2nd battery vs. 2nd alternator

I don't want to start a primer war type of discussion, but I'm about to receive my dual G3X system from Stark for my RV-6 slider O-360-A1A C/S, and I would value any input from members of the forum as to their suggested configuration. I fly about 100 hours per year, and I file IFR whenever the situation requires. I've already made up my mind to remove the current vacuum system and vacuum instruments. I will use a different vendor to supply an alternate electronic ADI source, such as a Dynon product , Tru Trak Gemini, or MGL Xtreme.

I'm currently debating the merits of a second alternator, like a B&C SD-8, versus a backup battery such as the one (IBBS-12v-4ah) offered by TCW Technologies, or both.
Here's the equipment that I either have or will have - one magneto, one Jeff Rose electronic ignition, dual G3x with engine sensors, tru trak GX autopilot, Garmin 240 audio panel, Garmin 430W, Garmin SL40, King KT76A xpnr and ACK blind encoder.

I obviously want some redundancy is case of a main alternator failure (plane power 60A). I had a previous alternator failure (Van's alternator) on a cross country from Florida to Texas and I limped home VFR the next day on a freshly charged battery, a Garmin 496, and a King handheld, but with a good vacuum system.

So let's hear some suggestons. Thanks to all.

I would think that a spare alternator is more useful. Combine that with replacing the battery before failure and you should be golden.

I have one each alternator and battery.

2 batteries vs. 2 alternators

My experience has been that failed batteries have shown up before I even got the engine started where as failed alternators have happened in flight, both times! Therefore, I vote for two alternators and one battery. That is how I am designing my new panel and electrical system.

Both - sort of

There are proponents of both approaches, but in my opinion, the way to make a decision for your system is to compare the two options in terms of the number of reversionary modes you can achieve.

Like you, I opted for a two-vendor panel to reduce systemic failure modes. In choosing what type of redundancy to build into the electrical bus, I opted for an SD-8 instead of a backup battery for the following reasons:

1) The SD-8 can be configured for self-excitation, so it can provide power without a charge in the battery.
2) The small Dynon displays (and others) can be equipped with internal backup batteries anyway.

In my system, these capabilities provide more reversionary modes than a second main battery could provide, so I'm happy with it.

The more research you do, the more opinions you'll uncover. Good luck!

Both alternatives are well described in Bob Nuckoll's

book and the diagrams are here: http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/
Z-12 for two alternators/two batteries and z13/8 for two alternators/one battery. I am going with the Z13/8 for a few reasons: cost and simplicity. Also, both my Skyview's will have their own batteries for backup (along with the D6 backup EFIS with its own battery) The sd-8 can really put out about 10A (per Nuckolls) and should get anyone back on the ground if the main alternator goes south. The other thing about the SD-8 is that even if the remaining equipment pulls over the 10A, all that happens is the battery makes up the needed amps, with a decline in voltage and no harm to the sd-8. Again, this works for my type of mission. The 20A backup seems to me to be more of an additional alternator sharing the loads all the time with cross-over connectors needed if the main goes out.
Just my $.02
Bob

I can start my engine with my back up battery a second alternator will not do that.

Z13/8

N184JG said:

I can start my engine with my back up battery a second alternator will not do that.

Click to expand...

I'm doing the Z13/8 scenario to allow comfortable completion of flight for the duration of on board fuel. The lack of second battery prevents me from starting a flight when something is not right.

Two batteries is no different than one larger battery. Whatever causes one to discharge will likely cause the second one to follow. The only exception I can think of is a failure due to age which shouldn't happen anyway cause were going to replace it regularly. Z13/8 for me.

Bevan
RV7A wiring.

I've built two Rockets, one with dual alternators and one with dual batteries. FWIW, my opinion is that the dual battery system is less complicated to wire, fewer connection points and equipment, less expensive to buy, install, and maintain, fewer switches, less wire, and generally just a better way to go AS LONG AS YOU STAY UP ON YOUR BATTERY MAINTENANCE. For me that meant replacing the batteries every two years as part of the condition inspection.

I did buy a dual battery controller from Blue Sea to manage the charging of the two batteries from one alternator but that part was less expensive than buying another alternator controller from B&C.

Both configurations worked exactly as they were intended and both followed the Aeroelectric philosophy. Feel free to PM me if you have any specific questions.

BTW, this is just one person's opinion. Either works and both have pros and cons. It was my experience that the cons of the dual battery system were fewer than that of the dual alternator system. Do what makes you happy no matter what the opinions are expressed here on this forum.

My second battery is not on unless needed it is turned on thru a second master type relay and will power my panel for 4 to 5 hours.

IMO a BUG (back up generator) is the way to go, I run a the B&C 40 amp with SD-8 back up.

It depends on your mission and systems in your aircraft, so each plane will be different.

There are also many ways to wire and install a second battery. It could be a small battery to provide aux power to an EFIS/AHRS, or it could be a larger battery to provide an alternative power source to a Subaru, rotary, Chevy, etc. engine that needs electric power to run. The aux battery can be charged with a diode, or you can tie it in with an aux battery contactor. So there is no right answer other than ?it depends.?

In your case, it sounds like the mission is IFR and cross-country flying, and you want the ability to fly home if your primary alternator fails. IMO, if you?re flying IFR you need a second attitude source with its own backup power source.

Based on those assumptions, my recommendation is to install a backup alternator (SD-8 or -20) and install a backup EFIS with its own internal backup battery. If you have a primary alternator failure, you can load shed and fly across the country on your backup. If you have an alternator failure while IMC, you?ll have plenty of juice in the ship?s battery to land safely (and load shed anything non-essential). If you have a main electrical bus failure your backup EFIS has its own battery to run independently.

IMO that is the simplest architecture to match your mission.

Self Excite SD-8

foka4 said:

1) The SD-8 can be configured for self-excitation, so it can provide power without a charge in the battery.

Click to expand...

Has anyone actually made the SD-8 self excite? I have heard of this theory and read many posts, but I don't know anyone that got it to work reliably. Not at least without a small (i.e. 9V, etc.) battery to trip the alternator field.

I only ask because it would have been really helpful when I left my master on while at a remote field with no maintenance services.

Greg

I went with 1 B&C alternator, 2 batteries, 2 master switches. Start on both batteries, taxi, runup on both, turn off 1 battery just before take-off. Next flight, fly/charge the other battery. The only time I will use the 2nd battery in flight is if the first failed for some reason. The first will be turned off, before the 2nd is turned on and the 2nd will be in the battery only (no alternator) position.

Just one way to do it.

Jeremy Constant

gvouga said:

Has anyone actually made the SD-8 self excite? I have heard of this theory and read many posts, but I don't know anyone that got it to work reliably. Not at least without a small (i.e. 9V, etc.) battery to trip the alternator field.

I only ask because it would have been really helpful when I left my master on while at a remote field with no maintenance services.

Greg

Click to expand...

I checked with B&C and you need to follow Bob Nuckolls revised circuit for the SD8 to "self excite." This diagram can be found here: http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z25L.pdf

The SD-8 is a permanent magnet alternator that does not need excitation. It is constantly generating voltage as long as the engine is turning. The issue is the relay that connects it to the rest of the electrical system - this used to need to be energized from another source of current. The modified circuit given by Bob Nuckolls derives this current from either the airplane power or from the voltage generated by the SD-8, so it can be turned "on" even if the battery is dead.

The SD-8 is a useful and almost foolproof alternate energy source. I have one in my RV-7A, and it is still working great after over 600 hours.

Mark Olson N407V RV-7A N16XV F1-EVO Rocket

thanks for clarifying that.

I mistook the meaning of the terminology "self excite" off his drawing. At any rate, that is how I am going to wire it.
Bob

If you go with a second battery then swap one of them out every year and alternate so that you always have one that is less than 1 year old.

Yes, two batteries is easier to wire, but I'm not sure it's more fool proof in flight because you need a way to isolate them, then you need to sort out which one to charge and fly from.

I think overall the SD-8 is the best solution. In my wiring diagram I put it through it's own relay on the back side of the master contactor along with a separate relay for the emergency buss. This means that in the event of a master contactor failure, I can flip a switch and my SD-8 will power my emergency buss.

Also, I'm pretty sure the SD-8 is lighter which was very important to me.

schu

Has anyone experience with the GAMI Supplenator?
John

akschu said:

Yes, two batteries is easier to wire, but I'm not sure it's more fool proof in flight because you need a way to isolate them, then you need to sort out which one to charge and fly from.
schu

Click to expand...

That's what the BatteryLink ACR does. You don't need to do anything, it's all automatic. It is easier to wire, and simplier to operate than two alternators, but it is heavier no question.

In the end, do what makes you happy.