Data Sheet

#33 A guide to leisure batteries

#33 #33 A guide to leisure batteries
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7: Factors affecting battery performance
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Factors affecting battery performance

  • Temperature – The stated capacity of a battery is expressed in Amp hours (Ah) but the quoted figure is only applicable when the ambient temperature is 25C. In reality, there are many occasions in the UK when it’s much colder than 25C and for every one degree drop in temperature, there’s an approximate one per cent reduction in a battery’s Ah capacity.

Let’s take an example. If a battery which is nominally rated at 60Ah is operating in temperatures of 15C, this ten degree reduction will bring about a 10 per cent reduction in its capacity. This now means that it effectively performs like a 54Ah battery. The temperature issue can have adverse effects for caravan and motorhome users who enjoy winter sports. Not only will a battery face greater demands when providing their lighting, fan-assisted heating, and evening TV entertainment during long, dark evenings; its effective capacity is significantly lower than it would be in warmer conditions.

Needless-to-say, if a battery box is mounted inside, heat in the living area will help to improve its performance. However, many batteries are mounted externally where ambient temperatures can be much lower.

  • Battery age – As a battery gets older, its performance deteriorates and it needs re-charging with increasing frequency, therefore, it is unlikely to last more than five years.
  • Higher rates of consumption – If numerous appliances are being used simultaneously, a battery’s increased discharge rate affects its ability to provide power. This is not just a matter of saying that running appliances with double the wattage results in a battery becoming discharged in half the time. In practice, an extra demand on a battery has an even greater effect on its power-providing potential. Or to put it another way, the faster your battery is discharged, the lower its capacity will be.
  • What size leisure battery do I need? – You will first need to work out what your consumption is going to be on the 12V equipment you use and for how long it is used then, if required, factor in what seasons you go away. An example is shown below:


Equipment(Typical example)

 Current(Amps)

 Multiply

 Hours of use

=

 Amp hour rating (AH)

One fluorescent tube 0.75        x              5 =          3.75
2 halogen lights 1.50        x              3 =          4.50
LCD TV 2.5        x              4 =        10.00
TV booster 0.05        x              4 =          0.20
Water pump 0.7        x             0.5 =          0.35
Motor mover 30 (Average)        x             0.3 =          9.00



Sub Total = 27.8

(For winter use add an extra 10%)



Add 20% safety margin = 5.56



Total Ampere Hours required per day = 33.36

This gives your typical daily consumption. It is surprising how much power is required and how we take it for granted. Heavy consumption equipment like motor movers will have a detrimental effect on the battery performance for the reasons explained below.

From this information decide what size battery will likely be needed depending on how long you are away from a charging source. It is likely that this is not enough to meet your needs and will have to consider a top up charge. This will be covered in more depth in the Battery Charging Data Sheet (36).

  • Rate of discharge – Following on from the above point, a battery’s ability to provideweb 20ah power will depend on the rate of discharge. Time plays a key part and that is why batteries built to meet EU 50342 standards have two or sometimes three Ah capacities marked on their labels. Normally a manufacturer’s quoted Ah capacity of a battery is based on a discharge time of 20 hours – referred to as ‘the 20 hour rate’. For instance, a battery designated as 95Ah at the 20 hour rate might only achieve 80Ah at a five hour rate – or as much as 105Ah if educed demand extends over a 100-hour period.

Unfortunately there are so many different factors involved in a battery’s actual performance, its Ah capacity can only be taken as a guide. The type of battery design and construction, ambient temperature, battery age, and consumption levels have already been mentioned. In practical terms, nearly all caravanners find their battery seems to need recharging far sooner than they expected.