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Amperage Calculation Spreadsheet- 912 Tanarg / Enigma

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By XC Triker
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Amperage Calculation Spreadsheet- 912 Tanarg / Enigma

So, I have a "6 watt" in dash radio.  I asked the dealer what wattage it uses = SIXTEEN !!!  Transponder is similar.  So is the Enigma (EFIS), and strobes.  My new landing light is listed as 21 watts.

Using the formula    Watts = Amp x Voltage  you can calculate amperage for each component.

The GPS I guessed conservatively at as 0.5 amps and reverse calculated 6.75 watts.  I googled iPhone 5 charger and found 4 watts or 0.3 amps (so a conservative 0.5 watts seems good).

Now, all added together = 7.54 Amps !!

The 912 is rated at 18amp / 250 Watt at Max RPM per the factory.  However, Lockwood gives a more conservative real world estimate of 14 amps continuous "at best!"

Ok, so 14 amps available minus 7.54 required to run the equipment, leaves 6.46 left over "at best" (of course the starter battery needs some love too).

No problem then!!  Oh, but what about at idle.  With everything running, at idle I will have only 1.46 amps left over.  So, basically in my mind, my system is pretty well loaded up.  I could add more, but would have to turn other things off.

I'm not a big fan of electrically heated items-- extra wires, expense, hassle, tangles, ?escape from accident/water?--  but to tell you the truth, there is not really that much juice left over to run all those things.  I also listened to a podcast by Alaska bush pilots on survival-  they recommended to dress for the crash-  so, since you won't have electricity to keep you warm if you go down in the cold, I figure it's best to just dress warm in layers.  But, then again, I don't fly in super duper cold stuff like some of you do, and I don't have an extra alternator, so ...

I did list the estimated draw of heated items, in case you are interested.

Comments

  • CraigV

    A few years back we used to have seemingly endless discussions about the electrical capacities of our 912's and the stated current draw of each item and how it was sucking up all of our capacity. Paul, a fellow trike pilot and a very practical electrical engineer, I think got tired of this hangar talk one day and said, "Let's see what the actual draw is on these various items." So we stuck a meter on each one and measured the current draw. We were all shocked at how low the actual draw is compared to the theoretical draw. 

    Perhaps on your radio for example, it MIGHT approach it's stated current draw while transmitting but what percentage of your flight time are you actually transmitting? Most things are like this. My landing light for example is a very efficient LED and I always use it on wig-wag, which uses a small fraction of it's low stated "on" current draw. 

    Gerbing heated clothing uses a pulsating thermostat (much like my super efficient landing light) and in normal usage a jacket will use a just a fraction of the current that it's rated at. My P&M trike has an idiot light on the dash that displays current charging state by a green/yellow/red light and I always have a green light under normal flight conditions except at low idle on the ground or the first few minutes of flight as a yellow (caution) light shows while the battery is recharged from starting.

    This system has worked well for me for a number of years now but if you read where I went down in flames someplace... perhaps it might be time to rethink my theory.

  • XC Triker

    Thanks @CraigV, good to know.  Good balance to the discussion.  I'm a little more concerned about the draw at idle as I'm probably going to install an LiFe battery (Ken & Tom both beat me to it and seem to love it, they say it really cranks their engine over fast (14lb weight savings)), but the LiFe's don't have a lot of spare juice (they're like a sprinter- fast/strong but not a lot of endurance). (Still, Ken tells me, he can crank his engine over multiple times without sign of slowing down).  Anyway, I digress, since I'm at a towered airport, I'm going to be running all those things at near idle and transmitting a lot-  I don't want to drain that expensive battery.  My LED landing light does not have wig-wag function :(   That's another great advantage of that function, besides extra visibility (I wonder if we can put in a circuit in the power line to cycle the power??)  Also another good reason to leave the transponder on Stby until take off. 

    This is like the 3 bears:  There's too little,  too much, and somewhere in there is a just right level  ;)

  • CraigV

    I've had the lithium-ion battery now for two winters and bought the very large capacity 36 AH. The standard lead acid battery for the Rotax 912S is I believe 14 AH, so there's never an issue of running low on juice. It's a great battery and I love it's power and deep reserves...however:

       . The cost! (YIKES!!!)

       . It behaves differently in cold weather than a lead/acid battery. A lead/acid will run down quickly in cold           weather and will run down the more you use it. The LiFe needs to be woken up because you won't initially get full cranking power. So in winter I roll the trike outside, crank it a couple times (mags off) then go inside to finish whatever and close the hangar doors which takes a couple minutes. Then I go out and start it normally. 

    The 912 doesn't have the compression issues that the 912S has so this hard starting might not be a problem for those pilots and it should also work to put it on a special LiFe battery tender. Not to charge it with that tender however, since it doesn't loose charge like a lead/acid but rather to warm it and get the internal chemical reactions moving (hope I described it correctly).

  • XC Triker

    Hey @CraigV--  I think there's an important typo in your first sentence (you said Lithium-Ion (the dangerous ones that overheat and catch fire) when I think you meant Lithium-Iron-  the probably not dangerous ones).

    In cold weather, I heard that you can also just turn on some electrical items (like the EFIS) and that will warm the battery up.  Also, my impression was that it was probably a good thing to put it on the special LiFe battery tender to balance the cells in the battery once or twice per year-- as you said, not to charge it as it maintains shelf charge very well (but will NOT support a small vampire drain like a clock or internal memory.  The Enigma  EFIS has it's own watch battery for memory (needs replacing every 2 years)).

    I had a Hawker SBS J16 Lead Acid Dry-Cell Battery (I believe it was the right battery)-  It's listed as only 16amp hours ~$230 (more specs HERE).  Ken sent me a photo of his new LiFe battery it has 21 AH, 315 CCA

    The price difference is now down to ~$80 for the LiFe battery versus Lead.  Plus the special balancing charger $50 (but since it's only needed a couple times per year maybe Ken (@knussear) and I can just share one?).

    In depth discussion on LiFe batteries HERE

     

  • CraigV

    What have others done about starting in cold weather?  Initially I just turned on a couple things to get the battery warmed up and that seemed to be better than nothing. Next I went to cranking it and waiting as I said before and now I'm turning things on AND cranking then waiting before starting. Luckily I can do this with the large 36 AH battery and I think this works best of all but it doesn't get real cold, consistently in Lodi so it's hard to say. How do guys in colder climates handle this?

  • XC Triker

    @CraigV That's what I was thinking, it doesn't get that cold in Lodi !  Was it elsewhere that you had the issue?  How cold are we talking about?  It's funny, you ask about what others are doing-- I didn't know anyone was using the LiFe batteries when I first posted my page on using them-- everyone acted like I was NUTS to think about it.  Now Tom & Ken beat me to using them and I haven't got around to ordering one yet, I also think there's a bit of life left in my lead battery ...   So, you're only the third person I know who uses one- and you've been using it longer than anyone I know.  Who else does?

    Contrary to what I researched, that LiFe batteries are sprinters, Ken was talking about cranking his new LiFE battery over and over during a 30 minute period and it continued to have tons of power.  Everyone tells me they really turn the motor over well.   What does it do when it's cold, and you haven't "warmed" it with a few test cranks?

    I also had been hand propping my engine to move the oil up into the engine before starting it, and to do that one better, I'd then give the engine a crank over with the battery but mags off, before finally starting it.  I thought that when I got the LiFe battery I'd have to stop that- as what I read said it may not have enough juice to start repeatedly, but I guess I was wrong ...