The comparative merits and treatment of lead-acid and NiCad cells is a matter of interest to all cavers. Putting aside considerations of safety from electrolyte leakage and therefore removing the necessity to consider Ni-Fe cells I shall restrict myself to discussing the electrical properties of the first two mentioned types of cell.

Lead acid cells have been much more popular than Ni-Cad cells until the recent introduction of the FX-2 unit (which is essentially two 'F cells', obtainable from RS for about £7 each, which have been cast in potting compound). Lead-acid cells have the advantages of longer life (in terms of charge/discharge cycles), better behaviour at low temperatures, generally greater charge capacity per unit (compared to an FX-2) and having good abuse resistance. It is quite hard to totally destroy a lead-acid cell, whereas a NiCad can be caused to explode by badly exceeding the charging rating. The NiCad however has the advantage of being a sealed battery in the case of an FX-2 (and at least having no liquid electrolyte in older NiCad cells) and having a greater charge density so allowing a more compact unit.

As might be expected the sign of cell deterioration is a reduction in discharge time. The cause of this is quite different for the two types of cell.

NiCad cells seem to develop a 'memory', that is if the charge in them is maintained at around a certain level for many charge/discharge cycles it becomes progressively more difficult to get a full charge into them. To prevent this it is necessary to periodically 'deep discharge' a NiCad (ie. run it flat). However the load used to discharge it must not be left connected as this can lead to a reverse in polarity of one of a series set of cells (2 series cells in an FX-2). This arises because all cells do not have identical capacity and so at the end of the discharge period the cell which has the remaining charge can 'force' the other into a reverse polarity situation. It is for this reason that NiCads should never be connected in parallel. It is possible to cure a cell which has suffered a polarity reversal by applying a large (ie. greater than the cell p.d.) voltage to the terminals, although this is a rather haphazard operation and could lead to permanent damage (ie. explosion) if a large current is allowed to flow for anything other than a short period of time. The best option is to avoid a polarity reversal altogether.

For lead-acid cells the major cause of ageing is the deposition of lead sulphate on the electrodes (often termed sulphation). This may be partially solved with a bit of chemistry. The method is:
i) Drain the acid and dispose of carefully
ii) Rinse the cell out and fill with a solution of sodium sulphate (200 g/litre of distilled water)
iii) Give this 'cell' a charge in the normal way to drive the reaction
iv) Empty, wash out and refill with fresh sulphuric acid.

Although this has not actually cleaned the electrodes, the effect of electrolysing the sodium sulphate is to cause the lead sulphate crystals to become larger in size, but therefore smaller in total surface area. Hence the available surface area on the electrodes for light generation is increased. In this way a minimally useful cell may be rejuvenated.

REFS:

BCRA Cave Radio and Electronics Group Newsletter Vol.1 No.1 (Phil Ingham)
RS data sheet No. 2854 (Batteries)