Securitron PSM_Series - Manual (Page 7)

PN# 500-16200
Page 7 Rev. D, 08/11
To get the load current, it is usually easiest to work from product specifications. For instance, if
you know that the installation consists of eight Securitron Magnalocks which are shown in their
catalog sheets as drawing 125 mA each, you would know that the eight together should draw
one Amp. The alternate method of getting the load current is to measure it with an ammeter.
The meter is connected in series with the output of the power supply.
A complicating factor occurs when the load is not constant. Many installations consist of a
number of devices that are randomly turned on and off at different times. In this case you have
to make an estimate of duty cycle. For example a load might draw 1/2 Amps all the time as a
base condition, but 25% of the time it would go up to 2 Amps. To roughly analyze such a
situation, you would add the 1/2 Amp baseline to 25% of the surge (increase over baseline).
25% of the 1 1/2 Amp surge is 3/8 Amps so the total average load is 7/8 Amps. It is never
possible to get an exact value of the load but a good estimate is all that's required when making
sure you have enough backup. This is because you should always err on the side of extra
backup by specifying an oversized battery pack.
Once you have the load current, you need the battery pack capacity. This is derived by simply
noting the capacity in Amp-Hours called out on the battery. If there is more than one battery
connected in parallel (such as two 12 V batteries in a 12 volt system) you add the capacities.
When two batteries are connected in series (which always happens in a 24 volt system), the
capacity is unchanged. In other words, two 4 Amp Hour 12 volt batteries connected in series
to back up a 24 volt power supply yield 4 Amp Hours at 24 volts. In 24 volt systems, you'll
often find parallel/series battery packs. For example a 24 volt battery pack could consist of
eight 4 Amp Hour 12 volt individual batteries. First, the eight batteries are connected in pairs in
series to make four 24 volt, 4 Amp Hour packs. Then these four packs are all connected in
parallel to boost the Amp Hour rating at 24 volts to 16 Amp Hours.
Once you have the total average load and the battery pack capacity, you divide the former into
the latter to get the base backup time. For example, a load drawing 1 1/2 Amps, backed up by
a 16 Amp Hour pack would yield a base backup time of 10.7 Hours. The base backup time has
to then be derated to yield the actual backup time.
A number of factors decrease battery capacity. They include: improper charging, low
temperature, age and the discharge rate. Taking these in order, you can ignore improper
charging since you're using a Power Supply Monitor that effectively guarantees proper charging.
If the batteries are not in a room temperature environment, their capacity will be reduced. For
example, a 20 degree F drop in temperature will reduce total capacity by about 10%. Generally,
this is not a factor as the batteries are typically housed within a power supply enclosure which
maintains an elevated temperature. Age will not be a factor so long as the batteries are
replaced at least every five years. The discharge rate, however, always needs to be considered.
Actual backup time will equal base (calculated) time only when the discharge rate is 20 hours or
more. In other words, if base backup time is calculated at 25 hours, you can take that figure as
being representative of the actual backup time you will get. When the base backup time is
shorter than 20 hours, you have to derate to determine your actual backup time. This is simply
because batteries do not perform as well when they are being discharged rapidly. The chart
below shows the derating factor for different base backup times.
BASE BACKUP TIME
% CAPACITY YIELD
20 Hours 100%
10 Hours 90%
4 Hours 80%
2 Hours 70%
1 Hour 60%
30 Minutes 45%
15 Minutes 30%
The chart makes it clear that the shorter the base backup time, the more the battery pack will
lose capacity so as to produce an even shorter actual backup time. For example, consider a 4
Amp Hour battery backup up for a 1 Amp load. The base backup time is four hours but you
have to derate to 80% so you'll only get 3 hours,12 minutes. If the load was 4 Amps, your base
backup time would be one hour. This figure would have to be derated to 60% or 36 minutes.