Electrical system for review

[mburch]

I figured out some more ways to incorporate feedback and "increase simplicity"...

Now there is no aux battery relay, just a switch. I also got rid of the "you forgot to turn the ignitions on" warning function, since it was just too smart alecky. I did keep the "you forgot to turn off the ignitions after shutdown" light, since it serves a useful purpose - keeping me from being stranded away from home with a dead battery if I have a brain cramp and leave the ignition on.

Latest revision is here for those who're interested.

Say Paul, did you happen to remember what that resistor value I PM'd you about was...?

thanks,
mcb

[Bill Dicus]

Your system looks excellent. The one in my -8 is planned to be very similar. However I don't have the B&C or Plane Power alternator, just the Van's, and I'm not sure how to get over voltage protection for that alternator. I have the B&C regulator for the SD20. Anybody know a good way to achieve the OV protection for the Van's? Would appreciate any advice/ Bill

[Radomir]

Bill, B&C has "OVM-14" (overvoltage protection module)... check with them if it'd work for you.

http://www.bandc.biz/cgi-bin/ez-cata....cgi?26X358218 (and scroll down)

[Ironflight]

Originally Posted by mburch

Say Paul, did you happen to remember what that resistor value I PM'd you about was...?

Dang Matt - those notes are out at the hangar, and I was just there but forgot to check! Let's see, reconstructing my thought process, I wanted to limit th current well below the wire capacity - and rally, it is just trickle charging while the plane sits - maybe 0.1 amp? You can figure your voltage drop across whatever diode you're using and compute the resistor from there - I am guessing I'm using a 100 Ohm or thereabouts.

Gee, now you've made me do math in public - I imagine the EE's are going to all jump on me now!

[mburch]

Originally Posted by Ironflight

Dang Matt - those notes are out at the hangar, and I was just there but forgot to check! Let's see, reconstructing my thought process, I wanted to limit th current well below the wire capacity - and rally, it is just trickle charging while the plane sits - maybe 0.1 amp? You can figure your voltage drop across whatever diode you're using and compute the resistor from there - I am guessing I'm using a 100 Ohm or thereabouts.

Gee, now you've made me do math in public - I imagine the EE's are going to all jump on me now!

Well, since I are one (though I mostly do software these days) I'll belabor the point a little longer if I may... By "charging while the plane sits", do you mean that the intent is for the aux battery to trickle charge from the main battery when the plane is sitting on the ground, powered down, with energy transferred from one battery to another? I didn't think that would actually work - I did a (very) brief stint designing battery chargers in a previous life, and we always used something north of 13V as the minimum voltage a lead-acid battery would have to see before it'd accept a charge. For what it's worth, Nuckolls has this to say on the topic:

In order to charge a battery, we're obligated to raise it's terminal voltage quite a way ABOVE the open circuit terminal voltage. Batteries DELIVER energy at 12.5 volts and below and just begin to accept significant charge at 13.5 volts and above . . this is why the alternator's output voltage is adjusted to 13.8-14.2 volts. . . one may measure tiny "leakage" currents between the two batteries hypothesized above but no significant amount of energy will transfer between them.

I could fully believe that these little VRLA batteries are different though. And obviously you are flying and happy with your setup, so you must be doing somethng right!

Of course if you meant "the aux battery trickle charges from the main bus while the alternator is running"... then I fully agree that ought to work great.

Thanks for the resistor value - I think I'll probably take a sacrificial VRLA battery from Walmart and do some experiments on it with my 13.8V bench supply to see how it really behaves when it's deeply discharged, using 100 ohms as a starting limiting resistor value. I'll post the results here if I come up with anything, in case anyone is curious!

Thanks Paul (et. al) for the willingness to help.

cheers,
mcb

[Ironflight]

You're basically correct Matt - I doubt that it really charges much while sitting there, unless it is really, really low. In that case, the voltage differential might allow current to flow, but heck, I'm one of those Aero E's, so what do I know about electrons? Someone once told me to think of them like are molecules, flowing from high to low pressure areas.....you can see I have a simple mind!

Bottom line is that it probably only really charges in flight, like you said - but the only time I have had the Aux battery go flat is when I left it on alone too long while doing ground maintenance on the Aux alone....

Paul

[mburch]

Originally Posted by Ironflight

Bottom line is that it probably only really charges in flight, like you said - but the only time I have had the Aux battery go flat is when I left it on alone too long while doing ground maintenance on the Aux alone....

Excellent, that's what I want to hear.

By the way, just to add another data point to the conversation, I found a site showing that Cessna uses a 24V 6.2Ah aux battery for their G1000-equipped aircraft. In 12-volt terms, that would be like a 12.4Ah job... almost as big as the little 17Ah PC680 we're all driving around with as our main battery! I guess it takes a lot of juice to keep those big-screen teevees turned on.

mcb

[Ted Johns]

Matt,

If you want 100ma charge current, it seems to me a good starting point would be 16ohms. I see the math as: (14.2v - 12.6v) / .1a = 16ohms.
This only applies to alternators, battery chargers are often unregulated and a fair bit higher than 14.2v.

[mburch]

Originally Posted by Ted Johns

If you want 100ma charge current, it seems to me a good starting point would be 16ohms. I see the math as: (14.2v - 12.6v) / .1a = 16ohms.

Thanks Ted. With the diode in there, you lose another ~0.7V of the charging voltage, so I think the resistor value would be more like: (14.2V - 0.7V - 12.6V) / 0.1A = 9 ohms. I'll use that as a starting point and see what the actual measurements look like.

Getting close to being able to start ordering components!

cheers,
mcb

[RV10Pilot]

Matt,
I was planning on putting in a very similar electrical architecture into my RV-10. I started with Z-13/20 and then added an aux battery to allow me to start up the avionics in advance of starting the engine and not have to worry about the voltage sag during start.

This thread has got me thinking about the complexity of the system and all the different flow paths between batteries, buses and alternators and now I am considering switching to the Z-14 Dual Battery, Dual Alternator, Split Bus design. The only modification would be to put the cross feed between the two buses since the small aux battery would not be used to start the engine. The efis, mfd and ahrs could be fed from both buses with individual diode protection so a single bus failure would still be able to provide power to critical systems.

Larry Rosen
RV-10
#356