A Different Battery Question

On Wednesday, October 16, 2019 at 1:01:35 PM UTC-7, Eric Greenwell wrote:
Dan Marotta wrote on 10/16/2019 9:48 AM:
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.
I'm at a loss.

I'm guessing you did the test with the 2.5 ohm resistors in series (5 ohm total),
for about 2.5 amp draw. In parallel (1.25 ohms total) the draw would be 10 amps.

How much current is needed to run your instruments?

Disconnect the battery, short the connector, and select the backup battery with
your switch. Measure the resistance of the wiring with an ohmmeter on the (+) and
(-) of the 12 volt buss at the panel. It should read less than 0.5 ohms.

If that wiring (panel to battery box through the switch) is OK, look closely at
the connectors (glider side and battery side). Check the ohms for each lead from
battery post to battery side connector. It should less than 0.1 ohms (essentially
a short circuit). If the (+) lead is high, maybe the fuse is bad, too small, or
the wrong kind, or the fuse holder is faulty.

That should be "short the plane side connector"! I know Dan wouldn't short the battery side connector and waste fuse, but somebody might on their system...