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Batteries

Bought five 110 leisure batts two years ago. I am mostly in a marina with shore power and the charge showing on the monitor is 105, having been out for 6 weeks I run engine for 3hrs, monitor shows 40-50%, when engine is turned off batteries will slowly rise to 80%, at night I turn everything off, except fridge, and monitor shows 55-60%, in morning it reads 0% and have to run engine to get it up to 80%. Is this a sign that I need new batteries? If so, what type? Maintenance free? Gel? Cheap ones every 2 years? Any advice welcome. Many thanks.

Asked by: Anthony Burnell  | 10.12am, Thursday 3 October


WW says:

Charge monitors are not always as accurate as you would like- and often have to be calibrated, depending on the type fitted. It is also worth using a voltmeter to check the battery bank as well- if the reading is much below 11V then you have effectively flattened the battery bank. Repeated flattening of a bank is guaranteed to shorten the life considerably.
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Having a bank of 5 batteries, the positive cable should enter at one end of the bank, with the negative at the other- this can help with reducing the effects of one battery receiving more charge/discharge than others. hopefully, the batteries are already connected this way.
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In a large battery bank, the connections between the batteries need to be substantial cables- at least 25mm2 cross sectional area, and preferably double this, to reduce resistance in the circuit.
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Also, it is worth checking that the elctrolyte levels in the batteries are correct- that there are no uncovered plates. If one cell has less water in than others, it might mean that it has failed. This will have a knock-on effect on the whole battery bank.
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You could also use a hydrometer to show the specific gravity of the individual cells, which gives a very good indication of the state and the capacity of an individual cell.
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Three hours engine running may not be enough to replace the charge you have used during the day. If there is no external voltage regulator, then very quickly the output of the alternator will drop, as the apparent voltage on the battery bank increases. Although a 110A alternator might put out 50A just above tickover, it may quickly drop back to around 10A or less, so the batteries don't receive the charge they should. Longer cruising times might be needed.
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As for type of batteries, it is a thorny issue. however, for most boaters, it is the way they are used, rather than the type, that cause the most problems. True traction batteries are usually only available in 6 or 2V cells, so require some thinking about when fitting- and they are very expensive. Maintenance free ones, however, are probably least suitable, as any battery will produce gas when charging and so lose water- this cannot be replaced in a maintenance free battery. They are generally designed for light use, rather than providing domestic supplies to boats. A reasonably good quality, wet-cell, leisure battery should, if carefully used and maintained, give good service and last five years plus. Apart from the hideously expensive lithium ion batteries, the internal chemistry of maintenance free, gel, open and AGM batteries is the same, with just the electrolyte (sulphuric) acid in a different format.
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Standard leisure batteries are probably the most cost effective. A 110AH battery only holds that at the 20hr discharge rate (i.e. providing 110Ah over 20 hrs). If you draw quicker, the capacity is reduced- and without an external charge controller on the alternator, probably means that you only have around 40Ah of suseable capacity per battery. Just don't try and connect more than 5 batteries in parallel, as with normal lead-acid batteries, the internal resistance can be too great for effective charging at 12V.
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Digital alternator controllers do increase both charging times and effective battery capacity, and are well worth considering if you are living aboard and relying on your engine for battery charging.
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If we can be of further help, please let us know.

Mark Langley  | 11.28AM, Thursday 3 October

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