On Thu, 21 Sep 2017 12:35:16 -0700 (PDT), wrote:

On Thursday, 21 September 2017 20:24:48 UTC+1, T i m wrote:
On Thu, 21 Sep 2017 12:05:59 -0700 (PDT), tabbypurr wrote:

snip


How accurate do you want the readings?

As accurate as the meter can be itself, ideally.

Your total error is all the errors added together,

I know.

so you'd need some exceptional circuitry to not add anything significant to the meter's error, which might be 1% or so. More realistic would be to keep the electronic errors within 1%.

Understood.

To get that you'd be using an opamp to knock 10v off,

Why not a zener or precision voltage IC?

with 0.1% resistors everywhere it matters.

As long as the final result calibrates correctly, does it matter what
the tolerances are / were?

Avoiding anything getting hot always helps keep the errors down,

With 250uA FSD I can't anything much getting warm?

so anything that gets warm heatsink it, and operate parts way under their limits. Not that that will be a problem with this meter.

Quite.

So, precision reference, zener, resistor bridge?

With a precision reference and zener my meter wouldn't start doing
anything till the measured voltage started to exceed 10V, even if I
was ramping from 0 to 15V (and that's fine for a 12V lead acid).

With a resistor bridge with a 1/3 split I should be able to get FSD at
my 15V but anything less wouldn't work (would it?) ... so it's going
to have to be either of the first two ... or something else?

Basically, as long as I hold the negative of the meter at the same
voltage as the positive at anything 10V and below, the meter won't do
anything. Then as the +ve of the meter exceeds 10V and goes to 15V, a
meter set to read 0-5 volts should follow ok, giving me reasonable /
best possible accuracy across the working range of the battery.

The only prerequisites are to ensure the -ve of the meter is held
exactly at +10V (from a supply ranging from 10.5 to 14.4V) and that
the meter current limiting resistor is correctly matched (to give the
250uA required for FSD at 5V PD).

As long as any reference would be suitably driving at anything above
10V and not overdriven at 15V then we should be good to go?

Cheers, T i m