Jay Summet via EV wrote:
I'm looking for an AH meter for my truck... I'm willing to pay extra to have it use a hall effect sensor (as opposed to a shunt), as I don't need it to be terribly accurate and for my mounting position a hall effect will be easier/cleaner.
Hall sensors are fine for measuring current, but they aren't much good for amphours. The problem is that hall sensors have small offset errors that get accumulate over time until the amphour reading is meaningless.
For example, suppose you have a 500 amp Hall effect sensor. It's a fantastically good expensive one, so its offset error is only 0.01% (not 1% which is typical of cheap ones; but 100 times better). That means it has an offset of 500a x 0.0001 = 50 milliamps. It thinks that a current of zero is really 50ma.
An amphour counter multiplies this current error by time. 50ma x 24 hours = 1.2 amphours per day, 8.4ah per week, 36ah per month. The amphour reading becomes largely useless.
Now try this with a real (affordable) Hall effect sensor, and the amphour reading becomes useless in a matter of hours. Cheap AH meters get around this by treating any low current (like under 1 amp) as zero. Then the amphours don't change when you're parked. But it won't take into account parasitic loads, like standby drain of your DC/DC converter, controller, the voltmeter on the pack, etc. It also won't take into account low charging currents from a float charger, PV panels, etc.
A shunt has the advantage of zero offset voltage. Its voltage is *exactly* zero at zero current. The shunt amplifier could have an offset error; but they usually use a chopper stabilized opamp that also has essentially zero offset.
-- Knowledge is better than belief. Belief is when someone else does your thinking. -- anonymous -- Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
