Ignition battery

[ceslaw]

Interesting

I look forward to what you learn

[stautz]

I have a similar problem. I replaced my battery with a brand new PowerSonic battery. During run up when checking the emergency ignition function the voltage would drop to well below 12 volts in about ten seconds. The engine continues to run, but the significant drop in voltage was concerning and I am not sure how long it will run.

It seems like the circuit has an additional load or short when the emg switch is turned on.

[Andy]

It seems like the circuit has an additional load or short when the emg switch is turned on.

The only load on the battery when the emergency ignition is on should be the LED and the ignition module itself. The switch disconnects the battery from the charging source and connects it to the ignition module.

My test data show battery discharge current of 1.2 A at start of a 30 minute discharge test and 1.5 A at the end of test. The increase in current as battery voltage drops appears to be consistent with the use of a linear voltage regulator in the ignition module.

If a new battery that has a full charge won't hold voltage during a normal emergency ignition test then the problem is likely that the new battery is no good. This is why I have decided to test and replace on condition rather than on calendar date.

Here is a sample of battery voltage plots for a the battery that has been in my FX-3 since it was built early in 2020. These are all preflight ignition tests not the extended 30 minute discharge test:

ign tst plots.png

[ceslaw]

Andy:

EXCELLENT information. It is great seeing data to back up a theory.

Would the take away be to test for ten seconds, confirm it is holding voltage, than end the test? Is ten seconds an appropriate length? Based on fairly consistent results over time with your data, it would seem to be the case.

Presumably only testing for ten seconds would have virtually no effect on total capacity of batter in a true emergency.

Knowing that replacement batteries can be bad (as was the case on one I replaced at the annual condition inspection), this would indeed seem to be a better approach than annual replacements.

Chuck

[Andy]

Would the take away be to test for ten seconds, confirm it is holding voltage, than end the test? Is ten seconds an appropriate length? Based on fairly consistent results over time with your data, it would seem to be the case.

I think the 10 second discharge test gives a reasonable indication of battery condition but it's not as good as a 30 minute discharge test. As I said in an earlier post I'm concerned about the emergency battery unregulated charging current. I'm going to hold off on doing another 30 minute test until I have a charge current limiter installed.

Since you are taking an interest in the topic here are the voltage and current plots for my most recent 30 minute in-flight discharge test:

[ceslaw]

I think the 10 second discharge test gives a reasonable indication of battery condition but it's not as good as a 30 minute discharge test. As I said in an earlier post I'm concerned about the emergency battery unregulated charging current. I'm going to hold off on doing another 30 minute test until I have a charge current limiter installed.

Since you are taking an interest in the topic here are the voltage and current plots for my most recent 30 minute in-flight discharge test:

Perhaps one could treat that back up battery like an ELT battery by way of comparison. In other words, if an emergency occurs and it is used for an extended period, consider replacement. If no emergency occurs, test it for 10 seconds and use it indefinitely as long as it passes the run up check. The need to rely upon it in an emergency should be extremely rare, so it should last a long time based on your data.

You noted substantial current draw to recharge it after a deep discharge. If it is only tested for 10 seconds on the run up I assume it would charge at only a few hundred milliamps and high current flow would be the rare exception following an emergency.

I'm just trying to come up with protocols that I can follow going forward consistent with data you have acquired. Thanks for posting.

[Andy]

You noted substantial current draw to recharge it after a deep discharge. If it is only tested for 10 seconds on the run up I assume it would charge at only a few hundred milliamps and high current flow would be the rare exception following an emergency.

I don't have a family of plots for charging current after a 10 second test but, in a quick check, I found that 0.9 A peak seems to be typical. Charge current exceeded 0.3 C for 7 seconds in one example. I doubt that this would be damaging but I'd prefer to keep charge current under 0.3 C (0.6 A).

[jmorrical]

Would someone explain how the emergency ignition battery is charged? Is the alternator the only charging source? Can I 'switch off' charging somehow? Could you include the circuit diagram showing it.

Earlier in the thread Andy said he had some concern about the emergency battery having unregulated charging. How so?

[Andy]

Would someone explain how the emergency ignition battery is charged? Is the alternator the only charging source? Can I 'switch off' charging somehow? Could you include the circuit diagram showing it.

Earlier in the thread Andy said he had some concern about the emergency battery having unregulated charging. How so?

The emergency ignition battery is charged from MAIN Bus via a 10 A circuit breaker, a blocking diode, the emergency ignition switch, and a 10 A blade fuse. The circuit diagram should be in the maintenance manual. It's "-107 CC11-160 IGNITION BACKUP BATTERY SYSTEM".

The ignition battery will be charged any time MAIN Bus voltage is greater than the ignition battery voltage plus the diode drop. That would normally only happen when the engine is running and the alternator is providing power. If the emergency ignition breaker is pulled the ignition battery will not be charged but the battery can still power the right ignition system when the switch is selected to emergency.

Charging of the ignition battery is unregulated except that the alternator regulates MAIN Bus voltage to about 14.1 V. If ignition battery has been significantly discharged then the charging current can exceed the battery manufacturer's recommended maximum charging current. The discharge caused by the normal pre-flight emergency ignition check is not sufficient to give excessive charge current. Excessive charge current will only be seen after several minutes of emergency ignition then a return to normal ignition.

I developed and tested an emergency ignition battery charge current limiter. (That's a grand way of saying I tested the current/voltage characterisitics of a car brake light bulb.) I have not fitted this current limiter because my data showed that it was not required if the ignition battery was only discharged during pre-flight ignition test.

If the ignition battery was used during a real in flight emergency it would be preferable to remove it from the aircraft and charge it on a properly regulated charger.

Comments are based on actual emergency ignition battery current measurements. My FX-3 has continuous monitoring and logging of emergency ignition battery current.

[jmorrical]

Andy,
Thanks for the explanation of charging.

I have chosen to use an EarthX ETX680C battery and their Optimate Lithium Battery Charger/Maintainer to optimize the health of the battery. Is a battery powered main bus providing enough volts to charge the ignition backup battery? From what you have said I think not??? While familiarizing myself with the Garmin G3X I will operate on ships battery power with the charger in what they call Maintenance mode. I wonder if I'm not charging that ignition battery during those times?

I have had my ignition backup battery on the shelf for six years while I've been 'slow building' my EX(1). Would you replace this OEM battery before first engine run and first flight even if it shows good volts? Or see how it tests during first engine run? I have seen Power Sonics says they have a 5 year shelf life.

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