Hi guys, I have my car running now (finally) and currently just have a multi meter for instrumentation. I'm running 96V which fully charged is around 104.5V. How do I convert a voltage reading to know what it means in terms of 3/4 charge, 1/2 charge etc?
All of the other posters outlined why you cannot accurately do what you are talking about. My post is interpreting your question or what's behind it. You wish to cheaply and accurately determine what you have left without spending a lot of money. Assuming you own a laptop this rather cheap voltmeter has usb conversion to feed a laptop voltage information along with other useful data:
http://www.ebay.com/itm/Digital-Mul..._Automotive_Tools&hash=item4d0d46e08e&vxp=mtr
Caveat: I don't own one of these I just did a quick search to find a computer interfacing voltmeter.
You would need a very simple program to sample and store the information over time to calibrate the results into something useful.
Since measuring current is actually done by measuring the voltage drop across a "shunt" or known resistance (normally) you would need a shunt or I believe you should be able to measure voltage from one end of your longest primary wire (either plus or minus, makes no difference, just pick the longest cable so as to get the highest signal to noise ratio, i.e. voltage indication) to the other end of that wire and feed that to the laptop as the current indication. Sample it every second or so, store the amount and seconds information and there is your accurate fuel gauge.
In case this is confusing, here is the barnyard explanation of what the simple program would do: You connect your Harbor Freight $5.99 voltmeter across say the wire returning minus from you trunk up to the controller "minus" in the front. You extend the voltmeter black lead to the rear with a small wire and to the minus battery terminal. The red lead gets extended to the controller minus exactly where the cable you are "shunting" connects to the controller. Looks like you are doing something redundant doesn't it? Shorting across the cable or something? Just do it. The voltage is your "uncalibrated" current indication. THE ACTUAL VALUE IS NOT RELATED TO PACK VOLTAGE and it does not tell you your starting, ending or any other "voltage" in the system. This voltmeter is being used as an ammeter period. More current will make your "relative" voltage go higher. Now that is the "current " part of the equation. Let's build a program that keeps track of how much current you are using over time. Wait were in a barnyard and we want to use wood and bailing wire. OK so you get one of your friends to ride along with you and they have a watch (with a second hand and it is wound up and running), a pen, an old manual (lever you pull on the side to sum) adding machine since we don't want to steal any power/range from the system) and a clip board with graph paper.
First you have to calibrate the "system" so your "logger person" puts a zero in the box on the bottom left hand side of the paper. You start moving your EV using five percent of the throttle and while you hold that you both see .05 volts on the voltmeter. Your friend writes ".05" in the next box up from zero on the left of the paper. You go to ten percent, he writes ".1" in the 3 rd box above the others. This calibrating is done until at full throttle you see "1 volt" and that goes in the twentieth box above all the others. You have now calibrated your HF 6$ voltmeter to read in "relative" amps. The actual amount is not important for this purpose.
You make a commitment to drive at a steady state in one minute increments (Like as if you are the only person on the road and traffic signs and lights do not exists, or basically as if you are from Boston
)
Every minute the logger places a check in the box corresponding to that voltage he sees at that time on the meter and is above the "minute" line at the bottom of the graph moving to the right on the graph. Since he has a whole minute to work he adds up the boxes below his "entry" and puts that total into the adding machine. For the first two minutes you drive at 10% throttle so each minute equal two boxes under the checked box or 2+2=4 and 4 is the total on the adding machine for the first two minutes. For the next ten minutes you drive at half throttle and each minute is at ".5 volts" (or at the end of twelve boxes across, 2 boxes two "up" and ten boxes .5 "up") the adding machine shows 54 boxes below the line so to speak total. And so on until you run out of graph paper or battery. When your pack has reached discharge voltage(measured on another Harbor Freight 6$ voltmeter) (the one time the voltmeter is accurate as to what you have left which is zero) You can now look at the adding machine and see the total number of boxes which represents the total "charge" you have available to use in the future.
After you have the total number of boxes or charge you can just drive (after recharging fully) as you wish as long as you hold the throttle steady in one minute increments. Your friend will keep a tally of the boxes you have used and let you know when your total used is getting close to the original total you had available.
Now in the real world that kind of friend doesn't exist and one minute increments does not fit legal or defensive driving. We have to increase the sampling and totaling thing and leave the friend at home. So what to do?
A computer fed this "voltage across a shunt" can do all of these functions and that is what is done.
I should add that a cheap meter like the ebay one could also measure RPM and many other things mimicking a lot of the expensive EV monitoring equipment.
