Amperage Draw

[zehutiman]

I’m wondering if anybody has any insight into a drastic change I recently noticed in my battery amperage indication on the GI 275 Engine Monitor.  The airplane just came out of the shop for some comm wiring issues and to recalibrate the CiES fuel senders.  Normally, I see 1 to 3 amps, but for this 40 minute flight I was seeing 16-24 amps.  Just to be clear, the 24a is what’s being pulled from the battery, correct?  The battery voltage during this last flight was 13.4v.

 

[skykrawler]

If the shop ran the battery down a bit the system will have to charge.  What was the bus voltage before starting the engine. The Garmin 275 pilots guide is not informative as to the meaning of the battery amp indication.    If the alternator is working correctly there should be no current pulled from the battery except during load transients.

If the alternator is not working the battery will be providing the current - but the bus voltage would go down fairly rapidly.

You can pull the alternator field breaker and watch the battery amp indicator to see what happens.  If the battery amp go higher it tells you something.

[takair]

How old is the battery?  Maybe it’s legit recharge attempt on a failing battery?

whats going on with oil pressure?

[1980Mooney]

3 hours ago, zehutiman said:

I’m wondering if anybody has any insight into a drastic change I recently noticed in my battery amperage indication on the GI 275 Engine Monitor.  The airplane just came out of the shop for some comm wiring issues and to recalibrate the CiES fuel senders.  Normally, I see 1 to 3 amps, but for this 40 minute flight I was seeing 16-24 amps.  Just to be clear, the 24a is what’s being pulled from the battery, correct?  The battery voltage during this last flight was 13.4v.

 

Negative amps means battery discharge. Positive amps means charging the battery.  You have 2 batteries wired in parallel in the tail with the Missile conversion. I think that you have one of the batteries bad in parallel. As @skykrawler said it should show zero amps in cruise after your batteries charge back up following start. If one battery in parallel has a bad cell and a low voltage it will cause the regulator to try to charge and to continue charging both batteries. But you didn’t notice classic weak battery on starting because the good battery has enough amps to turn the starter normally. 
 

Failing Batteries wired in Parallel are harder to diagnose. You need to pull the battery box from the tail  (no small task with that Missile mod - you should have a custom battery board to slide it) and do a load test on each battery individually. 

Edited November 2, 2025 by 1980Mooney

[zehutiman]

3 hours ago, takair said:

How old is the battery?  Maybe it’s legit recharge attempt on a failing battery?

whats going on with oil pressure?

Two batteries, and they’re both about 18 months old.  The oil pressure is why I took the photo.

[zehutiman]

57 minutes ago, 1980Mooney said:
 

Failing Batteries wired in Parallel are harder to diagnose. You need to pull the battery box from the tail  (no small task with that Missile mod - you should have a custom battery board to slide it) and do a load test on each battery individually. 

I’m hoping that the mx shop simply allowed the batteries to be depleted.  Though the engine started strong, I noticed that a picture I took 15 minutes after the one above, showed 16 amps.  I’ll have to go back out to the airport…I left the airplane on a trickle charger.

[Ragsf15e]

Many Mooney’s are wired with an amp meter to indicate exactly how @1980Mooney said.  It’s indicating amps going TO the battery when positive, FROM the battery when negative.  However, you have to be careful because some are wired as a loadmeter which shows the total load on the system.  My 252 has a loadmeter for each alternator and one for the bus.  So I see positive amps from the alt and maybe the bus has a couple less amps (also possible) showing, the “lost” amps are charging the battery.  
To be clear, I bet the OP is showing an amp meter as 1980 indcated, but you should carefully refer to the electrical diagram.  I’m not too good at electrical, so I usually ask @PT20J or @EricJ!

[1980Mooney]

1 hour ago, zehutiman said:

I’m hoping that the mx shop simply allowed the batteries to be depleted.  Though the engine started strong, I noticed that a picture I took 15 minutes after the one above, showed 16 amps.  I’ll have to go back out to the airport…I left the airplane on a trickle charger.

That’s possible. I am surprised that you normally see 1-3 amps. Flightaware shows that your flights are usually long - sometimes over 3 hours. You should be reading zero amps - your batteries should fully charge. I also have a 1980 Missile conversion. I don't leave my batteries on a charger. It’s generally only minutes after starting and takeoff before the ammeter settles back to zero. 

Do you normally leave it on a trickle charger? Or maybe you meant a tender/smart charger?

If your 18 month old batteries load test well then it might be your regulator.