Redundant BatteriesArticle courtesy of G.S. Air. With the ever increasing cost of the larger planes, as well as their ability to tolerate an extra 6 ounces of insurance, it has become common place to see redundant battery systems. They can be as simple as adding an extra battery and switch, to more elaborate systems with regulators and redundant switching electronics. I usually take the simple approach in believing in the KISS principle. Keep it simple, limit the hardware that can fail, and always provide two of everything where practical. Rather than add a electronic circuit, which still has only one path to the receiver, and more components that can fail, I provide two paths to the receiver with two batteries and switches. This setup will cover the most common types of failures such as connectors, solder joints, switches, cell to cell connections, bad battery packs, excessive discharge of a cell or pack, and so on. While it will not provide redundancy for a battery short, neither will a single battery pack setup. If you look at a quick table of the common failures, and what redundancy is provided by each, you can determine what cost vs. functionality you are willing to purchase.
Probably the first concern one might think of with a dual battery/switch system is the charging of one battery by the other since they are connected by the same bus. The second is the failure mode caused by a short in the battery system. After some testing for cross charging, as well as lots of reading, it appears that the cross charging isn't an issue after all. For example, I took my standard dual battery/switch configuration, and ran a couple of tests on it. In one test, one 1600mah battery was fully charged, and the other was almost fully discharged. I turned on the receiver and transmitter, and simulated a 20 minute flight. I had no problem with the Tx/Rx or battery system. After the test, the charged battery showed a normal loss consistent with a 20 minute flight. The discharged battery showed only very slight increase in capacity. Cross charging did occur, but was insignificant as far as the possibility of ever causing an in-flight failure. As long as you charge, maintain, and measure each battery independently, cross charging doesn't seem to be an issue at all. For further info, check the links listed below. A short in a dual battery/dual switch system will cause a failure of the redundant packs. With proper battery maintenance, a short is a rare occurrence. In a short situation, it will take a little bit of time for a complete failure of both battery packs, and that might even be just enough time to bring it down safely if you notice the plane starting to get sluggish, and attempt to land immediately. With a single battery system, the short will fail immediately, and that's the end of that. With an electronically switched system, a shot will no cause a failure, as the electronics will switch over to the alternate battery. Fortunately, shorts are very rare in flight, and usually happen when the batteries are inactive or sitting in storage. This allows it to be detected on the ground during routine pre-flight checks, thereby eliminating an in flight cell sort for all practical purposes. Implementation - dual battery/switch system
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