Hi Eric,

The two power resistors were in parallel for a total of 1.25 amps. Yes,
I should get around 10 amps (they sure got hot!) but I'll have to do the
measurement again to be sure, but I want to keep it charged up right now
to try a flight now that it's been on a charger for some time.

On 10/16/2019 2:01 PM, 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. 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.

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.


--
Dan, 5J