Specifications

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needs to have a high enough energy density to power a laptop and a projector for at
least two hours. In order to estimate the correct size battery and ensure its lifetime is not
compromised, a few calculations must be done. Most laptop power supplies consume
less than 120W and most projectors consume less than 450W. To be safe we can
round this up to a total of 600W power usage between the two, which would draw 5A
from a 120V source. This means we need a battery with an approximate capacity of
10Ah. Since draining a battery completely is bad for its longevity, it‟s best to abide by a
rule of thumb for the percentage of a battery‟s energy to be depleted in power storage
applications. This percentage fluctuates, depending on the number of cycles desired out
of the battery, but at the lower end of the spectrum is 50%. The effect of the depth of
discharge on the cycle life of the battery can be viewed in the figure below. Accounting
for an 85% efficiency of the DC to AC inverter, a battery with approximately 23.5Ah of
storage capacity is necessary. In most cases, batteries with this kind of storage capacity
are going to be of the smaller automotive sizes.
Figure 36 - Cycle Life vs. Depth of Discharge
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It‟s worthy to note that there are two different popular design types of AGM batteries.
The first AGM batteries were made with rectangular cells, but more recently battery
companies have been using spiral cells. Spiral cell AGM batteries are typically capable
of containing a higher internal pressure. This ability to withstand a higher internal
pressure reduces the sensitivity to overcharging, as they can be designed with a
pressure relief valve that opens at a higher pressure. The higher the pressure, the more
severe the overcharge will have to be to cause the pressure relief valves to open and
allow water to be lost. Spiral cell designs also tend to have lower internal resistances,
which increases charging speed and efficiency and decreases the effect of self
discharging. The figure below shows the construction differences between a spiral cell
AGM (left) and a typical rectangular cell AGM (right).