Low-voltage notification

[CGameProgrammer]

I think a LVC system is more complicated than what I want, but rather than be totally blind to the status of each cell, I'd like an LED array to light up when a cell goes low. Specifically, if I have N cells in series, I'd want N red/yellow LEDs such as these. Each LED should be off when the cell voltage is greater than X, yellow when it's below X but greater than Y, and red when below Y. Obviously this is N independent circuits; one LED circuit for each cell.

In my case I'm dealing with A123 cells (max voltage ~3.7V) and X may be 3.0V with Y being 2.5V, as an example. The idea is I head home or find a place to charge if a cell starts getting below X, and manually unplug the subpack a cell is in if it goes below Y.

Anyway, how complicated would this circuit be? What resistors would I need? I know nothing about analog circuits. I'm not even sure how to mount all this together. I assume there's nothing on the market already that works similarly.

 

[dogman dan]

Sounds good, but the first led would have to be pretty conservative, or else the time between led one and led two might be really short, like seconds. In a way, I would be handier if lifepo4 had a little more gradual a curve, but it is nice to have full speed for the entire ride too! Yesterday while riding to cutoff, I actually felt a cell group go off the cliff, I thought, hey I'm slowing do.... and before I could finish the thought, the pack cut off. About a half a second.

 

[CGameProgrammer]

Yeah you might be right. I'd either have to make the yellow LED turn on at a somewhat high voltage, possibly even more than three volts, or just omit it altogether and only use red LEDs.

What would the circuit be like for just a red LED turning on at say 2.7V?

Oh, and obviously the LED turns off when voltage goes below 1.9V just because that's the LED's limitation, if it's powered by the cell itself.

So another option is a more centralized approach where a separate 12VDC line powers the LEDs. But of course again I have no idea how to do this.

 

[slayer]

very good idea cc,
i made a 12 volts 4s 1p pack out of my spare cells and when i run a 12 volts halogen light i monitor each cells and only one of them is sagging a lot like going down to 2.8 while others stay at 3.3 and it will gradually go down to 2.5 volts over a period of 30 minutes, i did not want to go less than 2.5 volts cause i was happy with the 30 minutes dischaged time of the light and did not wanted to see my weak battery go down the cliff suddenly and loose it.

But knowing that maybe 3.0 volts and 2.5 volts would be great.

 

[slayer]

another thing i was thinking is, since most of the time there is only one or two that will sag you can just monitor that one or two on the handlebar with a simple voltmeter than you can decide with experience when to stop. cause the other healty batteries you don t really need to know they are doing well... just the bad ones and it is easy to know that just a couple of discharged test. well anyway this is just theoritacally i am going to try this with my pack when it is done.

http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=9017

 

[GGoodrum]

I think this will be more complex than simply cutting off the controller when one starts "off the cliff". The fact is if you have fairly healthy cells, and have multiple a123 cells in parallel, you wont ever see the LED turn yellow. I have seen 4p packs pull 90A and never drop below 3.1V per cell. You can do that all the way until the end, when the cells will dump in a hurry. The whole point is that you are trying to stop the cells from killing themselves, and you need to do it in a hurry. By the time you feel anything, and then look at the LEDs, chances are it is already too late. With a123 cells in particular, the discharge curve is pretty flat, and gets even flatter if you have a good number of them sharing the load. If you remove the load quick enough, when the first one is starting off the cliff, that empty cell's resting voltage will likely still be well above 3.0V. If the resting voltage ever drops below about 2.0V or thereabouts, the cell will start losing capacity, proportional to the amount under 2.0V the cell ends up at. I've recovered cells that have been between 1.0-1.5V, but they end up losing about 10-15% of their capacity. Cells that have been in the 1.8-1.9V range have lost about 5%. In all cases, the cells lose the ability to hold a surface charge at all, something a healthy a123 cell will do quite well. These damaged cells I call "stressed" cells. I have used stressed cells in combination with healthy cells, in lots of my a123 packs, but I'm in the process of reconfiguring all my packs and I'm weeding out the stressed/lower capacity ones for the new packs. I have a growing pile (50-60...) of these.

Anyway, the basic LVC circuit is three parts, a TC54 voltage detector chip, a resistor and an opto chip. Doesn't get much simpler than that. The opto outputs can be ganged together and tied directly into your controller to cut power if a cell starts to "jump".

-- Gary

 

[slayer]

Oh well good old Gary bringing us back to reality ...loll
maybe i will understand enough to try to build one of your lvc oneday but for now i am sorry especially the bms is way out of my skills

 

[CGameProgrammer]

Well what inspired this is I did have 6p go bad; one went down to 0 volts (I must have damaged it a while ago via short-circuit; there was physical damage evident) and dragged the five in parallel with it down to 0.1V. And for all I know there may be more cells damaged in such a way (though definitely less severely) or damaged through heat from soldering... I don't know. And if it does happen, the LEDs show you which one went bad. But in any case the discharge probably is too sudden for it to be useful.