Most Ah meters get around the offset errors by asking for a voltage level at which they can consider the pack fully charged and then resetting themselves to zero, so every full charge cycle they zero the error. This is of course not entirely accurate either, because typically as soon as the threshold voltage is reached they zero out and the pack is typically only 80-90% charged at that point, so there still is a risk that you discharge deeper than you realize, but it avoids the cumulative error effect - except when it is not fully charged for a long time, either because it is sitting unused or because you only do partial charges for a long time, then you again have no clue how far off it has gotten. This is the consequence of the bandaid of zeroing to try and hide the inaccuracy of low current measurement...
Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info http://www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of Lee Hart via EV Sent: Tuesday, January 19, 2016 12:46 PM To: Jay Summet; Electric Vehicle Discussion List Subject: Re: [EVDL] Looking for an AH meter, pack powered, circular 2" mounting hole 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) _______________________________________________ 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)
