MOD Upgrading the ignition backup battery in a Carbon Cub FX-3

[Cactus Charlie]

So here's another thought @Cactus Charlie. An electrical circuit, I would imagine, is designed to limit the current that "can" flow based on wire size, length, etc. As we know, this circuit is fused and breakered at 10A. It shouldn't matter if the "receiver" (battery) can take a charge at 2A or 100A. The circuit should be designed to not push current beyond the design. Correct?

The current that flows through a wire is controlled primarily by the source voltage and the load impedance. The resistance of the wire may limit the current but it will then get hot and may catch fire.

The wire must be sized so it can pass the expected maximum current. It is never intended to be a current limiter.

Circuit breakers and fuses should be chosen to limit current to less than the maximum value that can be safely passed by the wire.

First determine the expected maximum current either from source voltage and load impedance or from the load specifications. Then select the wire size so it has adequate current margin. Then select a CB or fuse with a lower current rating than the wire can safely pass.

If your EarthX "load" can draw 15 A from a 14 V supply then the CB, fuse, or diode may trip/blow. The circuit was not designed to pass 15 A. If you increase the CB and fuse ratings the diode may blow. I have not mentioned the switch rating before since I don't know what it is.

There are tables than can be used to find appropriate wire size in AC 43.13-1B.

 

[Cactus Charlie]

Found it. I don't have a large enough charger.

You don't want a charger for this test as it will have internal current and voltage control. You need a fixed voltage power supply with no currrent limiting.

 

[Cactus Charlie]

You need a fixed voltage power supply with no currrent limiting.

The battery of a running car may be a suitable alternative. Safer than trying to test with a running aircraft engine.

 

[Neal]

My test plan / goal is to discharge the backup battery. I can do this by powering my Starlink Mini using an adapter I have for the Starlink to connect to a 12V battery. Then I'll do a ground run and monitor the amps the plane will supply a discharged battery. What I'm trying to determine is what is the max amps the aircraft is going to try pushing to this battery. This is the worst case scenario I'm testing for in this installation.

The scenario that is of concern is aircraft charging the battery by the aircraft alternator. There is no concern of battery supplying power when called for.

I will also have alerts programmed in the G3X so I am aware if the ignition b/u battery goes offline which already should be programmed to alert below a certain voltage which indicates the ign b/u battery is unusable. I will verify this when I program the LiFePO4 numbers.

I would love to see a MOD thread on using the TCW IBBS 3AH battery in a E/FX-3 and how it would be wired. It's my next go to. I looked at the specs for the EarthX EBBS which is quite large and seems to be competing with the TCW IBBS 3AH product. I'd prefer TCW but don't understand the wiring needed so that's for another thread if someone wants to start that discussion. The TCW IBBS 3AH + Wiring harness seems to be around $500.