Batteries calculation ?

[Benzerari]

Guys;

For some reasons we are required to re-calculate the FAS's load, in every service visit, and see if the batteries are still reliable, to support feeding the FAS in quiscent and alarm conditions...etc

The formula is the following:

C = [ I1 . T1 + I2 . T2 . D ] 1,25

Where:

C: is the batteries capacity
I1: is the load’s quiescent current  
T1: is the stand by time (while mains disconnected) = 48h
I2: is the load’s alarm current
T2: is the minimum alarm time (while mains disconnected) = 0.5h
1.25: is further safety coefficient of tolerance for the batteries capacity

D: is de-rating factor = ‘2’  and  this is my main concern!

What is de-rating factor?  and  Why it’s set to ‘2’?

[Wiz]

Benz,

The de-rating factor recommendation in BS5839 part 1 2002 is actually 1 or 1.75 and not 2.

It is 1 when the system alarm load is equal or less than 1/20th of the battery's stated capacity. It is 1.75 when the battery's alarm load is greater than 1/20th of the battery's staed capacity.

BS recommends that you check any de-rating factor to be used from the actual battery manufacturers literature. If none is available, you should use the figure 1.75.

Using a de-rating factor in your calculations provides a calaculation as taking into account of the fact that the battery's stated capacity in AH is a nominal figure because most batteries can't actaully meet their capacity rating on high loads and are likely to exceed it on low loads. I.e a 24AH would probably be able to provide 1A for 30 hours but only 8 hours at 2.4A. The battery capacity is normally rated as if 1/20th is being taken out of it i.e if 1.2A was taken from a 24AH battery it would last 20 hours (i.e what you would expect from a 24AH battery!)

In respect of the BS formula, I know that you, Benz, can cope with formula math because I've noticed that you quote it quite often in your posts. Not everyone is as proficient, and the formula shown in BS i.e. 1.25(T1 I1 + D I2/2) fazes many people.

In trying to calculate the likely duty of the battery in an installed system, there is a simpler way of making a good calculation.

At the end of the day, you are trying to prove that the installed battery capacity is sufficient for the system load and you want to know how many hours standby the battery can provide (normally 24 hours in L installations).

The Fire Alarm Wizard's own method and calculation of doing this is carried out as follows:

1) Determine the system batterys' rated capacity in AH by reading what is printed on the battery.

2) Convert that figure into mAH by multiplying by 1000. i.e a 7AH battery becomes 7000mAH

3) Take 75% of the figure calculated in 3 above ( i.e. 75% of 7000mAH is 5250mAH) This figure (5250 in this example) becomes the new available battery capacity to be used in further calculation.

4) Take readings of both the standby load (in mA) and alarm load (in mA) of the systems *. For our example we will assume that our readings give a standby load of 70mA and an alarm load of 200mA

5) We now deduct half the alarm load reading from the battery capacity figure (we use a reading of half because the alarm load only has to operate for 30 mins). In our calculation this would be 5250 - 100 = 5150. This figure of 5150 becomes the new available battery capacity figure to be used in further calculation.

6) Divide the standby load reading into available battery capacity figure. In our calculation this would be 5150/70 = 73.57

7) The figure of 73.75 now equates to the number of hours standby duty available in our battery whilst still providing sufficient capacity to sound the full alarm load for 30 minutes and assuming our battery only has 75% of it's rated capacity.

* The practical method of calculating standby and alarm loads is to connect a ampmeter (reading mA) in series with the battery connection of the control panel/power supply. Disconnect the mains supply to the control panel and then take the standby load reading. Then initiate a full fire condition and then take the alarm load reading.

I hope you will agree that the above is the simplest practical method for an engineer on site to calculate the available standby capacity of battery in hours of any installed system and turns the BS 5839 formula into a set of practical and simply understood steps. Please note however that this method and calculation does not factor in any battery de-rating factor. I have never included the de-rating factor into my method because a) The de-rating figure is often 1 and therefore has no effect on the calculation and b) because it adds a further step to the calculation that blow's most fire aalrm engineer's minds! You could probably cope with it and adjust Wiz's on-site calculation method and include it.

I have tested my method over many years of use and have patents pending on it

[Benzerari]

Benz,

The de-rating factor recommendation in BS5839 part 1 2002 is actually 1 or 1.75 and not 2.

It is 1 when the system alarm load is equal or less than 1/20th of the battery's stated capacity. It is 1.75 when the battery's alarm load is greater than 1/20th of the battery's staed capacity.

BS recommends that you check any de-rating factor to be used from the actual battery manufacturers literature. If none is available, you should use the figure 1.75.

Using a de-rating factor in your calculations provides a calaculation as taking into account of the fact that the battery's stated capacity in AH is a nominal figure because most batteries can't actaully meet their capacity rating on high loads and are likely to exceed it on low loads. I.e a 24AH would probably be able to provide 1A for 30 hours but only 8 hours at 2.4A. The battery capacity is normally rated as if 1/20th is being taken out of it i.e if 1.2A was taken from a 24AH battery it would last 20 hours (i.e what you would expect from a 24AH battery!)

In respect of the BS formula, I know that you, Benz, can cope with formula math because I've noticed that you quote it quite often in your posts. Not everyone is as proficient, and the formula shown in BS i.e. 1.25(T1 I1 + D I2/2) fazes many people.

In trying to calculate the likely duty of the battery in an installed system, there is a simpler way of making a good calculation.

At the end of the day, you are trying to prove that the installed battery capacity is sufficient for the system load and you want to know how many hours standby the battery can provide (normally 24 hours in L installations).

The Fire Alarm Wizard's own method and calculation of doing this is carried out as follows:

1) Determine the system batterys' rated capacity in AH by reading what is printed on the battery.

2) Convert that figure into mAH by multiplying by 1000. i.e a 7AH battery becomes 7000mAH

3) Take 75% of the figure calculated in 3 above ( i.e. 75% of 7000mAH is 5250mAH) This figure (5250 in this example) becomes the new available battery capacity to be used in further calculation.

4) Take readings of both the standby load (in mA) and alarm load (in mA) of the systems *. For our example we will assume that our readings give a standby load of 70mA and an alarm load of 200mA

5) We now deduct half the alarm load reading from the battery capacity figure (we use a reading of half because the alarm load only has to operate for 30 mins). In our calculation this would be 5250 - 100 = 5150. This figure of 5150 becomes the new available battery capacity figure to be used in further calculation.

6) Divide the standby load reading into available battery capacity figure. In our calculation this would be 5150/70 = 73.57

7) The figure of 73.75 now equates to the number of hours standby duty available in our battery whilst still providing sufficient capacity to sound the full alarm load for 30 minutes and assuming our battery only has 75% of it's rated capacity.

* The practical method of calculating standby and alarm loads is to connect a ampmeter (reading mA) in series with the battery connection of the control panel/power supply. Disconnect the mains supply to the control panel and then take the standby load reading. Then initiate a full fire condition and then take the alarm load reading.

I hope you will agree that the above is the simplest practical method for an engineer on site to calculate the available standby capacity of battery in hours of any installed system and turns the BS 5839 formula into a set of practical and simply understood steps. Please note however that this method and calculation does not factor in any battery de-rating factor. I have never included the de-rating factor into my method because a) The de-rating figure is often 1 and therefore has no effect on the calculation and b) because it adds a further step to the calculation that blow's most fire aalrm engineer's minds! You could probably cope with it and adjust Wiz's on-site calculation method and include it.

I have tested my method over many years of use and have patents pending on it

Wiz;

Thanks for this good technical answer. The de-rating set to 2 is not my set, it is in the BFPSA (FIA) books, I myself haven't understood the de-rating factor since I had done BFPSA courses sometimes ago..., and thank you again for this technical answer.

The load’s currents I1 and I2 required in each service visit (for some reasons) are NOT the theory ones you get usually through batteries calculations for first install, while taking into account the individual current consumption of each device..., I1 and I2 are the actual and practical measurements of the full load, while fitting the Amp-meter in serial with the batteries, while mains disconnected, also in both quiescent and alarm conditions. And they could be different to the theory values for some reason or other.

Any way your answer does make many senses, thank you

[David Rooney]

Wiz;

Thanks for this good technical answer. The de-rating set to 2 is not my set, it is in the BFPSA (FIA) books, I myself haven't understood the de-rating factor since I had done BFPSA courses sometimes ago..., and thank you again for this technical answer.

The load’s currents I1 and I2 required in each service visit (for some reasons) are NOT the theory ones you get usually through batteries calculations for first install, while taking into account the individual current consumption of each device..., I1 and I2 are the actual and practical measurements of the full load, while fitting the Amp-meter in serial with the batteries, while mains disconnected, also in both quiescent and alarm conditions. And they could be different to the theory values for some reason or other.

Any way your answer does make many senses, thank you

Benz... you should always check I1 ans I2 on commissioning to see if they tally with the theoretical calculations.

The figures should always be checked on a service visit also as it could be an indication part of the system is drawing more current than it should and requires investigation.

We have also had instances in the past where tenants organise their own fitouts and and fims just add detection and sounders as they feel like it to a particular area in a building. By checking the quiescent and the load each visit we know if someone has been adding things they shouldn't.

[Benzerari]

Wiz;

Thanks for this good technical answer. The de-rating set to 2 is not my set, it is in the BFPSA (FIA) books, I myself haven't understood the de-rating factor since I had done BFPSA courses sometimes ago..., and thank you again for this technical answer.

The load’s currents I1 and I2 required in each service visit (for some reasons) are NOT the theory ones you get usually through batteries calculations for first install, while taking into account the individual current consumption of each device..., I1 and I2 are the actual and practical measurements of the full load, while fitting the Amp-meter in serial with the batteries, while mains disconnected, also in both quiescent and alarm conditions. And they could be different to the theory values for some reason or other.

Any way your answer does make many senses, thank you

Benz... you should always check I1 ans I2 on commissioning to see if they tally with the theoretical calculations.

I know that, but I am talking about in every service visit

The figures should always be checked on a service visit also as it could be an indication part of the system is drawing more current than it should and requires investigation.

Indeed, this is part of the matter !

We have also had instances in the past where tenants organise their own fitouts and and fims just add detection and sounders as they feel like it to a particular area in a building. By checking the quiescent and the load each visit we know if someone has been adding things they shouldn't.

We have seen the other way round, the customer had some alterations, and the builders cut off a lot of wiring..., when the fire alarm systems went faulty their electrician managed to run some bits of wiring to close the circuits back to normal but not as it was... they had tried to avoid further spending if the alarm company is called for that purpose…

[Wiz]

.......The load’s currents I1 and I2 required in each service visit (for some reasons) are NOT the theory ones you get usually through batteries calculations for first install, while taking into account the individual current consumption of each device..., I1 and I2 are the actual and practical measurements of the full load, while fitting the Amp-meter in serial with the batteries, while mains disconnected, also in both quiescent and alarm conditions. And they could be different to the theory values for some reason or other.........

Benz, What it means is that I1 and I2 should always first be calculated theoretically at design stage by using the number of devices and the cie and their supposed current consumptions for use in the battery capacity requirement calculations.

However, as we know, and David mentions, what you end up with on-site at a later date can be different from what was used in the original design calculations.

This explains your  question 'And they could be different to the theory values for some reason or other'.

Therefore the I1 and I2 values should be ascertained on-site each time a battery capacity calculation is required  and these used rather than the original 'theoretical' ones.

[Wiz]

....... The de-rating set to 2 is not my set, it is in the BFPSA (FIA) books, I myself haven't understood the de-rating factor since I had done BFPSA courses sometimes ago...,

Benz, the BFPSA de-rating figure of 2 you mention is very interesting. By my calculations of this in the BS capacity calculation formula this would calculate that they assume that the battery is actually half-as-good as the theoretical rating obtained by the capacity descritption i.e a 24AH should be assumed to have only 12AH capacity!

Surely this can't be right?

How do the BFPSA justify this?

Why do the BFPSA think they know better than the BS or rather why do they make recommendations that vary from the BS. Do they think the BS committee don't know what they are doing?

How can they say a de-rating figure of precisely 2 is always right? even the BS states that the any specific de-rating factor for any battery should be ascertained from technical info and in the absence of such, should be considered as 1.75.

I think you should take this up with the BFPSA and if they can't justify their recommendations with proven facts they should be asked why they are messing with BS5839 Part 1.

I'm getting sick of every man and his dog coming up with their own set of recommendations when BS provides a perfectly adequate recommendation. The Fire Services are particularly fond of doing this, particularly in the past, and it makes a mockery of the national/european standards.

[David Rooney]

Benz are you sure you haven't got it mixed up....

In order to simplify the calculation the official BFPSA recommended formula is:

            BATTERY    =   1.25 (T1I1 + I2)

I2 being the alarm load

Mimimum standby time (T1)
Quiescent standby current (I1)
Standby Capacity (T1 + I1)   
Load Capacity (T1I1 + I2)

1.25 being an aging factor (25% per year - estimated)

[Benzerari]

Benz are you sure you haven't got it mixed up....

In order to simplify the calculation the official BFPSA recommended formula is:

            BATTERY    =   1.25 (T1I1 + I2)

I2 being the alarm load

Mimimum standby time (T1)
Quiescent standby current (I1)
Standby Capacity (T1 + I1)   
Load Capacity (T1I1 + I2)

1.25 being an aging factor (25% per year - estimated)

It is always the same formula!

Indeed when it's simplifyed D = 2 and T2 = 0.5h, hence their product = '1'. As BFPSA sets D = 2 as constante value not like in BS as Wiz mentioned it could be less then '2', 1.75...etc

Threrefore C = (I1 T1 + I2) 1.25

There is no confusion in that

You have stated above, standby capacity is (T1 + I1) I assume it's by mistake, it should be (T1 * I1)

[Benzerari]

1.25 being an aging factor (25% per year - estimated)

This is another issue new to me as well, the batteries calculation formula should be then according to the above

1st year of install: C = [I1 T1 + I2 T2 D] 1.25

2nd year of install: C = [I1 T1 + I2 T2 D] 1.50

3rd year of install: C = [I1 T1 + I2 T2 D] 1.75

4rd year of isntall: C = [I1 T1 + I2 T2 D] 2

5rd year of install: C = [I1 T1 + I2 T2 D] 2.25

Is that correct? or am I missinterpreting?

[Benzerari]

Please note however that this method and calculation does not factor in any battery de-rating factor. I have never included the de-rating factor into my method because a) The de-rating figure is often 1 and therefore has no effect on the calculation and b) because it adds a further step to the calculation that blow's most fire aalrm engineer's minds! You could probably cope with it and adjust Wiz's on-site calculation method and include it.

I have tested my method over many years of use and have patents pending on it

Wiz;

In here, you are against both BFPSA and BS versions, by not considering the de-rating factor...etc

What sort of

[Wiz]

David, I agree with Benz - your comment 1.25 being an aging factor (25% per year - estimated) [i/] can't be right. The 1.25 in the formula relates to making a calculation that assumes a 75% battery capacity. But this is not per year. It is a one-off figure suitable for the whole life of the battery.

The second point I'd like to make is regarding the 'simplified formula' that you say the BFPSA recommends.  This is the same as the BS recommendation but doesn't include the de-rating factor and the 50% division of the alarm load. On the basis that the BFPSA suggest a de-rating of 2 according to Benzi, then this cancels out the 50% division of the alarm load. So this would be where the simplified formula comes in! - Ignore the derating figure and the division the alarm load because it achieves tha same result - but only if you assume the derating figure is 2!

Some examples:

The last part of the BS formula is  DxI/2 where D is the derating figure and I is the alarm load. If the alarm load is 3A and the de-rating figure is 2 (as Benzi says BFPSA suggests) then the calculation is:

                              2*3/2 = 3

The BFPSA simplified formula that you suggest for the above is just I, which, in our example, is just 3!
i.e. the simplified formula is the same as the BS formula if the de-rating figure is taken to be 2

However, if we use the BS mentioned de-rating figures of 1 or 1.75 we get:

                              1*3/2= 1.5     or     1.75*3/2= 2.625

I can see why BFPSA might want to use a de-rating figure of 2 to make the formula much simpler. But why should they do this and can they justify the impact this might have on the BS battery calculation.

If BFPSA is going to mess with BS recommendations, how far will they go?  I suppose they could even say something like you can ignore installing detectors in large voids, because it can sometimes be difficult to do! Where could it end?

Surely the BFPSA's job should be to ensure BS recommendations are followed and instruct their members in understanding how to comply with them - not to change the BS recommendations to make things easier!

[Wiz]

Please note however that this method and calculation does not factor in any battery de-rating factor. I have never included the de-rating factor into my method because a) The de-rating figure is often 1 and therefore has no effect on the calculation and b) because it adds a further step to the calculation that blow's most fire aalrm engineer's minds! You could probably cope with it and adjust Wiz's on-site calculation method and include it.

I have tested my method over many years of use and have patents pending on it

Wiz;

In here, you are against both BFPSA and BS versions, by not considering the de-rating factor...etc

What sort of

Not quite Benz, my method assumes that the derating figure is 1 because the alarm load on most systems I deal with is less than 1/20th of battery capacity. This complies with B.S. !

[Benzerari]

The de-rating factor recommendation in BS5839 part 1 2002 is actually 1 or 1.75 and not 2.

It is 1 when the system alarm load is equal or less than 1/20th of the battery's stated capacity. It is 1.75 when the battery's alarm load is greater than 1/20th of the battery's staed capacity.

According to the above, the de-rating factor is more related to (the batteries capacity per actual load), to keep a nominal supply current value not to its min and not to max…etc

I2 T2/ C <= 1/20, means the system will supply half hour alarm (while mains disconnected) for 20 times or more… is that what is required by BS?

[Benzerari]

Please note however that this method and calculation does not factor in any battery de-rating factor. I have never included the de-rating factor into my method because a) The de-rating figure is often 1 and therefore has no effect on the calculation and b) because it adds a further step to the calculation that blow's most fire aalrm engineer's minds! You could probably cope with it and adjust Wiz's on-site calculation method and include it.

I have tested my method over many years of use and have patents pending on it

Wiz;

In here, you are against both BFPSA and BS versions, by not considering the de-rating factor...etc

What sort of

Not quite Benz, my method assumes that the derating figure is 1 because the alarm load on most systems I deal with is less than 1/20th of battery capacity. This complies with B.S. !

I assume it is not particularly the one you deal with, but rather you have decided that your systems will supply 20 times alarm to go off (while mains disconnected) and for half hour each time and no more than 20 times...etc

Also your method is in fact your decision but it still the same method