What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Adding second Surefly

togaflyer

Well Known Member
I’m at the 500 hour mark on my Bendix mag but also considering adding a second Surefly mag. I’ve been running one for the past year and pleased with it. If I add a second Surefly, I need a continuous power source. Currently I have a
B & C primary Alt, a PP 30 amp backup, and a PC 950 battery with an hour of reserve. I’m not looking for an electrical system redo like doing a duel E system, but anything else to consider if I decide go dual E mags.
 
So if your only battery or battery cable develops a short to ground, will you have a working ignition system?
 
Small battery (3~4 ah) fed with a diode, with a voltage monitor and check circuit.
.
 

Attachments

  • Ignition Power Supply.jpg
    Ignition Power Supply.jpg
    54.8 KB · Views: 200
So if my battery cable failed. There are two 14 gage wires running from the battery in the back (RV10) to the engine compartment. Even if I kill the battery switch, I still have independent power to each mag from the PC 950, which has good reserve capacity. However, I could install a simple on-off-on switch to allow an emergency switch over to the bus bar, which is kept energized by either alternator. An IBBS could be added to power one mag in an emergency. Also thought about a mini battery like that earth x on a tray next to the main battery with an RV type dual battery isolator. But, between two alternators and a battery with excellent reserve amps though, I feel I’m already on good emergency power terms. In a failure situation, to loose all three power sources at once would be highly unlikely. If I was on only battery, meaning both Alternators have failed, I switch one mag off, run single mag and a direct route to the nearest safe landing. Not sure if one more battery really makes a difference. I have a battery backup G5 and Foreflight IPad panel display, so if I need to quickly shed all power by killing the battery switch an only running on one or both mags to the nearest airport, I still have safe power reserve.
 
I think the failure path that you’re not protected against is an inadvertent short. Maybe a main battery cable wears thru its insulation against the airframe, maybe a loose screw wedges between the airframe and main buss bar. In very short order your battery is destroyed. Any alternators feeding the buss (shorted to ground) will trip off. Without a battery they won’t re-start. That’s why most recommend a second completely independent battery/buss for one ignition.
 
Surefly x 2

Manny good suggestions by others.
An important thing would be that both Surefly have independent power supply.
If one of the two Surefly dies, no action is required, no switches to operate
to keep the engine running. I like Dan H thinking. Simple, low cost.
Just find a suitable airport for landing.

Good luck
 
Rich, the diagram I posted earlier includes a map switch circuit specific to the EDIS-based ignition I've been flying, as well as a cheap voltage monitor sourced overseas. It is unnecessarily complicated for your Surefly application.

Given you have two alternators, a power supply for the second Surefly could be simplified. Mostly you just want to be made aware if the IGN2 battery is disconnected from the alternators and primary battery. B&C sells a nice little voltage monitor...

https://bandc.com/product/over-under-voltage-sensor/

...which will flash a panel LED when the feed to Surefly #2 drops below 12.5V.

Note a few conceptual points. The little backup battery is truly just a storage vessel. During normal operation, all the electrons source from one of your alternators. Take the alternators off line, and both ignitions still access both batteries. You have the option of shutting down one ignition, doubling your ignition range. Worst case, you could throw both alternators and the main battery over the side, and still fly an hour or two on a 3 AH backup.

Note the fusible links in the ignition feeds. They are real, real reliable, and can be located right at the battery terminal so every inch of hot wire is protected. Less wire length, no breaker or fuse block. Components not installed never fail.

BTW, money, mouth, yada, yada....I have failed an alternator over the Smokies and continued to south Alabama without concern. Main is a PC925 (I need the mass in the back in a 390 powered -8), backup is a little 3.8AH.
.
 

Attachments

  • IGN2 Power with B&C BC207-1.jpg
    IGN2 Power with B&C BC207-1.jpg
    27.2 KB · Views: 148
First, thank you all for your information. Some excellent safety points were brought up. Dan, thanks for the schematic. It looks like something that works for me. So as I read it (with my IQ of 60) each mag has its own dedicated battery. The smaller, let say 6 ah battery, gets charged by the alternator, but it’s only draw is from the second Surefly (mag2). Mag1 is connected to the main battery (PC950). The B & C flags if the battery 2 has low voltage. The diode prevents back flow from the second battery. So for the second battery would I used a 6 gage wire, and what type of diode would I use.
 
Last edited:
So as I read it (with my IQ of 60) each mag has its own dedicated battery. The smaller, let say 6 ah battery, gets charged by the alternator, but it’s only draw is from the second Surefly (mag2).

Technically speaking, if one of the alternators is operating, the voltage at the diode is higher than the backup battery voltage, thus there is no draw from that battery. The little battery is merely a reservoir. It becomes the supply only when the alternator is off line.

Mag1 is connected to the main battery (PC950). The B & C flags if the battery 2 has low voltage. The diode prevents back flow from the second battery.

Correct.

So for the second battery would I used a 6 gage wire, and what type of diode would I use.

No need for a wire that stout. Normal operating current is equal to the ignition draw. I'd bet less than 2 amps, but Surefly specifies 14 ga wire and 10 amp protection, I suspect 14 ga is to ensure the installation is physically robust.

If you heavily discharge the backup battery, it will initially draw some significantly higher amperage in the early stages of recharging. That would be particularly true with a lithium phosphate iron battery. It would be a rare case, but given the possibility, sure, wire it all with 14.

B&C sells a classic diode configuration, but the net is loaded with suitable schottky diodes. Here's 25 of them for six bucks: https://www.amazon.com/10pcs-Schott...pcontext&ref_=fplfs&psc=1&smid=A13Q7CAPYJVEY5
 
I like this! Thanks Dan!

A batteries capacity to store energy erodes over time. Build into your regular maintenance routine a method to test its capacity or a regular replacement.
 
Back
Top