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7.15 SIZING THE INVERTER BATTERY BANK
One of the most frequently asked questions is, "how long will the batteries last?" This question cannot
be answered without knowing the size of the battery system and the load on the inverter. Usually this
question is turned around to ask “How long do you want your load to run?”, and then specific calculation
can be done to determine the proper battery bank size.
There are a few basic formulae and estimation rules that are used:
1. Active Power in Watts (W) = Voltage in Volts (V) x Current in Amperes (A) x Power Factor.
2. For example, for an inverter running from a 48V battery system, the approximate DC current required
from the 48V batteries is the AC power delivered by the inverter to the load in Watts (W) divided by 40.
3. Energy required from the battery = DC current to be delivered (A) x Time in Hours (h).
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, Formula 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 48V inverter is given below:
Let us say that the total AC Watts delivered by a 48 VDC input inverter = 1000W.
Then, using Formula 2 above, the approximate DC current to be delivered by the 48V batteries = 1000W ÷
40 = 25 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 48V batteries = 25 Amperes × 3 Hours = 75 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 48V batteries to deliver 75 Ah will be equal to: 75 Ah ÷ 0.6 = 125 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:
125 Ah ÷ 0.8 = 156.25 Ah (note that the actual energy required by the load was 75 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.
7.16 CHARGING BATTERIES
Batteries can be charged by using good quality AC powered battery charger or from alternative energy
sources like solar panels, wind or hydro systems. Make sure an appropriate Battery Charge Controller is used.
It is recommended that batteries may be charged at 10% to 13% of their Ah capacity (Ah capacity based on
SECTION 7 |
General Information on Lead Acid Batteries
