Both nickel-cadmium cells (NiCads) and the older nickel-iron cells (NiFes) have an alkaline electrolyte and need to be charged at a constant current for maximum efficiency. The cells must be supplied with about 40 to 60% more charge than they release on discharge. This means that with a 1A lamp they should be charged at 2A for 0.7 to 0.8 of the time they were discharged, or they can be charged at 1.75A for 0.8 to 0.9 of the discharge time. The charging current is not critical but it should not be above 2A for a NiFe cell or 3A for a NiCad cell. Both types of cell will withstand overcharging, but will gas rapidly and heat up if they are left on charge for too long. The vents on NiCad cells should not be unscrewed during charge. It is advisable not to unscrew NiFe vents either simply because the electrolyte will spill if the cell is knocked over. However if the NiFe is in bad condition the pressure buildup may cause electrolyte to be forced out through pinholes in the casing, (personal experience!!). Keep the battery upright during charging and for a while afterwards or the gas pressure may force electrolyte out through the vents. It is a good idea to wash or wipe the top of the cells after charging to get rid of any electrolyte that has bubbled out.
Good quality sintered-plate NiCads may be charged at up to three times the 10 hour discharge rate, that is, a 20 Ah cell may be charged for 5 hours at 6A. This may not be advisable for old or second-hand NiCads such as those usually found in caving shops. 'Varta Ltd' manufacture 10 and 15Ah sealed NiCad cells (types SD7 and SD15) which may be charged at 10 and 15A respectivly, but this needs a very specialised charger and if you consider doing this the manufacturers instructions must be followed exactly.
NiFes and NiCads are not easily damaged by moderate overcharging at reasonable currents (<2A) provided the plates are covered with electrolyte, but they will be damaged if they are discharged completely or very fast, or left without electrolyte in them for more than about 15 minutes when refilling. They may be left in any state of charge (unlike lead-acid cells which must not be left discharged) but old cells may have a fairly high self- discharge rate so they should be charged before use to be on the safe side.
Sealed NiCad cells should not require topping up since they are not supposed to leak. NiFe cells may require topping up with distilled water and refilling occasionally since they lose water when they gas. The electrolyte should be about 5mm above the top of the plates. Do not top up with electrolyte as the increased concentration can reduce the capacity of the cell by binding with the plates. After draining and filling a cell with fresh electrolyte it should be given a formation charge for twice the normal time.
The electrolyte is Potassium Hydroxide solution. This can be bought in half pint packets, enough for a 3-cell NiFe, from caving shops. True NiFe electrolyte has about 1% Lithium Hydroxide but this is not essential. Add the crystals slowly to cold water, stirring carefully to avoid splashing. Do not add water to the crystals. The voltage of a NiFe or NiCad cell is between 1.20 and 1.25V (ie. 2.4-2.5V for a 2-cell NiCad, or 3.6-3.75V for a 3-cell NiFe). This will drop during discharge, but more importantly, it rises during charge to about 1.5V typically. This means that a bulb may be blown or have its life reduced if the headset is turned on within a short time of charging. The lamp should never be turned on during charge as 2A will certainly blow it. For an old cell the voltage rise during charge may be much greater.
Lead-acid cells require more care in using. They can be divided into two groups; the free acid type such as 'Oldham' or 'Exide' batteries, and the absorbed electrolyte type such as the Berec 'Varley' battery.
Oldham batteries and other batteries of this type should never be overcharged or they may be damaged irreparably, thus constant current charging is only permissable if the state of charge of the battery is known exactly. For this reason a constant voltage charger is normally used. In this method the current automatically dies away as the cell reaches full charge. It is usual to limit the current to a known safe value. For an Oldham 2-cell battery the voltage is between 4.6 and 4.8V, but this is very dependant on the condition of the cell, and a current limit of between 1 and 2A should be set. The voltage of a lead acid cell is about 2.0V. This rises to the charger potential when on charge. If constant current charging is used it will rise to 2.5V per cell and higher.
Lead-acid cells should never be left discharged as the lead sulphide produced during discharge undergoes a generally irreversible reaction which results in reduced capacity and efficiency of the cell. A cell in this condition is referred to as sulphated. Periodically discharging and charging the battery helps to prevent sulphating; once a week is a good idea for an old Oldham battery. Periodically overcharging at a reduced current also helps restore battery condition and a severely sulphated cell may sometimes recover if cycled repeatedly or trickle charged at about 20mA for a prolonged period. Oldhams batteries should be topped up with distilled water periodically so that it appears at the filler hole when the cell is tipped through 10 or 20 degrees.
This type of battery is more robust than the free acid type. It is sealed, so virtually leakproof, it requires little or no maintenance and is tolerant of overcharging though it is not advisable. A large range, some of which are suitable for caving, are manufactured by 'Varta Ltd.' and 'Berec "Varley Batteries" Ltd.' (formerly 'Ever Ready'). These batteries are not widely available however and the manufacturers only supply to wholesale distributors.
'Varta Ltd' manufacture a 6V 5.7Ah unit (type Accu-Pb 6V 5.7Ah) which may just fit in an old NiFe case. The manufacturers specify constant voltage charging only, with no current limit needed. 'Berec' manufacture a 6V 12Ah unit (VPT 6.13/12) but it is the wrong shape (being cuboid) to fit in a NiFe case. Either type of charging can be used with this battery.
The voltage required for constant voltage charging depends on the temperature and type of use required. It is usually between 2.25 and 2.35V per cell but the correct value must be maintained to within 50mV. Since no current limit is needed the initial charging current can be made very large. At 300A the batteries will charge in minutes! This very high current does not damage the cells as it dies away rapidly as the cell charges (one reason why the voltage must be accurately maintained). The manufacturers instructions should be followed exactly. Sealed lead-acid batteries are also manufactured by 'Vidor' and one of these will fit in a NiFe case. (see Descent 39, pp 33-34, 'The Sealed Lead-acid Battery For Caving'). R.S Components Ltd. make a small range of sealed lead-acid batteries and their 6V 5Ah type is an alternative to the 'Vidor' type suitable for caving use. R.S. Components only supply to retailers but a local electrical shop may be able to order it for you.
If a charger is to be used to charge more than one battery at a time the rule is for constant current charging the batteries are connected in series, while for constant voltage charging they are connected in parallel.
The table below shows the lamps used with the various types of battery:
| Battery | Main Beam | Pilot beam |
|---|---|---|
| 2-cell NiFe/NiCad | 2.4V 1.25A | 2.5V 0.3A |
| 2.4V 1A | ||
| 3-cell NiFe | 3.6V 1A | 3.6V 0.3A |
| 2-cell lead-acid | 4V 1A | 4V 0.46A |
| 4V 0.75A | 4V 0.3A | |
| 3-cell lead-acid | 6V 0.3A* | 6V 0.1A |
*see Descent 39 p 34
The more serious burns are likely to occur underground where a cell leaks into a wetsuit. Such a burn may not be noticed for some time. Immediately it is discovered it should be washed with plenty of flowing water and the contaminated clothing removed. Try to cover the area with a reasonably sterile dressing and some padding to stop chaffing. Have the burn looked at on the surface as soon as possible. Dr John Frankland (letter to Descent 36 p 46) suggests that in the absence of water to wash the burn a suitable supply of sterile liquid may be readily obtainable and is better than nothing!!
Burns caused while mixing or using electrolyte should be washed immediately with plenty of water; alkaline solutions have a characteristic soapy feeling. Eye burns are potentially very serious; wash with copious amounts of water and see a doctor immediately.
Contaminated clothing should be washed thoroughly, perhaps with a neutralising agent (Boracic acid for alkali, Sodium Carbonate for acid), but do not use a neutralising agent for skin burns. A slight splash on clothing may cause mysterious holes and discolouration days later. Nylon is resistant to alkali, and polyesters (such as terylene) are resistant to acid (but the dyes in them may not be). Alkaline batteries should not be used with polyester sit-harnesses and acid batteries should not be used with nylon materials and should be kept away from nylon ropes.
Dave Gibson
| Varta Ltd. Hermitage Street, CREWKERNE, Somerset, TA18 8EY |
Berec (Varley Batteries) Ltd. Chandos Road, North Acton, LONDON, NW1O 6NF |
Index to Cambridge Underground
Table of Contents for Cambridge Underground 1979
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