adccommunitymod (AutomationDirect) asked a question.
Created Date: April 28,2009
Created By: jaspersk
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I need to continuously monitor the voltage on individual 6 volt batteries strung together in series for a 120V battery pack. I can't afford isolated analog inputs so I am thinking the least expensive method to do that is to use solid state inputs to multiplex the signals to an analog input and measure voltages incrementally up the pack one at a time. As long as the speed is fast enough, I should be able to get pretty fast updates. I'd use resistors for scaling and internal software for calibration. The circuit below would have 10mA = 6 volts. Will this work? Any thoughts on a better way of doing this without spending much more money? Right now, I think I can pull it off with a DL06, a 4 ch analog in, and a 10 ch DC out. http://i227.photobucket.com/albums/dd106/skskarda/BatteryMonitor.png
Created Date: April 29,2009
Created by: allukes
If the output is off, wouldn't the voltage on the terminal of the output exceed the output circuit's maximum voltage rating (27VDC) ? Have you tried this yet?
Created Date: April 29,2009
Created by: jaspersk
Thanks so much for taking the time to look at my diagram!!! I really appreciate it.
Darn it! I knew my circuit was too convoluted to work. I am not sure how to think about voltage drop across several outputs in series that are off but certainly you must be right. If there is no current flow, there will be no drop across the resistors so all of the drop will be across the outputs and it probably won't be evenly distributed so I will exceed the maximum voltage. Doh! :o Does the maximum voltage apply when the output is off? I suppose it does.
My plan B was to have one sinking and one sourcing output at each battery terminal so I could feed each battery individually to an analog input. I guess that woulnd't work either. :confused:
Any other ideas on how to pull this off without isolated analog inputs or relay cards? Can I get an optically isolated multiplexer at Automation Direct prices?
Is there an I/I transducer that I could get at low cost? If each output has a maximum rating of 50V, I suppose I could put a transducer in each common that would feed the analog input?? There has got to be a way.
Created Date: April 29,2009
Created by: allukes
How fast are you sampling? Can you use relays?
Created Date: April 29,2009
Created by: jaspersk
Relays are my worst case plan. Ideally, I'd like real time measurement. I would like to sample all 20 batteries in under a second. Are there any cheap 4-20mA isolators out there?
Created Date: April 29,2009
Created by: allukes
Why do you have to sample so fast? Does your data change that quickly?
Created Date: April 29,2009
Created by: bfitz
Could you get a voltage to current transducer? If you can, build voltage dividers for each circuit and just connect the voltage signal (with your solid state output switching) to your transducer/transmitter. The voltage dividers would maintain your ground connection so the inputs wouldn't float. You would have to adjust your calibration to account for the volt drop through the SS output, but it would be repeatable (I would think).
Brian
Created Date: April 29,2009
Created by: jaspersk
Yes, as the vehicle accelerates, the voltage across the battery pack drops rapidly and raises back up when the foot is taken off the accelerator. It is beneficial to have visual indication of how each battery handles spikes in current during acceleration.
http://www.youtube.com/watch?v=GTguQh_p5I8&feature=channel_page
Created Date: April 29,2009
Created by: jaspersk
Could you get a voltage to current transducer? If you can, build voltage dividers for each circuit and just connect the voltage signal (with your solid state output switching) to your transducer/transmitter. The voltage dividers would maintain your ground connection so the inputs wouldn't float. You would have to adjust your calibration to account for the volt drop through the SS output, but it would be repeatable (I would think).
Brian
Well, I could use a voltage input for that matter.
When you say voltage divider, are you just talking about a couple of resistors in series? I guess I was thinking that I didn't want a parasitic load to be on the pack all the time. I suppose I could use large enough resistors that the load would be inconsequential as long as I don't leave the pack resting for weeks on end. Is that what you were thinking?
Created Date: April 29,2009
Created by: bfitz
Well, I could use a voltage input for that matter.
When you say voltage divider, are you just talking about a couple of resistors in series? I guess I was thinking that I didn't want a parasitic load to be on the pack all the time. I suppose I could use large enough resistors that the load would be inconsequential as long as I don't leave the pack resting for weeks on end. Is that what you were thinking?
Yep. I didn't know if you needed a current input for your other items. I figured with a battery sized for an EV, the parasitic load of a volt divider wouldn't be a problem. Don't go crazy with the resistor values though. You need to take into account the input impedance of the voltage input/voltage transducer. You don't want the measuring device to "swamp " the divider circuit when it is switched in.
Brian
Created Date: April 29,2009
Created by: allukes
You could also use 1 relay between your monitoring circuit components and ground to float everything when the car is not in use. Because no path is found to ground, no current should flow through your resistor network and PLC modules to ground, however there would still be resistances across each battery, but these would be much higher than the path to battery ground and your currents should be very small if high resistance values are used. Its all a tradeoff as ususal cost/performance.