Ext 680 vs Ext900 choice ?

[Larry DeCamp]

I need to order a battery for 0360 8.5 : 1 . My ignition is SDS with isolated small battery for #2 cpu and coil.
I assume the cranking speed is the same for either size battery but the smaller one , with less AH , will deplete sooner than the big one. Not a priority.
My alternator is only 40 amp for simple VFR , so killing the 360 is not likely.
The main battery can supply #2 battery and ignition through a high capacity diode In normal operation, but, the main battery would be off line if the master is open to respond to smoke.
Therefore, I don't see an advantage from a 900 over a 680 except for extended cranking due to hot start issues .
Your thoughts would be appreciated.

 

[bjdecker]

Yup - just the Ah difference -- the voltage is what determines the speed of the motor, dV between a 12.8V Odyssey -vs- a 13.2V LiFePo and so on...

Another thing to consider with these LiFePo's is the amount of current they draw when discharged; An ETX 680 will easily draw 60+AMPs when depleted, an ETX 900 - 80+ AMPs etc. If you have a 40A alternator, with a 40A CB or ANL, it will open that fuse up creating a "load dump" at the alternator, frying it...and being really bad form.

On Ah/reserves - I've always believed that the thinking/calculus should be "How long can the airplane continue to fly to a suitable landing spot." and not "Can I continue to fly to my original destination + reserves." when I see that "ALT Offline" or "Lo Volt" message on the CAS.

 

[Larry DeCamp]

Yup - just the Ah difference -- the voltage is what determines the speed of the motor, dV between a 12.8V Odyssey -vs- a 13.2V LiFePo and so on...

Another thing to consider with these LiFePo's is the amount of current they draw when discharged; An ETX 680 will easily draw 60+AMPs when depleted, an ETX 900 - 80+ AMPs etc. If you have a 40A alternator, with a 40A CB or ANL, it will open that fuse up creating a "load dump" at the alternator, frying it...and being really bad form.

On Ah/reserves - I've always believed that the thinking/calculus should be "How long can the airplane continue to fly to a suitable landing spot." and not "Can I continue to fly to my original destination + reserves." when I see that "ALT Offline" or "Lo Volt" message on the CAS.

 

[Larry DeCamp]

Thanks much Brian. I was not aware that the B&C would put out more than rated. I have a 40 amp ANL between the alternator and buss/Battery contactor. Not sure where to go from here
with regard to AH reserves, that was my original delimma. If the alt died, you would have more time on both ignitions with a bigger battery. That said, the electrical layout with small ebattery assumed priority #1 is flat ground ASAP

 

[jclark]

Thanks much Brian. I was not aware that the B&C would put out more than rated. I have a 40 amp ANL between the alternator and buss/Battery contactor. Not sure where to go from here
with regard to AH reserves, that was my original delimma. If the alt died, you would have more time on both ignitions with a bigger battery. That said, the electrical layout with small ebattery assumed priority #1 is flat ground ASAP

A few additional points.

B&C, PP, and others will produce more current than the listed/rated amount when the circumstances are right. Discharged battery, low resistance, high engine rpm.
I have witnessed this with both of the listed alternators especially when I had done a long crank cycle and depleted the battery a bit. (Fuel not set right. Forgot to turn power on for electronic ignition. Hot start sequence not done properly.)

In early versions of Aeroelectric Connection, Bob recommended (I think) 60A ANL for 40A alternator and 80A for 60A alternators. This is the one in line from alternator to battery "+" post.

Additional Amp-Hrs for an electrically dependent plane can come in handy especially IF you lose your alternator at a location FAR FROM an airport. "Every little bit HELPS."

 

[dmattmul]

Thanks much Brian. I was not aware that the B&C would put out more than rated. I have a 40 amp ANL between the alternator and buss/Battery contactor. Not sure where to go from here
with regard to AH reserves, that was my original delimma. If the alt died, you would have more time on both ignitions with a bigger battery. That said, the electrical layout with small ebattery assumed priority #1 is flat ground ASAP

The ANL fuse was a concern for me but their specs like all fuses carry a pretty decent time at amps considerably higher than rating. For my 60-amp ANL fuse I've never seen 80 amps for more than a few minutes. Spec shows its rated at 90 amps for one hr.

Also, a 900 has considerably higher CCA and PCA than a 680.

https://www.renogy.com/content/files/Manuals/ANL-Fuse.pdf

https://www.batterymart.com/pdfs/ETX900-Product-Spec.pdf

https://www.batterymart.com/pdfs/ETX680C-Product-Spec.pdf

 

[Mich48041]

My interpretation of the Eaton Bussmann DATASHEET is that an ANL-40 will carry 80 amps indefinitely.
A 40 amp alternator will not output that much current even at high RPM and with a low resistance load.