STF question

I am using an oldeer Ilec SB-8 STF vario system, am very happy with it
and absolutely love the vario function (been an Ilec user for many
years) but in my Tetra-15 I am finally using a STF system, and have a
few questions.

Since this unit does not use GPS info , when I am in a headwind or
tailwind the P/S info is all the unit has available for speed
info...............yesterday I was in an area of extended strong sink,
along with a strong headwind............with no GS reference (per
GPS), what kind of accuracy does the SB-8 STF info have? And what can
I do to remedy this.

Another issue on that flight was a battery that dropped below
10volts............the STF needle was behaving rather badly!

Brad