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Old 02-19-2019, 09:57 AM
lndwarrior lndwarrior is offline
Join Date: Mar 2015
Location: Cloverdale CA
Posts: 304
Default Electrical Issue - low battery

I have been seeing a repeated problem with my electrical system that I am trying to figure out. Some things to know:
- I built the plane and designed the wiring system based on the Z-11 diagram.
- It has worked well for 300 hours
- engine is an O-235
- 40 amp B&C alternator with 300 hours on it
- B&C LR-3 voltage regulator with about 30 hours on it
- EarthEx ETX680 battery 300 hours as well
- Electronics International Volts/Amps meter set up in a LOAD Meter arrangement with matched EI shunt
- very low amp draw electrical system - about 10 amps with everything turned on (VFR panel, led lights, etc)

I have noticed my battery voltage dropping when it has not been flown in a while.

EarthEx says either I have a parasitic drain going on when the master is off or I have a bad battery.

So far I have not had the time to trouble shoot what might be draining the battery. I do have an Always On bus for LED cabin light, USB port and possibly other item that I cant remember right now.

My normal/average total amp draw in flight is around 7 amps.

Yesterday I checked the battery with a portable amp gauge and it was reading 12.9 amps - below the 13.3 that is typical for this battery.

The engine started just fine and I did a short MX flight directly over the airport. For the 10 minutes I was in the air the amp meter was reading about 25 amps, though this was varying up and down by an amp or two.

I have never seen this high amperage (alternator output) before. I have high and low voltage warning lights and a stuck starter light. None of the warning lights lit up during the flight.

So, is the 25 amps I saw simply just normal? Is this a case of the alternator charging a low battery? Again, the max load from my electrical system is around 10 amps, and everything was not turned on during the flight.

I know I need to check the system for parasitic drain but right now I'm just trying to understand that my charging system is doing.

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Old 02-19-2019, 10:15 AM
rocketman1988 rocketman1988 is offline
Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,831
Default If...

If the battery was substantially drained it could indicate that kind of amperage...normal charge rate can be between 5-15 amps...
Aerospace Engineer '88


Dues+ Paid 2021,...Thanks DR+
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Old 02-19-2019, 10:25 AM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,783

low resting state voltage could indicate a charging problem, a parasitic drain or a bad battery. First, about an hour after your flight check the battery voltage. If it is 13.3 or higher, like typical, disconnect the negative cable from the battery. Come back a day or so later and check voltage. If it is still 13.3, you have a parasitic drain. Reconnect cable and check in a few days to confirm it is then lower. If it is 12.9, you have a bad battery. If, 30 minutes after flight your voltage is 12.9, you have a bad battery.

25 amps for a minute or two after start would be typical. However, 20 minutes into your flight you are reading a current draw four times your normal would indicate a failing battery or a wiring problem, unless the parasitic drain is depleting a significant percentage of your charge, in which case it would be expected for 15-30+ minutes.

what voltage do you show 20 minutes into the flight?

You could have a wiring issue (failing connector or connection likely), but I would rule out a bad battery first. Obviously, a cursory check of battery and ground connections would be prudent.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 02-19-2019 at 10:30 AM.
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Old 02-19-2019, 11:16 AM
BobTurner BobTurner is offline
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,920

Correction: you checked your battery with a voltmeter. Amps is current, and that is what you need to know here. Keep the master off. Disconnect one of the battery connections, and, with the master off, attach one lead from your multimeter to the battery, the other to the wire you disconnected. Start with the meter on its highest amp scale, then turn it to its lower, more sensitive scales, until you get a reading, or still see zero. If the meter reads backwards, reverse the leads. If you see anything other than zero you have a parasitic load that you?ll need to track down and eliminate.
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Old 02-19-2019, 01:04 PM
Kalibr Kalibr is offline
Join Date: Aug 2015
Location: St John, Steamboat Springs, planet Earth generally
Posts: 102

Regarding possible parasitic current draws, an USB charging port can draw some current even if nothing is plugged into it
Building a Panther, but lurking here, like many others
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Old 02-19-2019, 02:59 PM
Paul 5r4 Paul 5r4 is offline
Join Date: Oct 2006
Location: Foley, Al
Posts: 638

Once I flew 15 miles from home base for fly-in breakfast. Everything normal. Was there about 90 minutes. When leaving I noticed my amps were reading +25/30. Normally after start up, (even after not flying for a couple weeks), I'll see maybe +5 or so until the battery is topped again by the alternator.

By the time I was back home the readings were normal again. I hadn't left anything on. Only thing I could come up with was the master solenoid had not opened when I turned the master switch off. From that point on, whenever I'm shutting down, I lift lift up an ear cup when switching the master off to make sure I hear the solenoid snap open. It is very audible.

I had left my master switch on overnight once a few years back and although everything in the plane was off the battery was dead.
Paul Gray
Foley, Alabama
N729PG..... 500+ hrs
RV 7A, Lycoming 0 320 D1A, Sensenich FP propeller
VAF supporter $$$
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Old 02-20-2019, 09:06 AM
ClayR_9A's Avatar
ClayR_9A ClayR_9A is offline
Join Date: Jan 2005
Location: Dallas, TX
Posts: 173
Default USB port

My guess is the USB port is drawing some power.

Easy test, just disconnect it and see if the battery drains.

Or, disconnect it, and put a multimeter between the power and the device to see how much current it draws. (easier than doing this at the battery, and tells you how much this specific device is drawing)
Clay R- RV9A - Flying since 2004 - 425 hours
VAF dues paid through end of 2022.
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Old 02-20-2019, 01:14 PM
Spinnetti Spinnetti is offline
Join Date: Jul 2018
Location: Frisco
Posts: 6

Do a amp test on the battery or take it somewhere for the test? Sure sounds like battery to me. Testers are cheap.
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