P-mag 9v Start

E-mag suggests a 9v battery will sufficiently power a P-mag for starting. Has anyone successfully tried this?

A friend's PC-680 internally failed when he attempted a start. Buss voltage was 12.6, but dropped to nothing with any load over 1/2 amp. Fortunately, he was at his hangar. So the question arose about how one would one get out of this predicament in remote location. MacGyvering a 9V patch would be easy enough (if you had such a battery). Biggest hurdle I foresee is priming a cold engine (the aux pump isn't juiced) or the mad propping for a hot start.

John Siebold

I did this one time.
It worked

Contact me off list and I'll send you a wiring diagram you can use to install a 9 volt battery holder for hand propping your P-mag equipped airplane.

The trick is wiring it so the 9 volt battery doesn't power everything on your buss and run down before you can spin the prop.

BTW, this may not work with a fuel injected engine due to the inability to run the electric boost pump.

Right you are, Bill.

My friend and I realize you must avoid backfeeding anything from the 9v battery. Keeping the panel P-mag power switch off prevents backfeeding the battery buss. We've two approaches so far. Permanently wire a 9v clip on a pigtail terminated at the P-mag +12 and gnd and accessible through the oil door. Snap in the 9v battery, unground and flip the prop.

Another idea was a +12v AA pack backfeeding the battery buss through the panel power point. The main battery negative lead would have to be disconnected, access through the oil door. All switches would remain off, clock and keep-alives being the additional drains. That's a clunky pack to carry around just in case, but it does have the advantage of powering the boost pump.

My make-it-as-difficult-as-possible scheme for lack of priming from the pump is unscrew the top plugs and pour in fuel from tank sumps.

None of this is ever suppose to happen out in the sticks, right?

John Siebold

I recommend putting a 9 volt battery box on the panel, in easy reach and keeping it empty until needed. Otherwise, the battery might corrode the box and render it useless when you need it most. Also, this keeps you from accidently turning the prop and having it fire.

The deal is you get the thing ready to fire and then add the battery, not before.

Here is one of the many 9 volt battery boxes I have found:



John, send me an email to bill (at) Repucci (dot) com and I'll forward the schematic we came up with.

The question I have with this version is it is possible that the diode might reduce the electrical current enough that you can't fire the P-mag. However, you don't have to flip any switches.
https://photos.google.com/u/1/share...?key=Y0pCeERtby0teUVOTjFiQlAyMmQwY2h4eHBjZXJn

This version requires a switch and I would probably leave off the diode for the reason stated above.
https://photos.google.com/u/1/share...?key=Y0pCeERtby0teUVOTjFiQlAyMmQwY2h4eHBjZXJn