A Different Battery Question

I've been using a 20 AH LiFePO4 battery as a backup in the Stemme. It's
portable and secured behind the passenger seat.Â* I charge it outside of
the aircraft.Â* When used, it is connected through an DPST switch (only
one half used) and switches the tail battery out of the system as it's
switched in.

The problem:Â* Even when fully charged it comes on line at around 11.4
volts or below and, if I key the radio, the radio display flashes and
the vario gives a Low Voltage warning.Â* When the tail battery is at 11.4
volts (it's an SLA battery), the radio works fine.

I've tested the lithium battery using a pair of 2.5 ohm power resistors
in parallel and measuring with a Watts Up power meter and it seems to
perform just fine, starting with voltage in the high 13s.Â* I ran it for
several hours that way, burning up about 15 amp hours without the
voltage ever dropping below the mid 12s.

Any ideas?Â* Everything seems to work just fine except for the low
voltage as measured at my ClearNav and at the Dynon D10a.Â* I'm using
good quality Tefzel wire of sufficient diameter, Power Pole connectors,
and the battery is properly fused.Â* I'm at a loss.
--
Dan, 5J

You need to be checking voltage with a voltmeter in the airplane.
sounds like you have something in the circuit providing more resistance than you want.

Power up the aircraft on this battery and measure the voltage at the battery, it should be up at in the 12s like you expect. if it is not then you need to measure the current being pulled and possibly test with another battery. to see why it is different than you bench testing. Are you pulling more current than your bench testing? What devices are pulling that current?

If it is up in the 12s then measure at your onboard voltmeter and confirm it is reading correctly. You said it has been reading about 11.4. You should not be losing a full volt between the battery and your voltmeter. Then start checking the voltage before and after each component between the battery and your volt meter to see what the offending component or wire is. You may find it is a combination of wires or components that add up to the total voltage loss you are seeing.

Brian

How about divide and conquer?

Get the system in the condition you saw with more volts at the battery than the clearnav. (Maybe disconnect the batt and make sure the clearnav powers down just to make sure you have what you think you have.)

Split the path in half by measuring the voltage drop between the battery + terminal and the clearnav +power in. Then the batt- to clearnav power-.


Choose which part of the path has most of the voltage drop and pick an accessable point in the middle. Measure the two halves.

Then repeat with smaller and smaller parts of the path until you have found it.

Yes, that's good advice.Â* I was hoping to not have to remove the rear
console where the switches are located to do the troubleshooting.

Oh well, soaring season is coming to an end and wave season is not yet
in full swing.Â* I guess I'll need something to do in the interim.

On 10/16/2019 12:09 PM, wrote:
How about divide and conquer?

Get the system in the condition you saw with more volts at the battery than the clearnav. (Maybe disconnect the batt and make sure the clearnav powers down just to make sure you have what you think you have.)

Split the path in half by measuring the voltage drop between the battery + terminal and the clearnav +power in. Then the batt- to clearnav power-.


Choose which part of the path has most of the voltage drop and pick an accessable point in the middle. Measure the two halves.

Then repeat with smaller and smaller parts of the path until you have found it.



--
Dan, 5J

On Wednesday, October 16, 2019 at 5:57:18 PM UTC-7, Dan Marotta wrote:
Yes, that's good advice.Â* I was hoping to not have to remove the rear
console where the switches are located to do the troubleshooting.

Oh well, soaring season is coming to an end and wave season is not yet
in full swing.Â* I guess I'll need something to do in the interim.

On 10/16/2019 12:09 PM, wrote:
How about divide and conquer?

Get the system in the condition you saw with more volts at the battery than the clearnav. (Maybe disconnect the batt and make sure the clearnav powers down just to make sure you have what you think you have.)

Split the path in half by measuring the voltage drop between the battery + terminal and the clearnav +power in. Then the batt- to clearnav power-..


Choose which part of the path has most of the voltage drop and pick an accessable point in the middle. Measure the two halves.

Then repeat with smaller and smaller parts of the path until you have found it.



--
Dan, 5J

Losing over 2V is much more than can be explained by a high resistance connection (it would have to be multiple ohms). I think you have a bad ground connection for some reason, and the battery is finding a ground path thru another instrument. Measure the resistance to your a/c ground bus to the negative battery terminal. Do the same for the positive connection to the 12V bus.

Tom

Really good advice, Tom.Â* I'll do that.

On 10/17/2019 6:26 PM, 2G wrote:
On Wednesday, October 16, 2019 at 5:57:18 PM UTC-7, Dan Marotta wrote:
Yes, that's good advice.Â* I was hoping to not have to remove the rear
console where the switches are located to do the troubleshooting.

Oh well, soaring season is coming to an end and wave season is not yet
in full swing.Â* I guess I'll need something to do in the interim.

On 10/16/2019 12:09 PM, wrote:
How about divide and conquer?

Get the system in the condition you saw with more volts at the battery than the clearnav. (Maybe disconnect the batt and make sure the clearnav powers down just to make sure you have what you think you have.)

Split the path in half by measuring the voltage drop between the battery + terminal and the clearnav +power in. Then the batt- to clearnav power-.


Choose which part of the path has most of the voltage drop and pick an accessable point in the middle. Measure the two halves.

Then repeat with smaller and smaller parts of the path until you have found it.


--
Dan, 5J
Losing over 2V is much more than can be explained by a high resistance connection (it would have to be multiple ohms). I think you have a bad ground connection for some reason, and the battery is finding a ground path thru another instrument. Measure the resistance to your a/c ground bus to the negative battery terminal. Do the same for the positive connection to the 12V bus.

Tom

--
Dan, 5J

On Thursday, October 17, 2019 at 5:59:58 PM UTC-7, Dan Marotta wrote:
Really good advice, Tom.Â* I'll do that.

On 10/17/2019 6:26 PM, 2G wrote:
On Wednesday, October 16, 2019 at 5:57:18 PM UTC-7, Dan Marotta wrote:
Yes, that's good advice.Â* I was hoping to not have to remove the rear
console where the switches are located to do the troubleshooting.

Oh well, soaring season is coming to an end and wave season is not yet
in full swing.Â* I guess I'll need something to do in the interim.

On 10/16/2019 12:09 PM, wrote:
How about divide and conquer?

Get the system in the condition you saw with more volts at the battery than the clearnav. (Maybe disconnect the batt and make sure the clearnav powers down just to make sure you have what you think you have.)

Split the path in half by measuring the voltage drop between the battery + terminal and the clearnav +power in. Then the batt- to clearnav power-.


Choose which part of the path has most of the voltage drop and pick an accessable point in the middle. Measure the two halves.

Then repeat with smaller and smaller parts of the path until you have found it.


--
Dan, 5J
Losing over 2V is much more than can be explained by a high resistance connection (it would have to be multiple ohms). I think you have a bad ground connection for some reason, and the battery is finding a ground path thru another instrument. Measure the resistance to your a/c ground bus to the negative battery terminal. Do the same for the positive connection to the 12V bus.

Tom

--
Dan, 5J

It would also be helpful if got a smarter charger than the one you have. I use the Hitec X2 Pro:
https://hitecrcd.com/products/charge...harger/product
It will inform you how many AHr it put into the battery, which is a critical cross-check. It can also do a discharge test.

Symptoms would indicate a bad connection or too light wire used.
Bad enough in such simple wiring, tragic on the neutral in 3-phase 400V. Seen enough of that film.
Jim

Somewhere someone else mentioned that a LiFePo4 battery is not
necessarily fully charged just because the green light comes on on the
charger.Â* I left it hooked to the charger for 3 days, with a lamp timer
to cycle it off/on every three hours (I have other reasons for that
timing setup), and today I hooked it up in the Stemme and everything
seems to work just fine now.Â* I'm currently doing annual on our C-180
but, as soon as I have some free time, I'll fly the Stemme and see how
it works in flight

On 10/16/2019 12:25 PM, JS wrote:
Symptoms would indicate a bad connection or too light wire used.
Bad enough in such simple wiring, tragic on the neutral in 3-phase 400V. Seen enough of that film.
Jim

--
Dan, 5J

Good thoughts posted thus far.....

My $0.02....with even an inexpensive DMM/DVM.....start at the "common buss" on/near the panel with all batteries fully charged.
Measure D.C. Voltage at the buss main input with a battery connected....
Switch things on, read voltage....write it down....
Key mic and do other load things...record voltage under load.
Repeat with the other battery.....
If the large battery shows a bigger voltage drop.....work through connections back towards battery with same test....see where the voltage drops lessens....in between good vs. bad/poor voltage drop is likely a poor or bad connection.

Could be a bad crimp, poor solder joint, loose/poor screw connection, maybe even small wire gage (although not likely unless waaayyyyy tiny...).