LiFePO4 questions

[jdcburg]

Hi Folks – I have a pair of Thundersky 4-cell 20ah packs from Elite Power Solutions that power a 24v 350W brushed Unite on a shift kit mtb. I have done a search here and looked through the Thundersky pdf but I can’t find all the answers to these questions. I would like to know the voltage numbers for LiFePO4 at the particular points that are mentioned frequently, ie, full charge (3.60v?), the point where the discharge curve flattens (3.40v?), the lower point where the discharge curve gets steep again (voltage number and/or % DOD?) and the point beyond which one should never venture. I do not currently have a BMS but I do have a celllog 8M that can be programmed to alarm at any voltage. Also what is the highest the packs should be charged? Do the numbers vary by brand, age of the battery or other factors? Is it better to use Ah or Wh rather than voltage? Thanks again for sharing your expertise - jd

 

[morph999]

I have the same Thundersky cells. What is the amps that you are pulling from the batteries? If it's 20 amps that you are pulling max then you are only pulling 1C from the battery and from what I read, you'll have a very flat discharge curve if you only pull 1C so you can program the celllog 8s monitor for a 2.8v LVC and then maybe something like 4.15v upper voltage cutoff. I charge mine to about 3.65v - 3.75v. Anything under 3.8v is fine from my experience so far.

Below 2.5v, the voltage drops like a rock as you can see on almost any lifepo4 discharge graph. YOu have only 24v ? Have you thought about just getting 8 small chargers and charging them all at once. That'd be the ultimate balancing technique. Each cell would have it's own individual charger. These are out of stock right now but it says June 10th so maybe he's almost got them back again. http://www.all-battery.com/TenergyLiFePO4BatteryCharger01300.aspx

Voltage is the #1 thing you want to look at to keep your batteries protected and what usually ruins batteries is voltage sag. If it sags below 2.5v while you have the throttle on full then it will damage your batteries. You can use as much AH of the battery as you want but keep in mind that the 2000 cycles is for people using 80 % of the battery or less. I only use 50 % of my battery but I'm pulling 2C (40 amps) out of it and so I'm pushing my cells to the limit of what they are capable so I don't pull more than 50 % because I'm worried about damaging them but if you only pull 20 amps out of these, you can use up to 90 - 95 % of it as long as you keep it above 2.5v per cell.

Might want to put on a piezo alarm on the celllogs unless you think you can hear the alarm okay how it now. I used to be against having a BMS but I have wised up and seen the error of my ways. I tend to be pretty stubborn so maybe you read an older post of mine. You can still get away without a BMS if you are extremely careful but it's not a good idea.

 

[jdcburg]

According to my WattMeter 28 amps peak (1.4C), but mostly it's under 20 amps. Longest ride so far - 20.5 hilly miles that used 12.85 ah. All cells were 3.25-3.26 at the end after resting a few minutes. I don't think I'll be discharging much more than that except for testing purposes or maybe special occasions. I would like some hard numbers for the "knees" if they are available

 

[The Journey Guy]

You can still get away without a BMS if you are extremely careful but it's not a good idea.

If you use a CellLog 8* with an array of single cell chargers, topped off by a CA, then in effect, you have a BMS, yes?

I cannot think of one thing that a BMS can do, that any combination of the above 3 things cannot match.

TJG

 

[katou]

Can't cut off electricity from the pack. If you ignore the warning.....

I don't trust myself, so I know I need something to cut the power and then refuse to turn it back on.

That said, the cost of a BMS for big current is prohibitive (IMHO). Here's what I was thinking.

Circuit 1: wide open power from the battery, will supply as much as controller/ca/wires can deliver. No voltage monitoring at all.

Circuit 2: BMS limited max amperage, with hard cutoff at LVC

Switch between the two. For max fun, activate #1 for a short bit, then cut back to #2

Maybe a BOOST button that is intermittent? Then it only works while pressed.

That's my best shot right now.

40a BMS is $90
http://www.batteryspace.com/pcmwithequilibriumfunctionfor16cells512vlifepo4batterypackat40alimited.aspx

60a is 170
http://www.batteryspace.com/pcmwithequilibriumfunctionfor16cells512vlifepo4batterypackat60alimited.aspx

and 76v at 100a is $260
http://www.batteryspace.com/pcmwithequilibriumfunctionfor24cells768vlifepo4batterypackat100alimited.aspx

Aye Carumba!

Katou

 

[The Journey Guy]

Can't cut off electricity from the pack. If you ignore the warning.....

I don't trust myself, so I know I need something to cut the power and then refuse to turn it back on.

There are a couple of ways to cut off electricity from the pack. If you have a programmable controller, that is one way. You can also use the CellLog 8's alarm port to cut off the voltage from the pack. One way of using the CellLog would be to hook the alarm output port to a relay which is normally closed, then when activated would go to an open state, cutting off pack voltage. There are other options too, you just have to be creative.

My point is that there is a way to do everything a BMS does, AFAIK. If I'm mistaken about that, then I am more than ready to be schooled and learn. I'm always ready to learn!

TJG

 

[morph999]

oh...I mean BMS as in something like Celllog monitor. I'm not talking about balancers or any kind of charging method. I agree, I think balancers are overrated. I charge mine in groups of 4...one 4-cell pack at a time and the electricity goes to all 4 cells without any balancers needed. A celllog monitor is very useful especially in the first week when you don't know what the batteries are capable of but after you find out where the limits are, it kind of becomes less useful.

Also, the more batteries you have, the more valuable a BMS becomes but with 24v, with one signallab BMS being the same price as replacing an entire Thundersky pack, I'd stick with the $15 celllog monitors like you are doing now.

 

[The Journey Guy]

oh...I mean BMS as in something like Celllog monitor. I'm not talking about balancers or any kind of charging method. I agree, I think balancers are overrated. I charge mine in groups of 4...one 4-cell pack at a time and the electricity goes to all 4 cells without any balancers needed. A celllog monitor is very useful especially in the first week when you don't know what the batteries are capable of but after you find out where the limits are, it kind of becomes less useful.

Also, the more batteries you have, the more valuable a BMS becomes but with 24v, with one signallab BMS being the same price as replacing an entire Thundersky pack, I'd stick with the $15 celllog monitors like you are doing now.

Ah, I get you now! And I agree with you in that this is effective only for smaller packs. But, that's what we were talking about, 24v packs, and as you say, for that application, a CA and CellLog and charger array will do quite nicely. In fact, I think they do a much better job that relying upon a BMS that could go out at any time(which a LOT of them do for some reason).

TJG

 

[jdcburg]

I think I will look for a relay that will be triggered by the celllog alarm circuit. I have an inexpensive controller from TNC. I'm thinking I could plug a relay into the ignition plug. I now shunt across that for the controller to work, so I would want a relay that opens on alarm? Or should I use the brake connector? If so, would I want a relay that closes on alarm? What should I set the low cell alarm to? Thanks - jd

 

[The Journey Guy]

I set my low cell voltage value to 2.8v . In real life, the cell can discharge a bit lower than that without ill effect, but I want to keep cells and pack in top condition, so I don't push the ratings. That also means that I set my low pack voltage to 22.4v, again a very conservative value.

If you use the relay at the shunt, then yes, you would want a NC (Normally Closed) relay. You were also correct that if you use the relay on the brake circuit, then you want a NO (Normally Open) relay.

TJG

 

[jdcburg]

TJG - Are you familiar with the celllog 8m? It has 2 alarm output settings - NO and NC. Could I plug the alarm output set to NC directly into the ignition plug on the controller and use the alarm as an LVC? I have included 2 images from the celllog manual below, if that helps. I can probably find a relay at Radio Shack if necessary. Thanks - jd

 

[The Journey Guy]

I believe the most the CellLog 8M or 8S can handle thru the alarm port is 50v @ less than 500ma. So, to answer your question, bottom line is 'No'. If you are only running a 24v pack, then the voltage is within limits, but the amps passing thru the CellLog going to your motor would fry it in a heartbeat!

What you basically want to do is this: If you are using it on the controller you want to use the alarm port as an On/Off switch for the relay. You could get a relay that is low voltage to actuate, and connect the + side to your power source (small battery, voltage taken from the pack, etc..) and then run the - side to the CellLog alarm port. You just cut the - wire, and one side connects to one pin of the alarm port, and the other side connects to the other pin of the alarm port and then continues on to the power side of the relay. Set the CellLog to be NC at the alarm port, and you will have that part finished.

Then connect the larger side of the relay, the side that will actually let the battery pack volts and amps to pass through. Again, take the - side from the pack, attach to one side of a NC relay. Attach the other side of the - lead to the other side of the relay which continues on to the controller/motor/whatever, and you're done. Bob's your Uncle!! :lol:

In operation, what happens is this: In normal operation, the relay is in it's NC condition, which means the - side of the pack is letting voltage pass through, and on to the controller, motor, or whatever. If for some reason the CellLog goes into an alarm condition, that alarm port goes to an Open condition, and the Relay will also go to an Open condition, cutting voltage off from the pack.

Hope I explained that ok. I'm certainly not a good instructor! :?

TJG

 

[katou]

Interesting. I was not aware that the Celllog would activate an EXTERNAL alarm. That opens up a great number of options. Maybe just apply the bare wires to a bandaid and tape to skin? Probably hard to ignore that one.

Seriously though, that is excellent information, and you idea of triggering a relay is an excellent one. I wonder if they make appropriately sized relays for our purposes? It would need to be rated to handle full pack amperage, say 100a.

Katou

 

[The Journey Guy]

Interesting. I was not aware that the Celllog would activate an EXTERNAL alarm. That opens up a great number of options. Maybe just apply the bare wires to a bandaid and tape to skin? Probably hard to ignore that one.

Seriously though, that is excellent information, and you idea of triggering a relay is an excellent one. I wonder if they make appropriately sized relays for our purposes? It would need to be rated to handle full pack amperage, say 100a.

Katou

I like the piezoelectric bandaid idea, excellent! :wink:

Oh yeah, the relays are out there. Consider an Auto starter relay, the kind that used to mount on the fenderwell under the hood. A small amount of voltage and amperage operated the relay, but the actual battery side of the relay could pass a LARGE amount of amps to the starter motor.

I would say do a few minutes worth of searching, and you could find a relay to suit your needs.

When you find one, let us know the link. There are probably more people that just us three wanting to do this sort of thing. Matter of fact, I know there is another thread about doing this exact thing, but i'm on my pain meds right now, and not thinking real clear.

TJG

 

[jdcburg]

Thanks for the info. There must be a relay inside the controller as the tiny wires leading to the ignition plug would never take the current either. I just tried the probes of my digital multimeter in the ignition plug. The voltage is 27.02. The DC milliamps on the 200 ma setting read 37.9-38.0 whether the motor is turning under power or not. So I guess I can use a pretty small relay? - jd

 

[The Journey Guy]

Thanks for the info. There must be a relay inside the controller as the tiny wires leading to the ignition plug would never take the current either. I just tried the probes of my digital multimeter in the ignition plug. The voltage is 27.02. The DC milliamps on the 200 ma setting read 37.9-38.0 whether the motor is turning under power or not. So I guess I can use a pretty small relay? - jd

You know what? I completely mis-understood your question. The pain meds do that to me sometimes. OK, revised answer for you, in your particular situation. With those values you said of 27.02v @ ~38Ma, then the answer is YES. Just run one side of the ign switch through the alarm port, set to NC, and you're good to go!

sorry for the confusion.

TJG

 

[jdcburg]

Well THAT didn't work! Apparently the black and red wires coming from the alarm port actually mean they are + & -. Also the wires going to the ignition plug are +&- although they are the same color. When I plugged the black wire in, the sparks flew! The bike seems to work so I didn't burn up the controller and the celllog still works but I bet I blew out the alarm circuit. I'll do some more testing and let you know what happens - jd

 

[The Journey Guy]

Well THAT didn't work! Apparently the black and red wires coming from the alarm port actually mean they are + & -. Also the wires going to the ignition plug are +&- although they are the same color. When I plugged the black wire in, the sparks flew! The bike seems to work so I didn't burn up the controller and the celllog still works but I bet I blew out the alarm circuit. I'll do some more testing and let you know what happens - jd

Hi JD. If you'll look at the schematic that came with the CellLogs, you'll see that the alarm output leads, the black and red leads, come from a transistor. Think of the transistor as a switch. There are only two ways a switch can be, on or off. As you already know, you can set whether or not the switch is on or off in the System-Alarm setting. There is no voltage being supplied from the switch, it only completes a circuit or opens up a circuit.

When you say "ignition plug", I assume you to mean an ignition switch, similar to what is in a car. Am I correct when I'm thinking that the key in that ignition switch is either on, or off? You turn the key one direction, and the bike lights up. You turn the key in the other direction, and the bike shuts off. Am I correct there?

I'll wait for your answer, and we'll take it step by step from there.

 

[jdcburg]

Nope, it's a socket that takes a 2-pronged plug. I assume an ignition lock can plug into it. I shunt across it with a bent paper clip to activate the controller. That's the circuit that I gave you the 27v/38mA figures for. The celllog directions say it "short circuits" when the alarm is activated so I assumed it would shunt across in the NC position. Silly me! off to work now. Will write more later - jd

 

[The Journey Guy]

They say a picture is worth a thousand words, so I made a quick test circuit so that you can see what I'm trying to say.

I used some 28 gauge wire wrap wire to attach to pins 9 and 6 on the circuit board inside the CellLog where the balance plug comes in. That gives me ~10v for my piezoelectric alarm buzzer (1v-10v limit) when the CellLog is plugged into a pack. If you look closely at the top right corner of the CellLog just above the usb port, there is a nice exit hole for the wires to come out at. So, I now have a + lead coming from pin 9 and a - lead coming from pin 6, leading outside the CellLog.

As you can see in the photo, the + lead from pin 9 goes to the + terminal of the buzzer. The - lead from pin 6 goes to the black wire of the alarm port lead. It really doesn't matter which wire you use, red or black, I just happened to pick the black wire.

Now, internally, that black wire goes to the transistor that acts as a switch. I have the Alarm set to being NO, meaning the switch is open, and will not complete the circuit. The other side of the transistor (switch) coming from the alarm port is red, and it continues on to the buzzer - terminal.

When things are right with the pack, the transistor is open, and no voltage gets to the - pin of the buzzer. When things are out of spec with the pack, the transistor goes into alarm mode, and the switch is closed. The buzzer circuit is then closed, and voltage gets to the buzzer, making it buzz.

Hope this helps a bit with what I'm trying to express to you.



I left the photo in a large size so that you can see the detail. Just click on the thumbnail to see the original size photo.

TJG

 

[jdcburg]

TJG – Thanks for helping me with this. Things are becoming a little more clear now. First let me tell you what happened then I’ll tell you my new plan, if everything is still working. I have a JVC controller that has a bunch of plastic connectors coming out of it. When I first got it, I hooked up the battery, motor and throttle ones and found that I had to put a bent paperclip shunt in both holes of the one labeled “ignition” in order for the controller to be activated. I also found that if I did the same with the one labeled “brake” that it cut the power to the controller/motor

I don’t read schematics and I don’t do that well with Chinglish. When I read the instructions (see one of my earlier posts) about the “short circuit,” I thought the alarm circuit would make a shunt across the red and black wires, depending on the NO/NC setting and alarm state. First I tried attaching the wires to the brake connector in the NO position. The motor ran normally. Then I switched it to NC, which I thought would cut the power, but it didn’t. So then I decided to try connecting the wires to the ignition plug while the celllog was still set at NC. I randomly tried to plug the black into one side of the ignition plug. I happened to pick the + side of the ignition plug and because the celllog was NC, it created a circuit to the neg side of the battery. I jumped pretty good. I tried it the other way but it doesn’t work either.

I would rather have an automatic LVC than a buzzer, so I’m going to keep trying. Let me run this by you. First I’ll check the black wire for continuity to neg in NO and NC. Should be infinity in NO and 0 in NC, right? Then I’ll check continuity of black to neg while an alarm goes off. If that checks out, I’ll see if there is any voltage between neg and each of the connections of the brake connector. If there isn’t, I connect one side to a neg terminal and the other side to the black wire of the alarm. With it set to NO, it should shut down the controller when an alarm goes off, correct? Thanks - jd

 

[The Journey Guy]

TJG – Thanks for helping me with this. Things are becoming a little more clear now. First let me tell you what happened then I’ll tell you my new plan, if everything is still working. I have a JVC controller that has a bunch of plastic connectors coming out of it. When I first got it, I hooked up the battery, motor and throttle ones and found that I had to put a bent paperclip shunt in both holes of the one labeled “ignition” in order for the controller to be activated. I also found that if I did the same with the one labeled “brake” that it cut the power to the controller/motor

I don’t read schematics and I don’t do that well with Chinglish. When I read the instructions (see one of my earlier posts) about the “short circuit,” I thought the alarm circuit would make a shunt across the red and black wires, depending on the NO/NC setting and alarm state. First I tried attaching the wires to the brake connector in the NO position. The motor ran normally. Then I switched it to NC, which should have killed the power, but it didn’t. So then I decided to try connecting the wires to the ignition plug while the celllog was still set at NC. I randomly tried to plug the black into one side of the ignition plug. I happened to pick the + side of the ignition plug and because the celllog was NC, it created a circuit to the neg side of the battery. I jumped pretty good. I tried it the other way but it doesn’t work either.

I would rather have an automatic LVC than a buzzer, so I’m going to keep trying. Let me run this by you. First I’ll check the black wire for continuity to neg in NO and NC. Should be infinity and 0, right? Then I’ll check continuity of black to neg while an alarm goes off. If that checks out, I’ll see if there is any voltage between neg and each of the connections of the brake connector. If there isn’t, I connect one side to a neg terminal and the other side to the black wire of the alarm. With it set to NO, it should shut down the controller when an alarm goes off, correct? Thanks - jd

Morning, JD. No problems helping you, as much as I am able to. I'm not an instructor, so at times I have difficulty expressing what I'm trying to say. Anyway, here we go.

First, it's a personal thing, but I don't use the word 'shunt'. I would rather use the word 'switch', because to me, that represents a means of opening and closing a leg of the circuit. In our case, we want to open or close a particular wire in the circuit. That's where the CellLog alarm port comes in. It is a switch.

It either opens the wire so that no voltage goes through it, or it connects the two sides of a wire, so that voltage does go through it. So, let's refer to the alarm port as a switch for the sake of this conversation, yes?

The paper clip you used on the 'ignition' plug was another example of a very basic switch. A manual switch, but a switch none the less. When you put the paper clip into both ports of the 'ignition' plug, you were using your 'switch' in the NC position. Right? Right!

If you remove the paper clip, the 'switch' then goes to the Open state, and the controller shuts down.

So in your situation, you want to replace your 'paper clip switch' with the CellLog's 'switch' ( the alarm port ).

In order to do that, you need to attach one of the 'ignition' plug's wires to one of the 'alarm' wires, let's say the black wire. You then attach the other 'ignition' wire to the other 'alarm' wires, the red wire.

Now, set the CellLog to have a NC alarm port. Now you have replaced the paper clip, which was completing the circuit, with the alarm port, which is doing the same thing, completing the circuit. Your controller should work normally this way.


OK, so now the CellLog is hooked up to the 'ignition' plug, and all is working as it should. You said you want the CellLog to act as a LVC, and it can do that very well. Double check the values you have the CellLog set at for LV situations. Low cell voltage, low pack voltage. Have them both set to what you want them to be at.

Now double check your bike, and make sure all works as it should. If it does, then it's time to test the CellLog to see if it will do it's job.

The way I test for that, is to go back and change one of the LV values. Use the pack voltage for this test. Change the value to a value that you know for sure is above what your pack is reading now. ( when you are changing values, it's best to do it with the CellLog powered by the usb plug, and not have it attached to the bike at all). When you have changed the Pack LV value, re-attach the CellLog to your bike, and it should go into an alarm state almost immediately. The 'alarm' switch will open, and your 'paper clip' will have been effectively removed from the 'ignition' plug because the 'alarm' switch went to an Open condition. The controller should go off, and you have your LVC circuit complete.

Give that a try, and let me know the outcome. Also, this may not need to be said, but just to cover all bases, when you are doing your testing, be sure that the balance plug is also attached to the CellLog. Seems obvious, but like I said, just covering all bases.

TJG

 

[jdcburg]

In order to do that, you need to attach one of the 'ignition' plug's wires to one of the 'alarm' wires, let's say the black wire. You then attach the other 'ignition' wire to the other 'alarm' wires, the red wire.

That's what I thought as well. But if you look carefully at the schematic, you'll see that the switch (transistor) is not between red and black but between black and ground. So when I set the celllog to NC and plugged the black wire into the (unknown to me) + side of the ignition plug - poof!!! Now I see that, before I didn't.

I think what is needed is a relay that is switched by the celllog. If I didn't destroy the circuit, could I get a NO relay and (pardon my broken electro-speak here) hook the "switching on-off" side to the red and black celllog alarm wires and the "switched by the relay" side to the brake plug, or else do the same thing with a NC relay and the ignition plug? Thanks - jd

 

[The Journey Guy]

That's what I thought as well. But if you look carefully at the schematic, you'll see that the switch (transistor) is not between red and black but between black and ground.

If that were true, then the test circuit I built as an example for you would not work. If what you are saying is true, then nothing would ever come out of the red alarm wire, because you have said that the black side goes directly to ground when the alarm is in a NC condition. There would never be a need for the red wire, if that were true. All that the alarm would do is to take the input signal, voltage, or whatever, and connect it directly to ground when it went to a NC condition. Nothing out on the red wire at all, at any time.

But as I said, I'm not a good instructor, and I'm not doing a very good job this time either. So, I'll leave this as is, and maybe someone else that is better at this than I am will step in and make it all clear, to the both of us.

In the meantime, if you come up with a solution that works, please let us know what it is, yes?

TJG

 

[jdcburg]

I'm afraid my alarm circuit is burned up. Can you do some testing for me please? From my reading of the schematic, there should be a voltage reading across the red and black wires when in NC and none when in NO. Likewise there should be 0 resistance between the black wire and the neg battery terminal when in NC and infinity when in NO. If what you say is correct, there should never be voltage across the red and black, just a change in resistance from 0 to infinity when switched from NC to NO.

I get no voltage across the red and black in either position but also no change in resistance across the red and black and also across black and neg on the battery. I do get 3v from red wire to batt neg. That makes me think there is a constant 3v in the red wire and the switch is between the black and neg. Whaddya think? - jd ps I've ordered a new celllog. next time I'll be more careful