Hi everybody,
thank you for all the kind and very valuable comments!
First, I used two NiMH batteries in series for testing purposes and the shown voltage is the actual one.
So the charging starts close to the nominal voltage of 1.2V. Since the cell voltage is rising slowly at this point I thought I take this example to show the effect. Sorry for not making this clear in the first place!
@ jremington: The OP AMP I am using is a LM 358 with a maximal supply voltage and difference Voltage of 32V – so I don’t think it that,
I also attached a complete circuit diagram of the current controlled battery simulator. Yes, the battery is floating. But since the OP Amp gives me only the differential voltage between the battery poles it should be ok.
@raschemmel: I compared the battery voltage with the Arduino outputs before with a reasonable precision. Like I said I charged the cells close from nominal voltage and expect a slow rise. Also it cannot be the cell because the “jump” also occurs with other cell chemistries, following the same pattern. The start of the “jump” varies a bit (2.43V – 2.443V) but the upper value is always 2.46.
I don’t understand how a bleed resistor could solve that. Furthermore I want to get reasonable precise capacity values by integrating the current over time. For this any unmeasured current will increase my error quite a bit.
Also thank you for that suggestion with the correction factor. But I did this already (not as accurate as you) and got reasonable results for the hole voltage range, except the little “jump” always at this position.
I used 56k Ohm for R1 and R2 and 112k Ohm for R3 and R4
@MarkT: Decoupling sounds interesting. However I have no idea how to do this. Would you mind giving me an example or an online link for that? Also suggestions how to make the circuit more robust would be nice.
I tried today to use different ports of the Arduino, no success. But I narrowed down the error a bit: Next to the the figure with the whole circuit I attached my latest measurement: I connected the Vout of the OP AMP directly with the Multimeter and the Arduino (both measured against ground). The result is shown in test3.jpg. The OP AMP output is not governing the “jump”.
So it must have something to do with the Arduino or how it is connected? Should I upload these connections as well?
Thank you again for your help!
