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en | Planning information
4.3
Effects of discharge rate on battery capacity and battery life
The rate at which a battery is discharged also has a profound effect on its capacity and life.
Figure 4.1 shows the effect of discharge rate on battery capacity. The figure shows that a
battery -when discharged at a low rate- will be able to deliver a higher capacity than a battery
discharged at a high rate.
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Figure 4.1 Capacity vs discharge rate
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4.4
Depth of discharge (DOD)
A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be
discharging from 100% to 20%, and then back to 100%. However, there are often ratings for
other depth of discharge cycles, the most common ones are 10%, 20%, and 50%.
Battery life is directly related to how deep the battery is cycled each time. If a battery is
discharged to 50% every day, it will last about twice as long as when it is cycled to 80% DOD.
If cycled only 10% DOD, it will last about five times as long as when it is cycled to 50%. The
most practical number to use is 50% DOD on a regular basis. This does not mean that you can
not go to 80% once in a while. It is just that when designing a system when you have some
idea of the loads, you should figure on an average DOD of around 50% for the best storage
versus cost factor.
Also, there is an upper limit: A battery that is continually cycled 5% or less will usually not last
as long as one cycled down 10%. This happens because at very shallow cycles, the lead
dioxide tends to build up in clumps on the positive plates rather than in an even film.
Figure 4.2 shows how the battery life is affected by the depth of discharge.
Figure 4.2 Battery life based on depth of discharge
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180110011Aa | V1.1 | 2011.05
Battery capacity
Discharge time in hours
Number of cycles
Daily average depth of discharge in %
Installation and Operation manual
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Battery Charger
Bosch Security Systems B.V.
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