I believe a Green LED has a typical forward drop of about 2V (depending on the color = construction materials, different combination = different voltage) and the typical opto-coupler uses an infra red LED, which has a low forward voltage drop, just above 1V if I remember correctly. This means that the circuit will pass current (and the opto output will conduct) as long as the battery voltage is above about 3V. That may be too high for LiFePO4 cells if you like to discharge them to 2.5V (especially under load they may sag well below 3V even if they are mostly full, no point in constant false alarms because then you learn to ignore them). To lower the forward voltage drop you need to select an LED with longer wavelength (orange or red). You will need to do your own due diligence in finding the data sheets of the parts that you are ordering and verifying their actual voltage drop and temperature dependence (no point in your BMS failing when it is hot or cold) NOTE that there are dedicated 3-pin devices that are used for purpose of battery monitoring - they come in different voltages and switch the level of their output pin if the voltage between the other 2 pins crosses the threshold. You'd need an opto coupler to get the signals out from a high voltage pack, but the construction is super simple and has guaranteed correct threshold if you order the right parts...
The circuit is super simple: On each cell, you have a series connection of the (green) LED, a resistor and the input of the opto-coupler (which is another LED) The outputs of all opto-couplers are wired in series (and isolated from the inputs, as the inputs carry the HV pack but the output is connected to chassis and to the 12V battery to light a "GOOD" LED as long as the voltage across the series of all opto-coupler outputs is low enough. (The series string is connected to ground and to the GOOD LED via a resistor, the LED connects to +12) If the voltage across the series of opto outputs increases, an alarm is sounded (connect the alarm trigger input to the point where the series of opto coupler outputs connects to the resistor that goes to the GOOD LED). Hope this helps, Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: [email protected] Private: http://www.cvandewater.com Skype: cor_van_de_water XoIP: +31877841130 Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203 -----Original Message----- From: [email protected] on behalf of Peakfoto Digital Photo Still n Video Sent: Mon 3/3/2014 10:55 PM To: [email protected] Subject: Re: [EVDL] cell alarming circut. thanks for your help n ideas again. have a circut daigram for this ? Peakfoto Digital Photo Still n Video wrote: > Is there something cheaper simplier than 2-8 cell log. So I can monitor my > 22 LFP cell pack. For like if 1 cell drops below like 50% DOD or a choseable > voltage that alarm me while riding. I guess it depends on what you call "cheap" and "simple". eBay advertises some Chinese ones intended for R/C models that are dirt cheap. But you get what you pay for... Cedric Lynch made a very simple system for his lithium cell pack. Each cell had an LED, a resistor, and an optocoupler. The LED, resistor, and LED inside the optocoupler were wired in series. They were chosen so the optocoupler was "on" if the cell was above the desired threshold, and "off" if it went below that threshold. The isolated output side of all the optocouplers were wired in series. They controlled an LED and alarm buzzer. If all cell voltages were good, the "good" LED was on and the alarm was off. If any cell voltage dropped below its threshold, it broke the series chain, turned off the "good" LED, and sounded the alarm. The three parts cost about 30 cents. Cedric put each circuit in a scrounged prescription pill bottle, so they were free. It's hard to get much cheaper than this. -- Ingenuity gets you through times of no money better than money will get you through times of no ingenuity. -- Terry Pratchett -- Lee Hart's EV projects are at http://www.sunrise-ev.com/projects.htm _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA) _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA) _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
