Choosing an ignition backup battery for an electronic ignition piston aircraft engine

[Neal]

You have an older IBBS. Are you sure it's LiFePO4 like the newer?

 

[Cactus Charlie]

I can read the IBBS label. I have dismantled the battery pack. What would it take to convince you. You seem to doubt much of what I post.

 

[Neal]

I'm just trying to grasp a LiFePO4 functioning below 10V. That's all. Does TCW not have a BMS in theirs?

 

[Neal]

I emailed TCW last night trying to determine if the 3 AH IBBS battery (2025 edition) has a BMS or not. I invited TCW to participate here but not sure if they will or not. I asked if there is a low voltage cutoff and if it has a BMS, this is the response:

• The IBBS transitions from aircraft bus power to its internal battery when the bus voltage falls to approximately 10.5–11.5 V. That is the point the unit starts supplying backup power from the internal battery pack.
• During backup operation, the output voltage will typically sit between about 10 V and 14 V.
• In the battery performance table (manual), duration figures are given for load down to 10.0 V — implying that useful output down to ~10 V is anticipated under load.

The interesting item, which we discussed but I'll resurface the conversation, with the IBBS battery is in normal operation it is "armed" and not supplying power. Like in our FX-3 with IBBS we turn the switch ON and it only supplies power when the main bus is not supplying power. This is why when I eventually found the sense wire disconnected from the IBBS breaker (which handles charging the IBBS) I saw a "IBBS in use" CAS alert in my G3X/GDU.

So if you're going to wire this for an ignition backup battery you need to understand this logic in how you wire it. Our current ign backup battery switch engages our PowerSonic backup battery but if you swapped to a TCW/IBBS it may simply ARM it and not actually show it powering your R ignition. It sounds like in the XCub they use a 3 position switch which is OFF, ON (armed), and OVERRIDE type of setup.

So the "discussion" between EarthX backup battery and TCW IBBS and low voltage cutoff is still in question. However, keep in mind the EarthX I have is a 4 AH battery in a small form factor as a drop in replacement for the PowerSonic (presumably) yet the TCW (3 AH) will require more engineering/design thought with wiring, voltage sensing, and the ability to test.

The next question that comes into play is below what voltage is insufficient for the Light Speed ignition module?

 

[Neal]

TCW answered my last question sent last night as most LiFePO4 batteries I am familiar with have a BMS to protect the cells from damage:

The TCW IBBS is equipped with a Battery Management System (BMS) that performs multiple functions, including monitoring temperature, voltage, and cell balance. While these parameters are carefully managed, the specific implementation details are proprietary and therefore not disclosed.

The battery is capable of delivering output down to approximately 9 volts, at which point the under-voltage lockout disables the output. At 10 volts, however, the battery is effectively depleted.

 

[Neal]

One other thing to keep in mind when choosing (or using) a LiFePO4 battery such as EarthX, TCW, etc. is that the BMS will (should) prevent charging below 32F. I don't know that EarthX does that, it should but the discussions concern me on this. There is something called "Lithium plating" that can occur if charging below 32 F and damage the cells from what I've read. So if you are in very cold climates and expect that your TCW IBBS batteries such as may be used for an ignition backup (or in my case EarthX) realize it may prevent charging until within allowed temps. This is one consideration of NOT changing out a PowerSonic AGM style ignition backup battery, or maybe consider a 4 AH AGM instead of higher quality that fits.