checked and all the cells had the same reading, showing that at least in part the treatment had been successful.

Over several months I found that by desulphating the batteries by this method the available amp hours did increase, which did help to lengthen the life of them and indicates that this method does work as the battery manufacturers claim.

 List of things to Make the batteries last.

I have examined the product technical sheets from as many battery manufacturers as I could and have found that there are a few fairly simple rules that need to be followed to make sure to get good life from your battery. 

1/ Make sure to choose the correct type of battery. When Ti Gitu was in Greece our Elecsol batteries prematurely failed and we needed replacements. Eventually we purchased four 12 volt Trojan 27TMH batteries, having been assured that they would give the same performance as the 6 volt T105 ones that we really wanted.

In fact this was a load of rubbish, which, now that I have read all the technical data, I have found that the 12 volt version has a considerably shorter cycle life than the 6 volt type.

For general yachting, use low cost lead acid batteries which can be topped up with water. So called leisure batteries are best.

For long term cruising or living aboard, then 6 volt traction types are best.

2/ Ensure that the battery does not run out of electrolyte. Make sure that the plates are always covered by topping up with distilled / de ionised water. There are some automatic top up systems such as the Trojan Hydrolink or to make manual topping up easy a top up bottle which automatically fills to the correct level can be purchased. An example of a top up bottle can be found here

3/ The less you discharge them the longer they will last. Trojan quote that the T105 will give about 3000 cycles if discharged to 80% capacity but discharged to 20% of capacity will only give 1000 cycles of discharge and recharge.

(There is a later version the T105 which is the 'T105 RE' which it is claimed will give 4000 cycles at 20% discharge.) 

Of course the problem aboard a boat is that it is very difficult to keep them charged at much above 80% of charge, so we start from a lower point to begin with.

There is a balance to be found here and I have decided that, practically, when away from shore power we do not discharge to more than 50% of capacity. But if possible not going as low as that too often.

We monitor this with a battery monitor which tells us voltage and amp hours used. It can also be done by reading the specific gravity of the electrolyte. 

4/ Watch the temperature. It is not often realised that batteries are generally designed to work at about 80 degrees Fahrenheit. At above this temperature they will give more power but not last as long. Below this they will not be so efficient but have a longer life.

Even when being hard charged the temperature should not go above 120 degrees F.

This means that even in temperate climates the battery box will need good ventilation and in the tropics where daily air temperatures can be considerably higher perhaps forced ventilation will be necessary.

Our battery bank is in a box next to the engine and I monitored the temperature inside while motoring through the Dutch canals. I found that it frequently reaches 90 degrees and so I am now experimenting with forced ventilation. I am fitting two computer fans into the box. One will draw air from the bilge into the box and the other will extract from the top of the box. I am using an adjustable domestic wall mounted heating thermostat to turn them on and off automatically. 

5/ Desulphate the batteries on a regular basis. The battery manufacturers recommendations should be followed. If the manufacturer will not readily give guidance regarding this then purchase from another manufacturer.

Desulphating them means raising the voltage to 15.5 to 16 volts for between 3 and 8 hours. The manufacturer should give guidance on this. If they don’t then purchase from a manufacturer who does.

Please carefully read the above section on how to safely desulphate your batteries as this process can be dangerous. 

6/ Keep the batteries clean, dry and regularly check all the connections. 

By following these six rules a yacht owner will exact the maximum use and life from the vessels batteries.

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