Main and Essential Buses with dual batteries
By Reg Nicoson
Basics
Some experimental aircrafts are designed with two electrical buses; one for normal operation (main bus) and one for operating essential equipment (essential bus). It is also common for each bus to have their own battery. The essential bus has limited current carrying capability for it only powers essential loads. As such, the essential (backup) battery is typically a smaller Ah than the main battery (see Figure 1 below).
Two important battery design considerations are: sizing the main and backup battery for discharge current and operating time (i.e. Ah) and maximum allowable charge current. With lead acid batteries, charge current is of little concern for a lead acid battery has a large resistance to charge current flow. This is not true with a lithium battery, due its low resistance to charge current flow. Lithium cells (batteries) require external mechanisms to limit charge current to acceptable levels. In the dual bus system shown in Figure 1, the backup battery is (or can be) directly connected to a 60 amp charging source. This is an issue for the 6Ah backup battery as it is rated for a maximum of 30 charging amps. The main battery being a much larger Ah is rated for the plane’s main alternator 60amp output. Charge current is directly proportional to the charging voltage; the higher the voltage the higher the charging current. Figure 2 below shows charging current versus voltage for a 6Ah LiFePO4 battery. In this example, the battery’s initial State of Charge (SoC) is 80%, 13.2V, so it’s charge current never exceeds the maximum allowed (in this example maximum rating is 30A).
But if the battery was drained to less than 20% SoC ( <13V) the current would exceed the maximum allowable 30amps (see Figure 3).
Main Bus and Essential Bus Isolated by Diode
When a diode is used to connect the main bus to the essential bus, a .5V drop across the diode lowers the charge voltage, thus limiting the charge current (see Figure 4 below).
With the isolation diode the backup battery’s charge current will be below the maximum allowed charge current (see Figure 5), even with a drained battery (20% SoC). This is because the backup battery’s charging voltage never exceeds 14.1V.
Maximum Safe Operating Limits
Based on extensive testing, the maximum charge rating will never be exceeded using this simple and reliable diode isolation design. A fuse could also be placed in line with the diode. There are other designs that would limit charge current. But no matter the design, what’s important is ensuring that the charge current never exceeds the battery’s rating for any given SoC.
References
Source 1 – EarthX Lithium Battery User’s Manual 2016
© 2017 EarthX, Inc. Application Note AN-1612 Rev A
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