ASK THE EXPERTS
Which type of battery; water pressure; smoke from a cruiser; how many batteries and heating hitches
What are the important things to know about the different types of battery that are available? FIONA, via the CB website
TONY REPLIES: Wet open cells have removeable cell caps that allow easy sampling for diagnostic purposes and topping up with de-mineralised water to replace liquid lost to gassing during high charging voltages (say above 14.5v).
They are good if you have an advanced alternator controller, an alternator set to deliver more than about 14.5 volts, or a battery charger that allows equalisation charging.
They come in two types: leisure batteries (cheaper) may use calcium which increases the voltage at which they start to gas, but they can only stand a fairly limited number of charge/discharge cycles. Traction and semi-traction batteries (more expensive) may be the best buy as they are good for many more cycles, but only if you really know how to look after batteries. You could destroy a £1000+ set as easily as you can a £200 set of leisure batteries if you do not keep them fully charged.
Sealed flooded cells are similar but you cannot get into the cells for diagnostics or topping up. You must never exceed their maximum charging voltage (typically 14.4 volts) and problems are harder to diagnose. The lead plates in the battery may be held apart by porous plastic separators (as in cheap open cell batteries) or they may be ‘pocketed’ which will be more expensive but last longer. Absorbed Glass Mat (AGM) batteries have the gaps inside the cells filled with glassfibre wadding impregnated with the acid. They are also sealed, so harder to diagnose – and you must never exceed their maximum charging voltage (from below 14 volts to above 14.5). They can charge faster, give a higher current, and have a longer life when looked after, but cost more. Gel batteries contain an acid gel instead of liquid. Some have their plates in a spiral form for more strength and potentially longer life. But they are expensive and despite their advantages, I feel, unlikely to be cost-effective.
For all batteries, three figures may be quoted: Cold Cranking Capacity is how many amps the battery can supply when new and fully charged for engine starting. This is only relevant for the start battery and rarely matters as most boats’ start batteries have a far higher CCA than needed.
Amp Hour Capacity (Ah) is how many amps for how many hours a new, fully charged battery can supply. A 100Ah can be assumed capable of supplying 100 amps for one hour, 50 amps for two hours, 25 amps for four hours etc.
Cyclic life is how many times you can charge and discharge a battery until it fails. Best practise is never to discharge domestic batteries below about 50% before recharging – and you will double cyclic life. In general, to maximise battery life keep them as fully charged as practicable for as long as possible. Consider them fully charged when the charging current has fallen to one or two percent of the bank capacity and it has failed to drop over about an hour.
An absorbed glass mat
A sealed flooded cells battery
A deep cycle version