Odyssey backup battery

[drmud90]

I have inherited an Odyssey EFIS. I intend to attach an 3.6 Ah backup battery. The objective is to provide avionics as well as engine monitoring. The Odyssey will power the SP-6, SP-7 and RDAC up to 0.5 A. I am not sure what is the load from the RDAC. Attached to the RDAC are the following sensors: coolant temp, oil temp, oil pressure, fuel flow, fuel pressure and fuel level. I can connect the RDAC to the Odyssey power through an inline quick burn 0.5 A fuse. I can test this by connecting the 0.5A fuse to the battery. Does this make sense?

[Bill Hertzel]

Fuses exist to protect the wiring. Not the device.
Assuming you power the EFIS and RDAC with 18 AWG wire.
18AWG is rated at 10 amps at 60°C.
A 5.0 amp fuse would be more than sufficient in this situation.
The EFIS to RDAC wire would not necessarily need an additional fuse.
But if you insisted, another 5.0 amp fuse located close to the Power Source end near the EFIS would be correct if you were still using 18AWG wire.
Using a 0.5 amp fuse to power a 0.5 amp load would result in a lot of blown fuses.

[rainier]

Fuses exist to protect the wiring. Not the device.

Actually - it's also partially to protect the device. There is a lot going on inside any piece of electronics these days - hundreds even thousands of individual components. Internal power supplies, regulation, protection etc can and does fail. Semiconductors do wear over time - some will last for hundreds or even thousands of years - other will be stressed a lot by heat/cooling cycles etc. Basically, things can and will fail.

An external fuse is required to prevent serious damage also inside the device by limiting the amount of current flowing into the device in the case of a fault that causes excess current flow. Without fusing, in a bad case this could result in a fire or at least unrepairable internal damage.

Our devices are designed in such a way that serious internal issues such as an input filter capacitor shorting or similar without and external fuse being present results in very minor damage by use of a sacrificial (but functional) component. The idea basically being to offer up a cheap component that will disrupt further destructive current flows and thus end up with something you can repair.

It is of course possible to use internal fuses and there are a number of small electronic fuse solutions - we do not use them due to bad experiences with them. They tend to wear and eventually blow without reason. Thermal self resetting fuses we also found unreliable and usually take too long to prevent damage further down the line.

So I definitely do recommend fuses to protect not just aircraft wiring - which is valid - but also the connected equipment. Choose the fuse rating wisely - both current and time it takes to blow a fuse. Keep in mind that electronic devices will usually have some form of large capacitors to charge as you apply power - so for a brief moment the current can go much higher than you would expect during normal operation - a fuse will notice than even if it does not blow - repeat the process often enough and the thin fuse wire will actually break due to the physical expansion and contraction of the wire every power cycle.