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Battery discharges with master off


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O.K. folks here's a poser for you.  My '69 M20C has decided to start discharging the battery even with the master off.  I've put a meter between the battery and the contactor and can discern no drain (not even a few ma) with the switch off.  The battery (Gill) is only 6 months old but it has done this twice over the past month.  Everything else works just fine.  With a freshly charged battery it starts right up, the alternator keeps the battery charged but let the plane sit for 10 days in the New England cold and battery voltage drops to about 9.8 volts and that's not enough to turn over the starter.  So, the possibilities are: 1.  a bad battery despite it being relatively new  2.  some parasitic load on the system but with a low enough drain to not register on my meter or, 3.  evil spirits?  Anyone have some thoughts?

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My initial thoughts would be a toss-up between #1 and #2, but would put my money on #1 if I were betting.  I'd suggest replacing the battery with a Concorde and see what happens.  Although many on here have had successes with Gill, the Concordes are superior and worth the small additional cost.

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Early on is when it may fail.  After you've flown and you know it's charged, disconnect it as Robert suggested for a few days.  Then see if it has lost much if any of the charge.

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Disconnect the battery, bring it home and check it over the course of a few days (if your not flying).  If your fancy you can hook it up to a voltage data logger and graph the voltage over a few days.  A few arduino options out there.

I've had many car batteries with internal shorts that drained the battery slowly or instantly.  The slow drains are frustrating because people would look at the rest of the car/plane for a fault.

Good luck.

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What others suggested to T shoot.
 

I had similar experience with my last few Gills.  Each new one lasted less and less.  The last one made it about a year.  5 years ago I switched to Concorde and just changed out that battery only because it was marginal when loaded......but it still started the engine fine.  The Concorde cost more, but lasted 5 times my last Gill.  Had heard perhaps Gill turned things around, so will be curious to see what you find.  Please let us know.

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The battery in my wife's car crapped out in just over a year. Free warranty replacement is a great thing!

I'd bet your battery is bad. Being a Gill, I'd probably bet on it.

Good luck and let us know what you find. Many auto parts stores will test it free if you take it to them (just don't mention where it came from, tell them you have some old vehicles).

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PP thoughts follow...

0) How old is your Gill battery? (Check the mfg code)

1) Remove Gill battery for testing...

2) Because the M20C doesn’t have anything that draws that much energy to be that noticeable...

3) While the battery is out.... order a spare battery from Concorde...

4) Install the Concorde to test long term...

5) Check the cigarette lighter socket... to see if it is always live... Somebody at Mooney thought this would be a good idea once...  leaving portable ADSB-in devices plugged in, can drain the battery in a few days...

6) If the Concorde doesn’t drain itself...  you have solved what is going wrong, and what to do about it in one swell foop...

7) older Gill batteries make good ballast... paper weights... and door stops...

8) It’s possible that Gill has changed their manufacturing techniques... it just isn’t showing up yet...

PP thoughts only, not a mechanic...

Best regards,

-a-

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Cloud, before you remove and do all that battery stuff above stuff. Have you flown with little fingers or some one not used to your aircraft? Check your over head cockpit light above your head. That light is wired directly to your battery with bulbs that draw juice.

 

I know a guy that had that issue. Since then I removed my overhead bulbs to prevent some inadvertent switch to on that would render the battery dead when we really need it.

Fly Smart

Ryan

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On 1/26/2021 at 12:48 PM, Cloudmirth said:

O.K. folks here's a poser for you.  My '69 M20C has decided to start discharging the battery even with the master off.  I've put a meter between the battery and the contactor and can discern no drain (not even a few ma) with the switch off.  The battery (Gill) is only 6 months old but it has done this twice over the past month.  Everything else works just fine.  With a freshly charged battery it starts right up, the alternator keeps the battery charged but let the plane sit for 10 days in the New England cold and battery voltage drops to about 9.8 volts and that's not enough to turn over the starter.  So, the possibilities are: 1.  a bad battery despite it being relatively new  2.  some parasitic load on the system but with a low enough drain to not register on my meter or, 3.  evil spirits?  Anyone have some thoughts?

#3 You missed a payment to your exorcist - you've been repossessed.

but seriously, #1, Gills don't have a long life reputation.

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There is a straightforward test for this....

  1. Turn the master off.
  2. Disconnect the negatived terminal. 
  3. Put a test light in series between the negative terminal and the lead. 
  4. If it lights up in the slightest...you have current flow on the lead and that’s why the battery is going dead
  5. Start popping breakers and such until the test light goes all the way out. When the light goes out, the last thing you touched is related to why your battery is dying. 

A more advanced method uses a multimeter to measure amps, but I’ve had good luck with a simple test light. It can’t light up if the the power or ground circuit is open. 

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There is a straightforward test for this....
  1. Turn the master off.
  2. Disconnect the negatived terminal. 
  3. Put a test light in series between the negative terminal and the lead. 
  4. If it lights up in the slightest...you have current flow on the lead and that’s why the battery is going dead
  5. Start popping breakers and such until the test light goes all the way out. When the light goes out, the last thing you touched is related to why your battery is dying. 
A more advanced method uses a multimeter to measure amps, but I’ve had good luck with a simple test light. It can’t light up if the the power or ground circuit is open. 

I would use a multimeter in ammeter mode, it can read milliamperes which would not be enough to light the bulb.
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3 hours ago, Cruiser said:

buy a Concorde. 

You know, I always thought it was spelled Concord.  I was going to make fun of you for telling him to buy a supersonic transport :D

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On 2/7/2021 at 5:56 AM, ArtVandelay said:


I would use a multimeter in ammeter mode, it can read milliamperes which would not be enough to light the bulb.

That's probably a good idea as our planes sit longer than cars between use and the technique I described I used on cars and I was usually looking for significant current draw. 

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Hot circuits in many Mooneys...

1) Keep alive circuits... things with memory in them like clocks, radios, timers... are kept alive using the battery all the time...

2) Some Mooney lighter sockets are always hot...

3) Some Mooneys have interior lights that are alway hot... with switches put on dumb places...   kids are supposed to read the POH before using the light over their head...  :)

PP thoughts only, not a mechanic...

Best regards,

-a-

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As far as I know, the only "always hot" circuits are for the clock and the interior lights. Avoiding the problem of drain from the interior lights is easy. The only fix for the clock "draw" is to disconnect the clock.

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13 hours ago, DonMuncy said:

As far as I know, the only "always hot" circuits are for the clock and the interior lights. Avoiding the problem of drain from the interior lights is easy. The only fix for the clock "draw" is to disconnect the clock.

And not all clocks are created equal!   My clock wasn't working and the avionics shop just grabbed a different used one and put it in.   It would kill my ship's battery in a few days.  My original clock was an Astrotech LC-2, so the hangar fairies refurbished it and it got put back in.   Runs a few years on a AA battery.   Much better.  ;)

 

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