Why do lead acid batteries slowly die and can they be recovered?
First we need to understand how battery chargers actually charge a battery. Switch Mode or ‘Intelligent’ battery chargers use a three stage process to recharge the battery most efficiently and safely. These stages are commonly described as Bulk, Absorption and Float. The Bulk stage makes up about 80% of the recharge and the current is held constant and the charging voltage steadily increases as the battery fills.
When the charging voltage reaches about 14.1 volts it switches to the Absorption stage. The charger then maintains the voltage and the current decreases until the battery is fully charged. When the battery is fully charged the charger switches to the Float stage and maintains a reduced constant voltage of about 13.0 Volts which will maintain a full battery charge compensating for the small amount of self discharge that will occur.
All lead acid batteries will gradually lose power capacity due to a process called sulphation which causes a rise in the batteries internal resistance. When batteries are left at a low state of charge for a long period that process can be rapidly accelerated. A typical good battery has an internal resistance of about 4 ohms. A sulphated battery has higher electrical resistance which can be up to 50 ohms, by which time a battery is effectively dead.
As current is the ratio of voltage to resistance a sulphated battery will have much lower current flow and, in the charging cycle, the charger will reach its pre-set limit long before the battery has had time to accept a complete charge. In this case the battery charger will assume from the current flow that the charge cycle is complete and switch to the Float stage.
So, what is Sulphation and how can it be prevented? Flooded cell lead acid batteries commonly used on yachts consist of a number of plates of alternately lead and lead oxide in a cell filled with an electrolyte of weak sulphuric acid. Each cell produces about 2.1 volts so a typical 12V battery consists of six cells connected in series producing about 12.6 to 12.8 Volts when fully charged.
The lead oxide reacts with the sulphur to give the plates a positive charge. The lead plates react with the acid making them negative and thereby creating a potential difference between the pairs of plates. This chemical process also creates a solution of lead sulphate in the acid and as the battery is discharged the density of the lead sulphate increases.
When a battery is discharged to below about 11 volts the lead sulphate will begin to precipitate out of the acid electrolyte and form a soft film on the plates. When the battery is recharged the film will theoretically dissolve back into the acid. However, if the battery is left at the low state of charge for an extended period the soft film will slowly crystallize into a solid. This is the process known as Sulphation.
This process can be minimised in some batteries that are branded as Gel type or AGM (Absorbed Glass Mat) type. In these the acid electrolyte is held in a gel or in a saturated glass mat that effectively limits migration of the lead sulphate to the plates. These batteries are more expensive but should give a longer service life and more resistant to deep discharge.
In theory the sulphation process can be reversed and some devices are available called desulphators or desulfators. These work on the principle that the lead sulphate layer can be dissolved back into solution by applying very much higher charging voltages. Pushing high voltage into a battery with a high internal resistance will cause it to rapidly heat up. All battery charging will cause the release of hydrogen gas but boiling of the electrolyte can cause hydrogen gas release at explosive concentrations.
Therefore, these devices use pulses of high voltage that are too short to cause overheating but long enough to reverse the crystallization process. This recovery process can take days or even weeks to repair a batteries performance. The crystallization of lead sulphate has taken a long time to form on the plates in the first place so reversal is therefore also a very slow process, and it is not always successful.