AA battery load tester

Circuit is simple. AA battery holder connected to ground on the negative side, and to 2 1W resistors on the positive side. Each resistor is connected to the drain of a beefy N-channel mosfet (controlled by Espruino - pins A1 and A0). Voltage is monitored before the resistors, and after each resistors, in order to measure current (so I can calculate current without knowing the resistance of the fet).

Logs are kept on SD card for graphing in excel. The idea is to be able to generate a discharge curve under various conditions, to get an idea what kind of battery to buy for every-day household electronics.

The idea of using analog write is that I could later expand it to try to keep the current constant.

I also have some constant current sinks coming I may try with this.

Untested, so far.

//Analog inputs
//C3: Battery positive voltage
//C2: Voltage after R1
//C1: Voltage after R2

//A1: R1 switch
//A0: R2 switch


var fetstate=[0,0];
var resistance=[4.7,15];
var endV;
var inter;
var timeout;
var Wsec=0.0;
var Asec=0.0;
var runtime=0;
var logname="logtest.txt"
var fs=require("fs")

function getMeasure() {
	var retobj={};
	var vbat=analogRead(C3)*E.getAnalogVRef();
	var cur=0.0;
	var pwr=0.0;
	if (fetstate[0] > 0) {
		var tv=vbat-analogRead(C2)*E.getAnalogVRef();
		tcur=(tv/resistance[0])
		pwr+=tcur*vbat;
		cur+=tcur;
	} 
	if (fetstate[1] > 0) 
		var tv=vbat-analogRead(C1)*E.getAnalogVRef();
		tcur=(tv/resistance[0])
		pwr+=tcur*vbat;
		cur+=tcur;
		cur+=(tv/resistance[1])
	}
	retobj.cur=cur;
	retobj.pwr=pwr;
	retobj.vbat=vbat;
	return retobj;
}

function upfet() {
	analogWrite(A1,fetstate[0]);
	analogWrite(A1,fetstate[1]);
}

function startTest(fetst,endV,maxtime) {
	fetstate=fetst;
	runtime=0;
	Asec=0.0;
	Wsec=0.0;
	var timeout=setTimeout("if (inter) {clearInterval(inter);endTest();}",maxtime*1000);
	var inter=setInterval(runTest,1000);
}

function runTest() {
	runtime+=1;
	var obj=getMeasure();
	var logstr=runtime+","+obj.vbat+","+obj.cur+","+obj.pwr+","+fetstate[0]+","+fetstate[1]+"\n"
	console.log(logstr);
	fs.appendFile(logname,logstr);
	Wsec+=obj.pwr;
	Asec+=obj.cur;
	if (obj.vbat < endV) {
		clearInterval(inter);
		clearTimeout(timeout);
		console.log("minimum voltage reached")
	}
}

function endTest() {
	digitalWrite(A0,0);
	digitalWrite(A1,0);
	console.log("Power: "+Wsec/3.6+" mWh Current: "+Asec/3.6+" mAh")
}

Looks like a really good idea - do you have a graph yet?

I love the idea of controlling the current draw by varying the voltage on the FET.

I'd actually really like to do an Espruino battery charger/conditioner - it'd be great to have something that'd keep my NiCd electric drill batteries in a good condition. It could even do cool things like cycle them while always making sure that one of them was fully charged :)

Nope. Haven't looked at this since I posted this. I've been kinda busy this week - last week before I'm on vacation, and will have more time to play with this stuff. I'll try to remember to bring the parts with me.