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Battery Size For 912 iS?

  • geoffrey klestadt
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2 months 4 weeks ago #16686 by geoffrey klestadt
geoffrey klestadt created the topic: Battery Size For 912 iS?
I note the installation manual calls for at least 16Ah of battery capacity. Since in the event of a dual alternator failure, the engine will require about 12 amps to run a fuel pump and ECU this is understandable.

Is there any data on the likelihood of a double alternator failure?

Can the engine be started in the "backup battery" emergency mode?

I am trying to decide on a battery size.

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  • richard Boslaugh
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2 months 3 weeks ago #16694 by richard Boslaugh
richard Boslaugh replied the topic: Battery Size For 912 iS?
I am using an Earth X 36C .
Has 36Ah and 405CCA
weight 3.9 lb
Lithium-ion
Meets Rotax requirement for an internal battery management system in accordance with Rotax service instruction.
Is the battery selected by Kitfox factory in response to the Rotax instruction for 912iS engines

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  • Paul Simmonds-Short
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2 months 3 weeks ago #16721 by Paul Simmonds-Short
Paul Simmonds-Short replied the topic: Battery Size For 912 iS?
Are you sure about those specs, Richard? I've been looking at the EarthX site for batteries specifications and the 36C says it's 12.4Ah and 320CCA. I'm looking towards the ETX 900, which is 16.0Ah/400CCA and weighs 2.2Kgs.

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2 months 3 weeks ago - 2 months 3 weeks ago #16723 by geoffrey klestadt
geoffrey klestadt replied the topic: Battery Size For 912 iS?
i went for the earthx 1200 - 24.5 Ah. that in theory gives me two hours of flight after total alternator failures. I hope I never have to find out if this is enough.
Last Edit: 2 months 3 weeks ago by geoffrey klestadt.

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2 months 3 weeks ago #16726 by richard Boslaugh
richard Boslaugh replied the topic: Battery Size For 912 iS?
The Ah figure is "lead acid equivalent".

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2 months 3 weeks ago - 2 months 3 weeks ago #16735 by Paul Simmonds-Short
Paul Simmonds-Short replied the topic: Battery Size For 912 iS?
Hmmm, I hadn't thought about the lead acid equivalent. But if your 36C is actually chucking out 12.4Ah even though it's the equivalent of 36Ah with a lead acid battery then the engine is only going to get 12.4Ah instead of the 16 recommended by Rotax.

I will be extremely interested to hear if the 16Ah requirement is 'lead acid equivalent' and that it is okay to 'step down' a Lithium battery to the same (or better) equivalent instead of what it's listed as providing.

I THINK I'll still need the ETX900 (16.0Ah) but I will be extremely happy to be told I'm wrong. I believe I'm basically getting the 16Ah at 1/3 of the weight of a conventional battery but I'll still need the 900 to satisfy Rotax.
Last Edit: 2 months 3 weeks ago by Paul Simmonds-Short.

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2 months 3 weeks ago - 2 months 3 weeks ago #16744 by Bill Hertzel
Bill Hertzel replied the topic: Battery Size For 912 iS?

Is there any data on the likelihood of a double alternator failure?

Probably much less likely than a total engine failure.

Can the engine be started in the "backup battery" emergency mode?

EVERY Engine start is in a "Backup" Battery Start.
"Start Power" and "Backup Power" are identical except "Start" passes through a relay that removes the Start Power from the ECM Bus once the generators come on line.
Backup Power attaches the battery directly to the ECM Bus.

~ ~ ~

The "Lead Acid Equivalent" only applies to high Current/ Starting applications.

A 16Ah Lead Battery will deliver 16Ah if discharged at 0.8A over a 20 hour period.
If the same battery has a 400CCA rating, that means 400 amps for 30 seconds before it drops to 7.2volts.
400A x 30 Seconds = 3.3Ah. About 20% of the 20-hour rating.

A lead acid battery with a 400CCA rating would typically be in the 30-35Ah range.
So the 12Ah Lithium battery is the 36Ah "Lead Acid Equivalent" in the 30-second 400A contest.
This is all fine for starting, which is a battery's typical intended use.

But this is not what our concern is.
If an auto battery becomes depleted, you just pull to the side of the road.
In an aircraft, it gets a little more exciting.

Unless you can stay in the air for another 20 hours after the generator dies, The 20 hour Ah rating means little.
What you want is more like the 1-hour rating which is never specified.
Assume it to be about half of the 20 Hour rating just to be safe.

A 16Ah rated battery (8Ah) is going to last about 40 minutes on a 12 amp draw.
A 12Ah rated Battery (6Ah) is going to last 30 minutes.
If you need a 30-minute fuel reserve, you should have at least 30 minutes of battery to go with it.
Are you ever more than 30 minutes from the nearest airport?

The Ah rating is all that matters when you are trying to get on the ground without undue delay.
You are not going to be worrying about over discharging either battery.
You will discharge it until it the engine quits or you land, whichever comes first, no matter what.

Buy all the Amp Hours you can get, and only expect to use half of them.
Last Edit: 2 months 3 weeks ago by Bill Hertzel.
The following user(s) said Thank You: RotaxOwner Admin, Paul Simmonds-Short

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  • richard Boslaugh
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2 months 3 weeks ago - 2 months 3 weeks ago #16757 by richard Boslaugh
richard Boslaugh replied the topic: Battery Size For 912 iS?
Bill, Thanks for the very informative discussion about Lithium ion batteries. I can only hope that every owner of an 912iS engine reads the thread. You have cleared up some issues I had about the ratings of Lithium battery specifications. In my case I had a areovoltz 16 cell before the Rotax battery instruction. I then changed to the Earth-x 36C. However I had this nice 16 cell so I mounted it behind the pilot seat on my Kitfox and wired it in as a standby emergency battery. It does not have the wire size to be a starting battery but has wire size to support engine operations. I connect the 16 cell to the main buss with an appropriate circuit breaker type switch which can put the standby battery in parallel with, or in the event the main battery is depleted , the mains switch can be turned off to isolate the depleted battery and the ECU source will continue with the emergency battery. I realize this arrangement complicates the issue however after 40 years of working on complicated avionics systems I am quite comfortable with the procedures involved. My EFIS has provisions to monitor two 12v sources so I see the main battery status as well as the standby battery status.The standby 16 cell is idle throughout the flight unless called for. Thanks again for the great thread.Richard
Last Edit: 2 months 3 weeks ago by richard Boslaugh.

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2 months 3 weeks ago #16760 by richard Boslaugh
richard Boslaugh replied the topic: Battery Size For 912 iS?
Almost forgot!
Thanks to Geoffery for starting the thread

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2 months 2 weeks ago #16782 by Glenn Martin
Glenn Martin replied the topic: Battery Size For 912 iS?
Yes good thread...
My Aircraft came with an 11Ah lead acid !! It has been "fine", however I am trying to buy and EarthX EXT900 (16AH) for months. The issue is finding a way to freight it here...everyone just sees the word "lithium" and panics.

I think the LED light to show a battery issue is a great idea too.

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2 months 2 weeks ago #16783 by Bill Hertzel
Bill Hertzel replied the topic: Battery Size For 912 iS?
Glen,
The EarthX Dealer Search says you have 3 dealers nearby.

Downunder Stol Viking Aircraft
64 Williams St.
Christchurch, New Zealand 7630

Precision Motorbikes Limited
807 Jones Rd.
Rolleston, New Zealand 7675

Motomox Ltd
25C Peraki St.
Kaiapoi, South Island 7630

They will likely have to order it.
While not permitted as Cargo on Passenger Aircraft, They can be transported on Cargo Only Aircraft in a 70% discharged state.
EarthX ships batteries every day.
They will know the requirements to get you your battery,

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2 months 2 weeks ago #16785 by Glenn Martin
Glenn Martin replied the topic: Battery Size For 912 iS?
Hi Bill,
Thanks

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2 months 2 weeks ago #16821 by Ross rynehart
Ross rynehart replied the topic: Battery Size For 912 iS?
Glenn,

I ordered mine direct from Earth X and they shipped to me in OZ no problems.

r
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