Battery Capacity Test ? Electronic Ignition

I have been flying my RV8 with Subaru 6 cylinder engine conversion for about 10 years and just under 700 hours. Thought it was time to perform another battery capacity test as my ignition is 100% dependent on electrical power. My configuration is two Odyssey PC625 batteries. I have the ability to completely isolate the batteries for catastrophic electrical failures but for this test I wanted to simulate normal flying conditions with an alternator failure. Using my normal operations the batteries are managed via an EXP Bus.

Test conditions. Outside temp was 79F. Alternator turned off for entire duration of the tests. I tried to make the test somewhat worse case by adding a bunch of loads that I would likely be shed in real alternator failure. The electrical load was two LED landing\taxi lights, 4 Amp heater blower running, Grand Rapids EIS and EFIS running, MT electric prop control, both electric high pressure fuel pumps running and all the power used by fuel injectors, ECU, etc.. Prop RPM was around 800 -1000 RPM so engine RPM was around 2,000. I

Test Results: The first item to fade was the MT Prop controller which began indicating a failure mode at about 11.3 volts. In this condition, the prop stays at same pitch. I was able to manually cycle the pitch at this point by taking the controller out of Automatic mode , and using the ?manual? toggle switch I did not check this capability later on, so not sure how long I would have the ability to manually adjust the pitch.

I did have some data collection issues as below 10 volts at the EIS and EFIS started to drop out (this was my data collection source using the EFIS to log data). I am assuming the noise in the data (constant up\down by ? volt ) is a result of the Exp Bus switching between batteries but I am uncertain. Next time I think I will hook up the voltmeter directly to the batteries and obtain data in that manner vs. using EIS\EFIS.

Happy to see, the engine was running fine down below 10 volts after about 65 minutes. The two PC625 batteries are a 5 years old. I think I will try to repeat this test every year or so. For planning purposes, I will try to be on the ground in 30 minutes after alternator failure as I may only be using one battery if I am forced to go to the backup bus architecture.

Would appreciate the input of others in regards to the difference in electrical load of the injectors when running the engine on the ground at 1,000 RPM vs cruise RPM of about 4,400 RPM?

Very good idea to load test primary and backup batteries once a year, especially with electrically dependent fuel/ignition systems. Good data on what items start to sign off at what voltages too.

Generally injector current draw increases about linearly with rpm at a given MAP.

Some approximate current specs for an SDS EFI system at 4000 rpm/ 25 inches MAP on a 6 cylinder-

ECU- .15 amps
Walbro 393 fuel pump at 40 psi- 4.5 amps
6 injectors- 4 amps
Coils- 2.2 amps

Total fuel and spark current draw- about 11 amps

One PC625 should keep the engine running for at least 45 minutes even if degraded by age somewhat. Two should double that duration. Be aware in cold weather, battery capacity is reduced somewhat.

Good plan to be on the ground within 30 minutes though as you're likely to have your comm and transponder on a bit at least in this sort of situation.

Vitally important to be able to isolate primary and backup batteries somehow.

BTW, the SDS ECU will run down below 7.5V where most other electronic devices will have signed off at 9V or so.

rv6ejguy said:

Generally injector current draw increases about linearly with rpm at a given MAP.

Some approximate current specs for an SDS EFI system at 4000 rpm/ 25 inches MAP on a 6 cylinder-

ECU- .15 amps
Walbro 393 fuel pump at 40 psi- 4.5 amps
6 injectors- 4 amps
Coils- 2.2 amps

Total fuel and spark current draw- about 11 amps

Click to expand...

Ross, thanks for the estimates on electrical load and info about the linear function of amps with RPM, very helpful for my future ground tests.

I need to call you as I am working on an RV10 and plan to go dual electronic ignition. Few years out but I need to become more knowledgeable on the topic and get some ideas on which engine builders have experience installing your system.

Just did a similar ground test.

2x100 watt 110v box fans powered by a 500 watt inverter. This is about a 20 amp load considering inverter efficiency. A 4 year old Concorde RG25XC took 1:15 to hit the 10.8v cut out of the inverter.

I was surprised it tested about the same duration of a new battery, as the new battery cranked much stronger and gave an easy 10 extra seconds before starting to drop off at all.

Both started from a full charge and 24 hour rest.

No connection were cleaned or tightened before hand, just the battery swap