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1.4.15 Depth of Discharge of Battery and Battery Life
The more deeply a battery is discharged on each cycle, the shorter the battery life. Using more batteries than the
minimum required will result in longer life for the battery bank. A typical cycle life chart is given in the Table 1.4.6 below:
TABLE 1.4.6 TYPICAL CYCLE LIFE CHART
Depth of Discharge
% of Ah Capacity Cycle Life of Group 27 /31 Cycle Life of Group 8D Cycle Life of Group GC2
10 1000 1500 3800
50 320 480 1100
80 200 300 675
100 150 225 550
NOTE: It is recommended that the depth of discharge should be limited to 50%.
1.4.16 Series and Parallel Connection of Batteries
Refer to details at Section 3.4.
1.4.17 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 specic 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 rst 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 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.
SECTION 1.4 | General Information – Battery Related
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