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1.4.13 Series and Parallel Connection of batteries
refer to details at Section 3.4.
1.4.14 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 an inverter running from a 12V battery system, the approximate DC current required
from the 12V batteries is the AC power delivered by the inverter to the load in Watts (W)
divided by 10 & for an inverter running from a 24V battery system, the approximate DC
current required from the 24V batteries is the AC power delivered by the inverter to the
load in Watts (W) divided by 20.
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 op-
erate in hours (H). The AC watts are normally indicated in the electrical nameplate for each appli-
ance 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 12V inverter is given below:
Let us say that the total AC watts delivered by the inverter = 1000w.
Then, using formula 2 above, the approximate DC current to be delivered by the 12V batteries
= 1000W ÷10 = 100 Amperes, or by 24V batteries = 1000W ÷ 20 = 50A.
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), or by the
24V batteries = 50A x 3 Hrs = 150 Ah.
now, the capacity of the batteries is determined based on the run time and the usable
capacity.
from Table 1.9 “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 the actual capacity of the 24V battery to deliver 150 Ah
will be equal to 150 Ah ÷ 0.6 = 250 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:
fOr 12V bATTErY: 500 Ah ÷ 0.8 = 625 Ah (note that the actual energy required by the load
was 300 Ah).
fOr 24V bATTErY: 250 Ah ÷ 0.8 = 312.5 Ah (Note that the actual energy required was 150 Ah).
SECTIOn 1 |
Safety Instructions & general Information
