Guys,
The 1.25 figure in the formula is the one that will give you the 75% of the capacity which we are all calling the 'ageing factor' for want of a better name! Multiply, for example, the figure 8 by 1.25 and you get an answer of 10. i.e instead of installing an 8AH battery install instead a 10AH battery to compensate for the 'ageing factor'
The de-rating figure, variously mentioned previously, as 1, 1.75 and 2 is a figure which is meant to adjust for a feature of all batteries where the actual capacity achievable from a battery is dependant on the discharge rate. I have previously mentioned this, but just to summarise; a 10AH cpapcity battery should, in theory, provide 1A for 10 hours or 10A for 1 hour. In fact, batteries normally only give their rated capacity at a discharge rate of 1/20th of their rated capacity. i.e. For a 40AH battery it will give 2A for 20 hours.
Furthermore, if your discharge rate is less than 1/20th, the battery is likely to last longer than the supposed capacity i.e for a 40AH battery it will give 1A for more than 40 Hours. Conversely if the discharge rate is more than 1/20th, the battery is likely to last shorter than the supposed capacity i.e for a 40AH battery it will give 4AH for rather less than 10 hours.
Benz, This 1/20th figure is that which my previous posts mentions and it is not what your answers to me are suggesting. It has nothing to do with expecting the alarm to go off 20 times etc.!
Benz, in answer to your question; the BS de-rating figures of 1 and 1.75 are based on typical batteries. De-rating 1 can be used where the alarm load current is low and thereby taking a small current from the battery (less than 1/20th) so the battery is likely to meet it's rated nominal capacity. Where the alarm load current is high and taking a larger current (more than 1/20th) the battery is unlikely to meet it's rated nominal capacity and the de-rating figure of 1.75 will compensate for this.
Graeme, you make the comment that using a de-rating figure of 2 instead of, say, 1,75, will make very little difference. I absolutely agree. However there is a difference. And this difference will be even larger if the de-rating figure should only have been 1.
I understand the BFPSA suggestion of always using a de-rating figure of 2 because this simplifies the formula. It allows the de-rating figure to cancel out the 50% division of the alarm load. Therefore the simplified formula doesn't even mention the de-rating figure (of 2) they have used. However, all of you who have taken the course mention a de-rating figure of 2 and the simplified formula, when the formula has already taken a de-rating figure of 2 into account. It seems to me that this aspect of the simplified formula has not been properly explained on the course.
However, I have strong opinions on the right of the BFPSA to suggest a formula, that whilst easier to use, does not tie in with the recommendations of BS. In fact, I have strong opinions on anyone who varies the recommendations of BS without making sure the impact of doing so is fully understood and explained.
In the case of this battery capacity calculation, I have already admitted that a de-rating figure of 2 against 1.75 is a small difference. But what happens when you calculate using the BFPSA simplified formula that a standby battery will only last 23 hours and fail a system on commissioning, when the system designer has designed used the correct de-rating figure for that battery/system of say, 1, and this provides a battery standby duty of 25 hours? What if the discussion got legal, what would the judge accept? The BS recommendation or the BFPSA's version?
Is it right to follow the BFPSA training if it varies from BS unless the BFPSA highlights those things they have unilaterally changed and explained the potential ramifications?
