08-10-2017, 11:56 AM

(07-10-2017, 03:18 PM)Trickle Charge Wrote:(07-10-2017, 03:10 PM)Wiz Wrote:(07-10-2017, 09:57 AM)Trickle Charge Wrote: So just to confirm the following formula is correct?

[(Q x 24) + (A x .5 x 1.75)] x 1.25 = Ah Required

Cmin= 1.25 (T1 I1 + D I2/2)

is that a yes or a no?

Basically, yes, but you should really use the BS specified formula terms because these are universal and therefore understood by everyone. For example, your Q, which I presume stands for 'quiescent' is not a universally understood term for 'quiescent' or 'standby' load. Also A could be construed as Amps, when in your formula it stands for Alarm load.

There is nothing wrong with the BS formula and and it should be easy to remember that I1 is the standby load and I2 is the alarm load measurements in Amps and T1 is the standby time measured in hours.

Most importantly, you should remember that your 1.75 figure is not the constant that figure appears to give it but is a variable, shown in the BS formula as D. It also only applies when the Alarm load is greater than the value of the battery capacity divided by 20 (i.e. a 24AH battery gives a value of 1.2A.) The explanation in the BS describes it as follows:

When Cmin/20 is less than I2 the value of D should either be based on the battery manufacturer's data or should be 1.75.

This means D can have a value of 1, 1.75 or a figure determined by the battery manufacturer. I would accept that if you use 1.75 you are likely to be using a figure which will give you a battery capacity calculation which almost always errs on the 'safe' side. However, what happens if you use the 1.75 figure to calculate a battery capacity in an argument against someone who used the proper BS formula in a system with a low alarm load reading and calculated a lower standby battery capacity would be sufficient. Who would be correct?