SamplexPower PST-300-12 Manuel Du Propriétaire page 26

seCtIOn 7 |
The first step is to estimate the total AC watts (W) of load(s) and for how long the
load(s) will operate in hours (H). The AC watts are normally indicated in the electrical
nameplate for each appliance or equipment. In case AC watts (W) are not indicated, for-
mula 1 given above may be used to calculate the AC watts. The next step is to estimate
the DC current in Amperes (A) from the AC watts as per formula 2 above. An example
of this calculation for a 12V inverter is given below:
Let us say that the total Ac Watts delivered by the 12V inverter = 1000W.
Then, using formula 2 above, the DC current to be delivered by the 12V batteries
= 1000W ÷10 = 100 Amperes.
next, the energy required by the load in Ampere Hours (Ah) is determined.
for example, if the load is to operate for 3 hours then as per formula 3 above, the
energy to be delivered by the 12V batteries = 100 Amperes × 3 Hours = 300 Ampere
Hours (Ah).
now, the capacity of the batteries is determined based on the run time and
the usable capacity.
from Table 7.3 "Battery Capacity versus Rate of Discharge", the usable capacity at
3 Hour discharge rate is 60%. Hence, the actual capacity of the 12V batteries to
deliver 300 Ah will be equal to: 300 Ah ÷ 0.6 = 500 Ah.
And finally, the actual desired rated capacity of the batteries is determined based on
the fact that normally only 80% of the capacity will be available with respect to the
rated capacity due to non availability of ideal and optimum operating and charging
conditions. So the final requirements will be equal to: 500 Ah ÷ 0.8 = 625 Ah (note that
the actual energy required by the load was 300 Ah).
It will be seen from the above that the final rated capacity of the batteries is almost
2 times the energy required by the load in Ah. Thus, as a Rule of Thumb, the Ah
capacity of the batteries should be twice the energy required by the load in Ah.
for the above example, the 12V batteries may be selected as follows:
- Use 6 Group 27/31, 12V, 105 Ah batteries in parallel to make up 630 Ah, or
- Use 3 Group 8D, 12V, 225 Ah batteries in parallel to make up 675 Ah.
26 | SAMLEX AMERICA INC.
General Information on Batteries for
Powering Inverters