Sensing of battery voltage....battery which is connected to half a metre leads.

[treez]

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.
Ultimately, A microcontroller ADC reads the battery voltage.

During discharge, the battery voltage needs sensing because the battery is composed of five NiCd D 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.
https://i46.tinypic.com/351btye.jpg

(Also attached as PDF)


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

MCP601 datasheet:
https://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.

 

[crutschow]

That circuit should work fine. Since your criteria is the battery voltage during charge or discharge then you don't need to be concerned about the battery or contact impedance. It should be negligible for your purposes.

 

[treez]

That circuit should work fine. Since your criteria is the battery voltage during charge or discharge then you don't need to be concerned about the battery or contact impedance. It should be negligible for your purposes.

..sorry but its a 1 amp discharge and the battery Z(internal) is 75 milliohms, so its not negligible

 

[betwixt]

In an emergency light is 0.075V really that important?

Brian.

 

[treez]

we really do need to be able to be sure we are discharging right down to 5.25V before shutting off the emergency led driver.....this is all to do with our customers, who insist on "4 year" discharge performance, and if its only "3 years and 300 days" then it gets classed as "not four year capable" and we take a dive....so we need accurate cell monitoring, though we fear the worst thing is the cell hysteresis, where a cell which whilst supplying 1 amp measures 1.05V, it then measures 1.15V when its been stopped discharging for a few 100 milliseconds.

 

[BradtheRad]

unfortunately, we don’t know what the resistance of the cell to cell interconnects is

You can measure this by sending a known current through the battery pack, then measure volts across an interconnect. This is done by putting a voltmeter lead on one battery plus tip, and the other lead on the neighboring battery's negative tip.

Divide by the known current to get resistance.

 

[treez]

thanks Bradthe Rad, ill rip off the battery sock and try and do this, do you think there'll be enough room between the d cels to get the probe tips in there?

 

[crutschow]

Don't you just want the total resistance of the cells and interconnect? For that you just need to measure the voltage drop at the output for a known current from a charged battery pack.

 

[treez]

sorry crutschow no-can-do, the battery voltage creeps about when charged or discharged, its called hysteresis, and is not definable.

 

[BradtheRad]

thanks Bradthe Rad, ill rip off the battery sock and try and do this, do you think there'll be enough room between the d cels to get the probe tips in there?

It will be tricky.
I keep a couple of safety pins in my meter case, to poke through heat-shrink-wrap or wire insulation.