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

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Neal

Forums Chief Pilot
Staff member
Joined
Oct 31, 1996
Posts
1,324
Type aircraft owned
Carbon Cub FX-3
Base airport
KFCI
Ratings
COMM, IFR, MEL, SEL
A few discussions on the forum about choosing an ignition backup battery but this thread will document the actual mod of switching to a different ignition backup battery from the PowerSonic AGM installed in my FX-3. Anyone else that wants to contribute with their mod to their Carbon Cub of any flavor feel free to use this thread as well.

This evening I decided to pull the right forward panel off and begin the brainstorming process of changing out the PowerSonic battery to another option of higher capacity and likely LiFePO4. I didn't take any pictures but here are my discoveries from the start of this process.

The EarthX almost fits in the hole of the PowerSonic bracket but it does not. I thought about removing the bracket used to hold the PowerSonic but that would be a challenge due to the small nuts behind the bracket that holds the custom built bracket in place. I also would prefer to leave the bracket intact should I ever want or need to go back to the factory setup. So that area is out.

Now where can I put this small EarthX battery? I pulled the GDU and it will not fit behind the GDU so that option is out.

Back to the right forward panel. There is a lot of space below the Light Speed ignition modules and forward at the firewall end. After maneuvering the battery around I think that's the best spot. I need to design a mount that will likely use Adel clamps to secure it in place. It's going to take a few iterations I'm sure and I surely want to make sure I never touch or impact the boot cowl and cause a dent on that thin aluminum so I'll also want to put a piece of foam pad between the battery and the aluminum for safety. Again, there's plenty of room but I don't need any more drama with this plane so I am being extra careful.

An alternative but far less desired location would be possibly mounting it below the pilot seat underneath the aircraft from the access panel on the bottom.

The TCW IBBS battery is an ideal size and it's tempting but it too needs a bracket of some type. I'll see what I can come up with for the EarthX I have first. Time for some CAD work to see what I can design for a PETG bracket.
 
@Cactus Charlie when disconnecting the PowerSonic battery I noticed a blade fuse holder connected to the wire. I didn't open it to check the size of the fuse. If you have your schematic handy can you tell me what size fuse is inline to the ignition backup battery?

If it's less than 10A this may be an issue with putting a pig tail on the EarthX battery to directly charge it with my Optimate charger that pushes 10A if that current can go downstream of the battery and through that fuse? Then again if nothing is energized to pass the current maybe I'm incorrect but I just want to make sure charging the EarthX directly at 10A max won't be a problem with the wires connecting to the battery that go to the backup ignition circuit.
 
I'll ask my question in advance. Let's say the fuse is a 2A or 5A fuse. I have a pigtail connected to my EarthX battery to charge while in the hangar which can push up to 10A with my current charger. I "believe" that current will not flow through that blade fuse as no circuit is open while the aircraft is not in use, correct? So charging the battery at 10A with that wire downstream with a smaller fuse is not at risk?
 
The ignition battery blade fuse is 10 A. The fuse will not pass current from an external charger if that charger is connected directly to the battery terminals.
 

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