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

[Cactus Charlie]

CB and fuses should be sized to protect the wiring not the equipment at the end of the wire. You can't increase the CB and fuse rating without considering whether the wire is rated for the increased current.

I don't know what blocking diode CubCrafters uses but the 90SQ030 diode shown in the LSE ignition schematic is rated at 9 A.

 

[Neal]

I may put a 15A fuse in place of the 10A as that's behind the panel and not something I can reset in flight. I believe that limit is only affected by charging and not discharging of the battery for demands such as Light Speed. It's really just to protect the incoming current (charge). The breaker I can reset but again should not be affected by charging but only in use activities?

Am I thinking this through correctly as my only concern of going over 10A is during charge.

 

[Cactus Charlie]

The 10 A fuse is in circuit for charging and discharging. The 10 A CB and the blocking diode only pass current when charging.

When I was concerned about the charge rate of the AGM I devised and tested a simple current limiter but I never fitted it to the aircraft. I stopped doing in flight testing as I found the 10 second run-up test was a good indicator of battery condition.

The limiter was based on the characteristics of tungsten filaments. They have low resistance when cold and high resistance when hot. In series between source and load they act as a current limiting device. More current, filament gets hotter, resistance goes up, voltage drop increases, current goes down.

My limiter used a car tungsten brake light bulb. I do not know if such a limiter would be compatible with the LiFePO4 BMS but it could easily be tested on the bench and I doubt it could harm the battery.

 

[Neal]

Has anyone checked the specs for max charge rate of the IBBS 3AH battery or the EBBS? Good info to know before diving into this mod. I hope to test mine today with a ground run.

@Cactus Charlie can you post the electrical schematic of the ignition backup circuit showing the charge path so I can see what you mention about going through the breaker, Light Speed, etc.? Just want to have it here in this discussion for educational purposes of anyone considering changing the factory setup.

Thanks!

 

[Cactus Charlie]

Has anyone checked the specs for max charge rate of the IBBS 3AH battery

Yes, I did as part of the research of my plan to replace the AGM with a 3 AH IBBS.

"Pin 5 provides charging and bus voltage sensing for the IBBS unit and must be connected to an aircraft power bus. When the voltage on pin 5 falls below about 11 volts the system automatically transfers the load current on the output pins to the internal back-up battery source. Pin 5 maximum current draw is 2.5 amps during charging."