Accurate battery voltage sense (+/-30mV)

[eem2am]

Hello,

We are designing an Emergency Light, which powers a lamp from a 6V battery when the normal power supply fails. Otherwise, the Emergency light PCB charges the battery.
The battery is at the end of a wire lead, which is half a metre long. The resistive drop in this lead during battery charging (1 Amp) and discharging (400mA) means that we are not correctly sensing the battery voltage.
Therefore, we wish to run separate “sense” wires from the battery terminals to the PCB, so that we can more accurately sense the actual battery terminal voltage.

During discharge, the battery voltage needs sensing because the battery is composed of five NiCd cells in series, and it is dangerous to discharge such a battery pack to below 5.25V as individual cells may become damaged.

During charging, the battery terminal voltage must also be known, because the battery terminal voltage must always be below 8V.

The internal impedance of each cell is 15mR. (so that’s 75mR for the battery, -unfortunately, we don’t know what the resistance of the cell to cell interconnects is)

Schematic of Battery voltage sense circuitry.
http://i49.tinypic.com/e9t8rd.jpg

(Also attached as PDF)


..Please forgive me using LT1006 opamps in the schematic, in the actual circuit , MCP601’s would be used……………..

MCP601 datasheet:
http://ww1.microchip.com/downloads/en/DeviceDoc/21314g.pdf


Please could you state how accurate the sensed battery voltage would be with this setup?……we would prefer not to have the expense of an (opamp) instrumentation amplifier if at all possible.

 

[Harald Kapp]

I'd say the accuracy of the sensed voltage depends almost entirely on the accuracy of the resistors used. The OpAmps will contribute some minor offset (max. 4.5mV), but the gain being 1 this should be negligible.

I wonder why you are using 2 OpAmps. The 1st stage is unnecessary.

The OpAmp selected should be suitable for this application.

 

[eem2am]

The source resistance is very high (2K2//12K) and so i need the buffer to make it lower.

It is this high source resistance which makes the differnetial amplifier not-so-suitable for this use.

I wonder if there is a cheap instrumentation opamp? All the ones i see are expensive. I'm lloking for something around <1.5$ for 5000 pieces.

 

[Harald Kapp]

Use 100ks for the diff-amp and you'll be fine with that source resistance.
Or leave out the diff-amp completely. It makes no sense. What difference are you going to evaluate that is not alreaady seen by the first amp?

 

[eem2am]

i need an opamp in there to buffer the "signal" from the battery, i cant have power current passing down the voltage sense wire, as i'd get resistive voltage drop inaccuracies.

here is the refined mothod, there is surely no way i can do without the opamp...........

 

[Harald Kapp]

This circuit makes much more sense. Just 1 OpAmp. That is how I meant it.
I suggest you add some overvoltage protection (suppressor or zener diodes) in front of the OpAmp.