led cell monitors

Just been thinking about making up some led monitors to put on cobalt oxide cells, has been suggested that a simple way would be use zener diodes so that leds switch on/off at given voltages, or a comparator circuit ( more complex).
I think it would be very useful to have an overvoltage (when charging) and undervoltage indicator on each cell ( though alot of work to set up), would certainly make for a safe system and easy diagnosis of a faulty cell.
I'm guessing a bit but overvolt led lights at 5v? and undervolt an led lights at 2.4v?? only for cobalt oxide i'm thinking here ( same could apply to iron phosphate but different voltages).
anyway would anyone have any clue on how to use zener diodes to make an led turn on when cell goes down to 2.4v or if it goes over 5v when charging. I would imagine two leds on each cell would do the job.
recently got a stack of old cyclone lithiums , the cells are fine but the bms boards are very bad, so intending on puting some better bms boards on them and puting to good use. I've got quite a few cells here, which are 5ahr in metal cases, if anyone is looking for cells ( cobalt oxide give me a yell) the person who had them is looking to get a bit of money back on a bad investment in these batteries, he would be happy with about $220aud per 24v 15ahr pack, but the bms boards are not good, very dodgey.
Cells seem fine, charge up fine, some seem to have undergone some gasing as the cases have buldged out slightly, but dont seem to have lost capacity at all.
Just a note on the lithiums I was selling on my website, you guys were right , the damn company was selling me cobalt cells and telling me they were iron phosphate..........have been sourcing from different companies, there is an awful lot of new companies springing up making iron phosphates in china, prices seem to range from $1.60usd/ahr to over $2usd/ahr.

Yep.. alot of mumbo-jumbo in the battery world... finding good quality packs is no easy task.

I'd love to see a universal LED setup that could be ajusted with simple Jumpers, kind of like you swap Master/Slave/CS on PC hard-drives..

Having the option to set the rig up for SLA, Nimh/nicad , Lithium-x, High-Low V .. even an audible warnnig would be cool.. RC guys do this i think.

I've been thinking about doing something like this too, SolarBBQ.

I came up with two ideas - the LED idea, and a more complex system which involves multiplexors, a 555 timer and basic microcontroller (like a PIC - or maybe just an expensive PIC and no timer or multiplexors) and an LCD display. I'm leaning more in favor of the PIC. Among other things, it's just cooler. But I was thinking it would be neat to log the voltages measured over time... just for the fun of it. Plus I just understand microcontrollers better and feel more comfortable with them.

I bought a breadboard and LCD display and I've been sketching out circuits and reading up on PIC's.

Let me know what you end up doing, and I'll keep you in the loop on my system as well.

Cells seem fine, charge up fine, some seem to have undergone some gasing as the cases have buldged out slightly, but dont seem to have lost capacity at all.

Click to expand...

Cell "puffing" (using the term from the RC world) is a very bad sign. It means that either the cell overheated, or it was overcharged or it was discharged at a higher rate than it's spec'd for (which led to overheating). Cell puffing is not a danger in and of itself - meaning a puffed cell is not more likely to explode than a non-puffed cell. But it's a sign that something in the system isn't working right (either cooling, charging or discharging). And it will lead to greatly reduced lifespan - you may not be seeing a capacity loss so far... but you will.

Just a note on the lithiums I was selling on my website, you guys were right , the damn company was selling me cobalt cells and telling me they were iron phosphate..........have been sourcing from different companies, there is an awful lot of new companies springing up making iron phosphates in china, prices seem to range from $1.60usd/ahr to over $2usd/ahr.

Click to expand...

I had a feeling that there was something wrong - the cells weren't heavy enough and the voltage was too high. It's a shame that those cells were "too-good-to-be-true". Thanks for mentioning it though, so that others know to watch out for it.

Those prices you quote match the prices that I have been seeing as well for direct-from-the-manufacturer pricing on LiFePO4 cells as well.

oh just for the heck of it, I hooked up a 12v 17ahr sla battery, directly to one of the 3.7v 5ahr cells. Assumed it would just catch fire rather quickly,
but to my suprise the wires that connected the cobalt cell to the sla battery, burned out before the cell caught fire, I didn't put an ammeter on there, but there was alot of amps going thru the wire ( I would say minium 40amps ).
The cell was flat at the start, i did though go to touch the cell after i took the wires off ( approx. 30secs i let current go into it) but noticed the air around it shimmering, so it was damn hot, did'nt put my finger on it.
I took a video of it, nothing much to see though apart from the wires smoking.
Afterwards cell charged up per normal, looks ok, bizarre!! not major expansion of metal case, some minor bulging, certainly not what you would expect.

I'm disappointed no explosion? Where's the fun in that?
Try smacking a fully charged one with a hammer (to simulate a bike crash).

A zener diode can be used to turn on a led when the voltage goes above a certain level pretty easily.

Getting an led to come on when the voltage drops below a certain level takes more circuitry. I've seen tiny devices designed to do this. I'll have to look around and see if I can find one.

Another idea I had was instead of an led (which requires that you watch all of them), you could drive an optocoupler, and have all the optocoupler outputs in parallel, so if any one of the cells dropped below the threshold, it could trigger a single, large led, or shutdown the controller. With optocouplers, any number of cells could be strung together.

Brett,

This is future project of mine as well.

I would like to know if a single cell in a battery pack drops below a set voltage. Hopefully this will show up a damaged cell.
Could also be used along with the DrainBrain as a low fuel indicator.

The device would plug into the balance taps plug on the battery pack and set off a light on the handle bars

Here are a couple of Low Voltage warning LED circuits and devices I have found hunting around on the web.

http://www.webx.dk/rc/lipoly/low-volt-alarm.htm
http://homepages.paradise.net.nz/bhabbott/lvw.html

http://www.dimensionengineering.com/CellShield.htm
http://www.bphobbies.com/view.asp?id=V936613&pid=EF-LVD-1T

and also a thread started by Reid Welch over at rcgroups,

http://www.rcgroups.com/forums/showthread.php?t=658784

Now I need to go away and workout what a Optocoupler is. :?

Greg

Here is picture of LED on battery box from Crystalyte

Think LEDs are battery strength meter? Never charged, or did anything with these batteries. Just sold them all. Go here for details
http://endless-sphere.com/forums/viewtopic.php?t=1112

thanks greg ( tailwind ) excellent leads there, enough info to get me started!!

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just a thought: throttle leds for battery voltage, already have three leds ( sometimes 4),
and all set up with nice circuit board, adjusting resistors to suit voltage cut off of lithiums would make for a nice unit for lithium cells.
Looks like they use a 5v reference voltage from controller, may or may not work not sure.