Question TCW IBBS (3AH) volts?

[Neal]

Everything has been working great, had an enjoyable golden hour flight tonight, OAT 60F, perfect conditions. I turned on IBBS and saw 12.7V wondering what the state of my IBBS battery is. That seems a bit low if it is in fact a LiFePO4 battery. Maybe it's because I haven't flown enough to get it charged? So I'm curious from others, when you turn on IBBS, realizing it's under load, what voltage are you seeing?

During flight I saw a max of 13.3 which seems odd as my other two LiFePO4 (EarthX) batteries show 14.1V at full charge. Maybe it's not charged yet? Maybe the battery has a bad cell? What are you seeing for a fully charged TCW IBBS battery?

 

[Cactus Charlie]

My FX-3 AFM supplement includes the following text -

"The normal IBBS charging cycle has IBBS volts gradually increasing to about 14.2 V then gradually dropping to about 13.3 V where the charging cycle starts again. IBBS will not be at maximum capacity until IBBS Volts has risen, at least once, to about 14.2 V after engine start."

and this graphic -

In flight the main battery and ignition battery display MAIN bus volts which is not a useful indication of battery condition. IBBS volts is the actual battery pack voltage not MAIN bus volts.

 

[Neal]

In flight the main battery and ignition battery display MAIN bus volts which is not a useful indication of battery condition

Not sure I fully agree. For example my backup ignition (EarthX ETX-104) doesn't just peg at 14.1V when on the main bus. The state of charge is not 100%. The voltage rises for example yesterday from 13.7 to 14.1. Granted, off charge the volts would show more like 13.0-13.3 and not 13.7. But there is still indication of a charge and then it peaks at 100% state of charge.

The IBBS is interesting. So what is your state of charge which is what I wish we would see instead of volts. This is how my RV (motorhome) is wired using a shunt to measure the current flow. Regardless of charging amps I see the actual state of charge of the battery. So with the IBBS I'm curious what the state of charge is and for those using it as a backup ignition battery this is important to know for what you can expect should you need to use it.

 

[Cactus Charlie]

Not sure I fully agree. For example my backup ignition (EarthX ETX-104) doesn't just peg at 14.1V when on the main bus. The state of charge is not 100%. The voltage rises for example yesterday from 13.7 to 14.1.

The ignition battery voltage readout is MAIN bus volts less any drop across the Schottky diode. Ignition volts will only be significantly different from MAIN volts is the battery charge current is very large. That does not happen with the standard 2 AH AGM battery. I have no data for your non-standard ignition battery.

 

[Neal]

My ignition battery matches the behavior of my starter battery. The odd ball is IBBS.

 

[Cactus Charlie]

So with the IBBS I'm curious what the state of charge is and for those using it as a backup ignition battery this is important to know for what you can expect should you need to use it.

State of charge can be 100% meaning that it cannot be charged any more. That does not tell you much about its capacity since capacity reduces as the battery ages.

The only way to know the battery capacity is to fully charge it and then run a discharge test. That tells you the capacity at the date and time of the test.

For an IBBS battery in good condition I assume the battery is not fully charged until the voltage rises over 13.2 V. I alert if alternator output is available and IBBS voltage is not greater than 13.2 V. I see this alert after engine start if IBBS was used for long time before starting. It has always gone away before I'm ready for takeoff.

More text from my AFM supplement -

"IBBS Volts - The color bands of the IBBS Volts scale have two different configurations. One set of bands is used when alternator output is available for IBBS charging. The other set of bands is used when alternator output is not available.

The upper end of the amber range of the charging scale has been selected to provide early warning that IBBS is either not charging or may not be fully charged. If IBBS was used for an extended time before engine start then voltage in the amber range and CAS alert IBBS VOLTS should be expected. The time it takes for this alert to be cleared will depend on how deeply the IBBS was discharged. Flight planning should assume that full IBBS capacity is not available while the IBBS VOLTS alert is displayed."

 

[Cactus Charlie]

My ignition battery matches the behavior of my starter battery. The odd ball is IBBS.

The IBBS battery pack is not charged as a series group. Each cell is charged individually. The battery pack voltage is not the IBBS charge terminal voltage.

 

[Cactus Charlie]

A while ago I accidentally deep discharged my IBBS battery. Long story shortened - the battery pack had one dead cell that appeared as a short circuit to a single cell charger.

I replaced the battery pack with a new one but IBBS voltage did not increase over 12.9 V in flight.

I sent the IBBS in for repair and an open circuit solder connection was found on one component.

I speculate that the IBBS internal charging circuit had, while attempting to charge the shorted cell, overheated a surface mount component and compromised its solder connection. (This was a rev G IBBS and current design is rev J or later).

 

[ve6yeq]

Using voltage to determine battery charge state is very hard for lithium based batteries which is why when I was working in the mobile phone business, the “BMS” always included a “charge counter” that measured the current in and out of the battery. Due to “drift” in this measurement, you needed to fully discharge the battery every few months to “reset” the charge counting.

With the IBBS, the best way that I can see of determining full charge is make sure that a full charge cycle (bulk - constant current, absorption - constant “high” voltage, float - lower voltage) has been completed not just that the “bulk” charge phase has completed (e.g. the voltage has increased to the high level).