Thunder Sky + Fechter/Goodrum BMS + PakTrakr + Zaurus = ???

AndyH

AndyH

10 kW
Joined
Jun 9, 2008
Messages
547
Location
San Antonio, TX, USA
This morning's quiz is: What do you get when you combine a 21-cell pack of 60Ah Thunder Sky cells, a Fechter/Goodrum BMS kit, a PakTrakr, a Sharp Zaurus, and an assortment of wire, solder, and shrink tubing?

Geek Art! - a fusion of solder, flux, matches on shrink tube, colorful squigglies, lithium, and Linux!

And hopefully an interesting view of what a BMS keeps from happening... :shock:

It started with this on one table:

View attachment bms.jpg

And this on another table:

View attachment lotsa_wires.jpg

The first one became this:

View attachment Complete_BMS.jpg

While the other, with the addition of these:

View attachment connectors.jpg

Became this:

View attachment fuse_blocks_installed.jpg
 
pgt400 said:
Andy, you do nice work! Question, did you have to modify the BMS to match the TS stated low and high voltage points (2.5 low, 4.2 high) ??
Click to expand...

These TS cells are LiFePO4-based, and like all LiFePO4 cells, has a CC/CV crossover point of 3.65-3.70V. No BMS changes are required.

Andy, I've not seen this PakTraker before. What exactly does it do?

-- Gary
 
The cells were charged with the T-S charger once but were not balanced. The first 3-mile ride with the default PakTrakr's 1-second data rate gave us a look at the 60Ah cells at rest and at about 1.6C spikes. The controller is set to 100A max and it worked pretty well - we saw a couple of 113A spikes that were quickly pulled back to about 98A.

(The PakTrakr's lithium sensors report voltage x100 and current x10.)
View attachment first_ride.jpg

Here's the T-S charge profile. I've confirmed so far that the charger will run for more than 8 hours and will continue the 2A pulses.
View attachment 2

Here's what charging looked like for this pack:
View attachment Initial Charge with Current2.jpg

The main charge is over and now it's doing the 2A 'charge blocks'. These cells can go up to 4.3V, but I want to keep them below 3.8V if possible. There's a weak teal line at 3.7V for reference. Notice how the pulse pushes the nearly charged cells into the low 4s but doesn't appear to do anything to the low cells. Notice also the gentle curve as voltage drops off after the charger stops pushing electrons.

 
But they do state a low cut off of 2.5v not 2.1? Are they being overly conservative?


GGoodrum said:
pgt400 said:
Andy, you do nice work! Question, did you have to modify the BMS to match the TS stated low and high voltage points (2.5 low, 4.2 high) ??
Click to expand...

These TS cells are LiFePO4-based, and like all LiFePO4 cells, has a CC/CV crossover point of 3.65-3.70V. No BMS changes are required.

Andy, I've not seen this PakTraker before. What exactly does it do?

-- Gary
Click to expand...
 
The plan was to connect the BMS into the 'daisy chain' so it's time to see if it works. The board was dumped into the bin and plugged in and the shunts immediately pulled the high cells back down to earth. So far so good!

View attachment 6

At this point, some of the cells in the pack were fairly well charged but more than 1/2 the pack was still pretty low. Pack voltage was about 73V and the charger stops at 75. The charger seems to have more brains than I did at 2:45 in the morning, so the best it would do was settle into it's 'cell balancing' pulse process. As we've already seen, without a BMS each pulse pushes charged cells to and above 4V while not appearing to do much for the low cells. I expect they're charging...slowly...but they're not making the lines move - we have to fix that! :wink:

The charger ran for just over 8 hours. I'll break the pictures up a bit so we can see what's happening.

(Remember that the PakTrakr doesn't log decimal points - current is displayed 'times 10'; voltage 'times 100'.)
View attachment 5

View attachment overnight_pack_Voltage.jpg

View attachment overnight_Remote_C.jpg

View attachment overnight_Remote_B.jpg

Finally some signs of life! Apparently, the T-S charger is programmed to just keep pulsing until eventually the low cells collect enough charge to move up.
View attachment overnight_Remote_A.jpg

View attachment close_pulses.jpg
 
pgt400 said:
Andy, you do nice work! Question, did you have to modify the BMS to match the TS stated low and high voltage points (2.5 low, 4.2 high) ??
Click to expand...

Thanks! No BMS mods yet. This is a V2.2 board with 1/2A shunts.

Thunder Sky's battery manual (http://www.thunder-sky.com/pdf/2007030222.pdf) says the cells can range from 2.0 to 4.3V. For 2000 charges, they say to keep things between 2.5 and 4.3. So far, it appears that the BMS will keep things below 3.9. The controller has a minimum pack voltage of 52.5V - 2.5V per cell. If the pack stays fairly balanced (once we get it there...), and with the maximum 1.6C discharges, I expect to hit the controller's LVC more often than hitting the BMS LVC.

That's the plan, anyway...we'll see if that's the way it happens in the real world. The next BMS version might be reset to 2.5V and will probably be built for a 1A shunt.
 
GGoodrum said:
...Andy, I've not seen this PakTraker before. What exactly does it do?

-- Gary
Click to expand...

Hi Gary,

The PakTrakr is a cell monitor that has a few alarm functions and spits out a serial data stream. http://www.paktrakr.com/

http://www.eghia.com/Bloghia/Entries/2007/6/24_The_Gauges_From_Hell.html and http://www.evconvert.com/article/dougs-paktrakr-with-zaurus is my longer-term plan for displaying the data.

Andy
 
Cool. 8)

What's the time base on those charging graphs? I'm trying to get an idea for the time between pulses there (or frequency).

It might take a really long time to bring the lowest cells up the first time. You could speed things by using a single cell charger on the low ones.

Once they all get equalized, the equalization time should drop quite a bit on the next cycle.
 
Interesting. If you looked at the voltage with an oscilloscope, my guess is it is actually oscillating at a much higher frequency. The PackTrakr samples once a second, so once in a while it will coincide with a peak and generate an artifact.

With large cells, depending on the charger voltage, you can sometimes see the orange LEDs flicker. With mine, the flicker is too fast to see and just looks steady.

The actual cell voltage when balancing is very 'noisy' and generally causes inaccurate readings with a digital voltmeter. If you stop the charging current, you can get an accurate reading.
 
Fechter - I think the charger's pulse is more like 2 minutes 'wide'. I'll verify that next time I'm sitting next to the charger.

Should/will/is the waveform from the 3.65-3.72V 'shunt turn-on' point be oscillating? Any thoughts on what the frequency range might be?
 
The larger the cell, the slower it will oscillate. 2 min would be much slower than my 10ah pack runs. With no cells attached, the upper limit on the frequency will be around 10khz.
I don't think the frequency is too important as long as it oscillates.

The maximum charger current will affect the speed as well. A higher charger current will pump the cells up faster and they will take longer to bleed back down. Is your charger running at 10 amps?

When the charge control circuit is on and you've reached threshold for at least one cell, the main LED should switch from red to green. When it's oscillating too fast to see, it just looks orangish. At a slower speed, you should be able to see it switch back and forth from red to green. So you're saying this is happening at about a 2 min interval?

Your graphs look much like the oscilloscope traces I saw on my pack, other than the speed.

An interesting observation from your discharge curves is that under high load, the cell voltage sags considerably. This is why we think the lower LVC voltage on the BMS will be fine. Once the load drops, the cell voltage bounces back up into the recommended range.
 
The charger will push 75V @ 15A all-out, but it's only 2A during these 'balancing pulses'.

I ran another 'overnight' charge thru the BMS last night. I caught one of the pulses this morning - 17 VERY bright orange LEDs and a fair amount of green on the 'charge control' LED. I don't think the charger to give me enough power to latch the BMS, but some of the pulses have pushed the pack over the 75V max listed on the charger.

I look forward to getting the pack balanced and taking another ride. I didn't expect to see quite as much sag from a 1.6C max discharge, but it's difficult to pull any one cell out of the 'colorful mess' of that discharge chart, and more than 1/2 the pack was at a much lower state of charge for that ride.
 
do you know if the PakTrakr will work with other lifepo4 cells like a123 and 40138 cells?

because i am thinking of getting one.


AndyH said:
The charger will push 75V @ 15A all-out, but it's only 2A during these 'balancing pulses'.

I ran another 'overnight' charge thru the BMS last night. I caught one of the pulses this morning - 17 VERY bright orange LEDs and a fair amount of green on the 'charge control' LED. I don't think the charger to give me enough power to latch the BMS, but some of the pulses have pushed the pack over the 75V max listed on the charger.

I look forward to getting the pack balanced and taking another ride. I didn't expect to see quite as much sag from a 1.6C max discharge, but it's difficult to pull any one cell out of the 'colorful mess' of that discharge chart, and more than 1/2 the pack was at a much lower state of charge for that ride.
Click to expand...
 
If they're all lit up, then you're pretty close.
Keep it cooking for a while longer and see if you can get it to latch.
It only takes one cell to be a bit low to prevent the auto shutoff.

The first balancing cycle on my pack took several days.
After that, it's now about 15 minutes.
 
what would cause a cell to be a bit low if lifepo4 does not suffer the same unbalancing effect that lead acid does?

i noticed the same thing happen on my a123 pack 4 cells had to charge slightly longer before all the lights came on.


fechter said:
If they're all lit up, then you're pretty close.
Keep it cooking for a while longer and see if you can get it to latch.
It only takes one cell to be a bit low to prevent the auto shutoff.

The first balancing cycle on my pack took several days.
After that, it's now about 15 minutes.
Click to expand...
 
ejonesss said:
what would cause a cell to be a bit low if lifepo4 does not suffer the same unbalancing effect that lead acid does?

i noticed the same thing happen on my a123 pack 4 cells had to charge slightly longer before all the lights came on.
Click to expand...
Hmm... I think they suffer from a different unbalancing effect then.
On my pack, I still have one cell that is consistently lower than the others. I think it was damanged in it's previous life.
 
OK - watching the TS charger whittle away at low cells with it's "2A for 2 minutes every 20 minutes" pulses is right up there with watching grass grow. I attacked a couple of cells with 2A voltphreaks chargers yesterday then did another overnight 8-hour charge with the Thunder Sky charger thru the BMS.

View attachment 2nd_overnight_RemC.jpg

View attachment 2nd_overnight_RemB.jpg

View attachment 2nd_overnight_RemA.jpg

The two lowest cells on Remote B weren't any better after all day in 'pulse mode' so I put them on voltphreaks chargers.

The BMS is doing a great job keeping the 'full' cells below 3.9. The time spent bending diode leads paid off! :lol:

Richard - with three low cells and 18 tripping the shunts, the bicolour control LED is easily 3/4 green, maybe more. Max pack voltage for this overnight run was 77.6V. As long as this charger continues to pulse as the last few cells come up, maybe it'll throw at least one pulse that will trigger Q2 and get the BMS to latch.

Back to watching the grass grow... 8)
 
ejonesss said:
do you know if the PakTrakr will work with other lifepo4 cells like a123 and 40138 cells?

because i am thinking of getting one.
Click to expand...

Hi Eric,

I have no doubt that it'll work. I don't know if it's the best way to monitor a 16S pack. I'm pretty sure it's not the least expensive!

What would you like to be able to do with the pack? Check-in on them from time to time and make sure they're doing OK, or monitor/datalog them constantly, or ?

The main reason I chose this is because I want some strong datalogging today from an off-the-shelf product. Longer term, I want to use my Sharp Zaurus as an 'instrument panel' to merge battery info with GPS and mapping data.

Andy
 
AndyH said:
Richard - with three low cells and 18 tripping the shunts, the bicolour control LED is easily 3/4 green, maybe more. Max pack voltage for this overnight run was 77.6V. As long as this charger continues to pulse as the last few cells come up, maybe it'll throw at least one pulse that will trigger Q2 and get the BMS to latch.

Back to watching the grass grow... 8)
Click to expand...

As long as the control LED is going part green, then it indicates you should have enough voltage at the charger. If the charger voltage is too low, it will stay mostly red. Do you know what the open circuit voltage for your charger is?

You might try testing the auto cutoff circuit if you haven't tried that already. Breifly jumper the all shunts line to the ground bus. This should make the control LED go green and latch until you disconnect the charger.

Juicing the low cells with a single cell charger seems like it should do the trick. I have seen bad A123 cells that just don't want to come up to voltage unless you really give them a lot of current. This might cause the cell to heat up, which could be bad. Their voltage drops right back to sub-normal as soon as you stop charging too. I threw these in a bin with a "?" on it. I do not have a complete picture of how bad cells behave and what all the failure modes are for LiFePO4 yet.

Since the low cells can only charge at 500ma and you have very large cells, it could take quite a while even with healthy cells. Once things get equalized, then it should be much faster from then on.
 
fechter said:
The larger the cell, the slower it will oscillate. 2 min would be much slower than my 10ah pack runs. With no cells attached, the upper limit on the frequency will be around 10khz.
I don't think the frequency is too important as long as it oscillates...
Click to expand...

It's definately oscillating! It might be getting some help from the charger... Here's a single 2 minute 'charge pulse' with 18 cells 'shunting' and 3 still low:

View attachment One_Charge_Pulse.jpg

And here are two pulses with no BMS installed:

View attachment Charge_Pulse_noBMS.jpg
 
fechter said:
I have seen bad A123 cells that just don't want to come up to voltage unless you really give them a lot of current. This might cause the cell to heat up, which could be bad. Their voltage drops right back to sub-normal as soon as you stop charging too. I threw these in a bin with a "?" on it. I do not have a complete picture of how bad cells behave and what all the failure modes are for LiFePO4 yet.
Click to expand...

Healthy a123 cells will hold a surface charge, up around 3.60V, for weeks, even months, but "stressed" cells will drop down to 3.35-3.38V pretty quick. Stressed cells will have lost about 10-15% of their capacity, and will have a reduced "C"-rating.

-- Gary
 
fechter said:
As long as the control LED is going part green, then it indicates you should have enough voltage at the charger. If the charger voltage is too low, it will stay mostly red. Do you know what the open circuit voltage for your charger is?
Click to expand...

The charger is labeled 75V 15A. I cannot get an open circuit voltage from the charger - it's 'too smart'. I have to connect the pack before I plug the charger into 110.

fechter said:
You might try testing the auto cutoff circuit if you haven't tried that already. Breifly jumper the all shunts line to the ground bus. This should make the control LED go green and latch until you disconnect the charger.
Click to expand...

The BMS works great on the bench. LVC kicks in on schedule, and the shunts range from about 3.65 to 3.72. Tripping the 'all shunts' line from any channel causes the BMS to latch green until it's unplugged. Life is good with the BMS.

fechter said:
Juicing the low cells with a single cell charger seems like it should do the trick. I have seen bad A123 cells that just don't want to come up to voltage unless you really give them a lot of current. This might cause the cell to heat up, which could be bad. Their voltage drops right back to sub-normal as soon as you stop charging too. I threw these in a bin with a "?" on it. I do not have a complete picture of how bad cells behave and what all the failure modes are for LiFePO4 yet.

Since the low cells can only charge at 500ma and you have very large cells, it could take quite a while even with healthy cells. Once things get equalized, then it should be much faster from then on.
Click to expand...

I think the 'smarts' of this charger might keep the pack from fully charging thru the BMS. As of last night the pack had 20 cells at or very close to 3.7V, and one cell still down at 3.35. I charged a bit without the BMS and the pack voltage climbed to over 82V, the higher cells climbed to 4.1. In this condition, the TS charger continued to cycle.

Connecting the BMS first pulled the high cells down to the 3.65 to 3.72 range for each channel. The charger would not 'latch' on and charge - not even to run it's normal 2A for 2 minutes pulses. Here's a short video clip of the way the charger behaves. It also shows the flashing shunt LEDs for the 'almost done' cells.:
http://www.rechargeablelithiumpower.com/media/TS_BMS.wmv

I'll finish the last cell with a single cell charger.

Once the bike starts collecing more miles, I should be able to use the TS charger for the bulk of the charge, and then switch to a fixed power supply for the final filling/balancing. That's plan one, anyway.

Maybe the TS charger can be modded to increase the power a few volts. There's a chance that this charger has already been 'tweaked' as a standard TLS60-15 is rated at 72.6V, while the TLS72-15 is rated at 87V.

Has anyone reverse engineered a TS charger yet? :wink:
 
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