Lee; Once again, thanks for all the Great Information !
Dennis Elsberry, MO Sent from my iPhone > On Jun 21, 2025, at 3:39 PM, Lee Hart via EV <[email protected]> wrote: > > One additional comment: If measuring amphours is important, the E-meter / > Link-10 / Link-Lite is the way to go. They have an exceptionally accurate > circuit for measuring the voltage across the shunt to determine current. > > A shunt is basically just a precision resistor. A typical shunt has a voltage > drop of 50 mv at 500 amps, which means it is a 0.0001 ohm resistor. At 1 amp, > that means the voltage drop is just 0.1 mv. You need an amplifier to boost > this to a measurable level. But most opamps have input offset errors of > several millivolts. A 0.1 mv signal gets lost in the noise. > > A 1 amp error when measuring current usually isn't that important. But to > measure amphours, the error gets multiplied by time. A 1 amp error in > measuring current means an error of 24 amphours per day. And it keeps > accumulating every day! > > Most inexpensive meters thus have a threshold below which they assume the > current is zero. That way, the displayed amphours won't accumulate when the > vehicle is parked due to errors in the meter's current measurements. But it > also means the meter won't accurately display amphours due to small loads or > charging currents. > > The E-meter family uses an expensive ZERO offset opamp in its current sensing > circuit. Its worst-case input error is two orders of magnitude lower than > that 0.1 mv signal from a 1 amp current. > -- > Excellence does not require perfection. -- Henry James > But it *does* require attention to detail! -- Lee Hart > -- > Lee A. Hart https://www.sunrise-ev.com/ > > _______________________________________________ > Address messages to [email protected] > No other addresses in TO and CC fields > HELP: http://www.evdl.org/help/ > _______________________________________________ Address messages to [email protected] No other addresses in TO and CC fields HELP: http://www.evdl.org/help/
