current limiting on Curtis and TS cells

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current limiting on Curtis and TS cells

Post by Peter C in Canberra » Mon, 18 May 2009, 01:05

Hi,
While my car is off being inspected and hopefully approved by the engineer I am thinking about my first impressions of driving the car for the last few days with a permit to drive it unregistered.
I have a Charade with ADC 8", Curtis 500Amp controller and 45xTS LiFePO4 90 Amp hour cells with the Rod Dilkes BMS. Overall I am very happy with the performance; entirely adequate for an around town runabout car. If I put my foot down a bit hard, harder than generally necessary, I can get the alarm on the BMS to go off which indicates I have pulled enough current to get at least one cell to droop to 2.5V. This happens with battery current around 300Amps. This is more than the 3C they are rated for so I can't complain but it would be nice to limit things so this doesn't happen too often, especially with other drivers who might not back off immediately on hearing a beep. The battery volts at this point are about 125V (even with a nearly full charge) so the mean for the other 44 cells must be about 2.8V. This makes me wonder if one cell is a bit weaker than the others?

Anyway, The curtis controller has an adjustable current limit but this is a limit on the motor side current. I have seen elsewhere on the forum a comment that one could have much higher motor current than battery current in some conditions. I have a vague understanding about the controller acting like a transformer so that a high voltage, low current battery side can result in a low voltage high current on the motor side. So, I might not want to limit it to much less than the 500A max.   If things were simple and motor current equalled battery current I could just turn down the current till I couldn't get the BMS to trip. However, in the real world, would that have the power to the motor limited in some conditions when the battery was happy to deliver more?

Feel free to explain this all to me!
Thanks,
Peter C.
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current limiting on Curtis and TS cells

Post by coulomb » Mon, 18 May 2009, 02:28

Yes, the battery and motor amps are rarely equal. In fact, when one is high, the other is usually low. So limiting the motor amps is really not the solution.

I think you should think about a simple circuit that fiddles with the potbox voltage when you get an overcurrent alarm. I imagine a simple circuit (possibly affecting the top of the pot, or maybe the wiper voltage) that quicky reduces the voltage (hence current demand) when the alarm is on, and allows it to climb up at a slower rate. I think it would be good to have two trimpots to set these rates. You want the current to be cut back automatically and quickly when there is an overcurrent, and you want that effect to go away slowly when the alarm stops.

So let's say you floor it, and request 400 A. The battery grudgingly provides it for say 5 seconds, and you get good acceleration. Then an alarm comes on. In say a half of a second, you want the demand cut back to say half, or 200A. At that point, or somewhere between 400 A and 200 A, the alarm comes off. Let's say 300 A is what the battery can provide for long enough for 90% of your acceleration needs. The voltage that the controller sees ramps up towards where it was, but it takes say 1 second (4 times as long) to get to 300 A. At 320 A, the battery chokes again, so there is a reduction of current to 280 A, a ramp back to 320 A again, and so on.

If you get the time constants right, this could be smooth enough. (If you get it wrong, this could cause "kangarooing", sudden lurches of power and lack of power). The stability of this depends on many things, like inertia of the motor and vehicle, slack in the gears (if any) and diff, how the batteries respond to loads, thermal time constants, etc etc.

Weber and I have been contemplating a BMS that can send an "analogue badness signal", so that instead of this surging on and off, the system can come to an equilibrium. Unfortunately, we won't know whether this helps or isn't needed until the car is almost complete, and then it's no fun rebuilding 208 BMS modules and rewiring and reconfiguring.

So we're interested in this as well.

I guess you're pretty much committed to the "digital" (all or nothing) BMS boards.

Anyone with real EVs and Lithium batteries with BMS care to provide some real-world data on this?
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current limiting on Curtis and TS cells

Post by acmotor » Mon, 18 May 2009, 03:04

Guys, I am working with TS cells as well and I would note that the 2.5V min cell voltage refers more to SOC rather than voltage drop due to ESR when you draw high currents.
i.e. the guideline with most chemical batteries is don't over charge, don't discharge too far and don't make them too hot or too cold either ambient or by charge/discharge currents.
So a TS cell going to 2.5V at 3C needs to be considered in relation to its SOC.
BTW, if your TS are cold then 3C is more likely to pull them below 2.5V.

I would accept 2V at 3C to 5C if I know the cell is not at low SOC.
That does not rule out one cell being the weak link of course.
This is where a BMS with voltage monitor comes in. (or latching <2.5V indication)
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current limiting on Curtis and TS cells

Post by Peter C in Canberra » Mon, 18 May 2009, 04:27

It was fairly chilly though not below zero for the few days I got to drive on the roads last week. So, the cells would not have been particularly warm, except to the extent I might have warmed them being lead footed.
Once the car is back from the engineer I will be able to test further but my initial impression is that I could do nearly all my driving (once the novelty has worn off) below the point that was setting off the BMS alarm.
It is a relief to hear that 2.0-2.5V is not necessarily damaging if the state of charge is OK. Still, I'll try to treat them gently.

It sounds like there is no benefit to be gained by reducing the curtis current limit then? I think I should just instruct other drivers to back off if they hear the alarm when they are being lead footed and to get to a charging point promptly if it keeps going off with moderate driving.

For the moment I would not want to be changing BMS or anything else on the car unless really forced to do so by some impracticality. I'm hoping to have it up and running and not do much more work on it for a while (subject to the engineer being happy), aside from the last couple of planned jobs (some work on the heater that didn't work out as well as hoped, fitting a power-window kit: blasphemy probably, and a couple of cosmetic items).

As for Coulomb's suggested circuit, I guess the simplest to implement with the bits I have would be to use the alarm circuit (which also closes a relay) to pull in a resistor that is parallel to the pot box wiper. It would give a power cut, and it would be abrupt, and it would cut out immediately too. I would expect if one keep a foot on the pedal it would rapidly cut in and out as fast as the relays could switch. I wonder about the safety of having a power cut just when one might really want as much as possible for some emergency. If the alarm keeps going off on other drivers I might add a bright red light to the dash to light at the same time to reinforce the message. We'll see how it goes.

thanks again for the helpful comments.

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current limiting on Curtis and TS cells

Post by Squiggles » Mon, 18 May 2009, 04:45

Just have an ankle bracelet attached to a piece of wire in the car, make any other driver wear it and tell them that 3 seconds after they hear the alarm they will receive a small electric shock if they don't back off.
It is an electric car after all :)

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current limiting on Curtis and TS cells

Post by antiscab » Tue, 19 May 2009, 09:30

acmotor wrote:
I would accept 2V at 3C to 5C if I know the cell is not at low SOC.
That does not rule out one cell being the weak link of course.
This is where a BMS with voltage monitor comes in. (or latching <2.5V indication)


Ive been drawing my 40AH cells down to 2vpc (average, my paktakr reports a couple of them dip below 1v) under hard acceleration.
now its cold, it only takes 4C to do that.
when the batteries are warm, it takes more like 5C to do that (the wiring voltage drop makes my fancy new controllers low voltage limit a little inaccurate, on the conservative side).

its 2 months and around 2000km since i made the change. these batteries have already done 17'000km (my original range was 50km@80%), so i will see if capacity and ESR degredation increases in the coming months.

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current limiting on Curtis and TS cells

Post by coulomb » Tue, 19 May 2009, 14:05

Trouble with this is, you don't know if discharging below 2.5 Vpc is reducing life. Suppose it's from 8 years to 6 years; how would you know without a long, expensive experiment?. Maybe you don't care, but I'd like the maximum life.

As Weber pointed out to me in email, some part of the cell (perhaps near the terminals) is surely experiencing chemical reactions that are not good for its life. Think of the equivalent circuit not as a single ideal cell with a single internal resistance, but say four cells and resistors in a ladder. The closest cell to the terminals will experience the worse chemical reactions, and the innermost cell the least. The outer cell will get immediately charged back up from the other cells when the load comes off, though I don't know if this would really happen in an actual cell.

So it's a bit of a gamble.

Edit: make clearer re reduction of life not easily measured.
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current limiting on Curtis and TS cells

Post by Electrocycle » Tue, 19 May 2009, 14:39

you could install a Cycle Analyst - which apart from giving you a nice LCD screen with your power consumption, Ahs consumed, distance, and efficiency, can also do battery side current limiting by interfacing to the throttle.
You can then set your current limit to whatever you want on the fly.
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Post by Peter C in Canberra » Tue, 19 May 2009, 15:42

Thanks for the suggestion. I'll look into the Cycle Analyst and note it for the future. For now, I can't really justify changing from the TBS meter system I have installed. I think I will just have to stick to instructions that the alarm might indicate harm and go easy on the battery. I agree that the innards of a cell are not the simple ideal model and I do want to maximise their life, even at the expense of having to drive a little more moderately. In any case, ordinary performance for moderate driving is all I need this car to do, any more is a bonus.
Thanks for the helpful comments.
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current limiting on Curtis and TS cells

Post by antiscab » Thu, 21 May 2009, 01:25

coulomb wrote: Trouble with this is, you don't know if discharging below 2.5 Vpc is reducing life. Suppose it's from 8 years to 6 years; how would you know without a long, expensive experiment?. Maybe you don't care, but I'd like the maximum life.


yup, thats why im doing it on a small scale.
If there is a measurable reduction in service life due to high sag at high currents, ill find it.
This type of damage would be done on a per cycle basis, rather than a time basis.
i do 600cycles/year on my bike.

ill probably be able to conclusively say whether the high currents low voltages im subjecting my cells to were damaging them by Christmas.
The cells that are dipping below 1v will show the signs first.



Peter - before you start heading down the road of buying stuff for current limiting, put a volt meter on a single cell and measure the actual sag while accelerating hard. do this on multiple cells.

a curtis 1231C throws up alot of noise while your accelerating hard (you only see this on the smaller cells, the larger cells "absorb" the noise alot better)
Rods BMS doesn't like alot of noise, and will report a cell low when it isn't really.

cells sagging to 2.5v under acceleration is fine, its going below that that service life hasn't been proven (yet).

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current limiting on Curtis and TS cells

Post by Peter C in Canberra » Fri, 29 May 2009, 17:33

Now I have my car registered, I have driven it for a few more days, and the weather is getting chillier it seems the BMS alarm goes off pretty precisely at 3C (270Amps). I have the cells and BMS bits all at the rear of the car. I have the controller and everything else at the front. I would think that noise from the controller would only get to the battery and BMS via varying current in the main high voltage cables rather than radiated from the controller directly. I could imagine some benefit in hanging a capacitor across the traction battery lines to shunt any such noise and perhaps reduce the source impedance to the controller for brief bursts. However, I assume any such ultracapacitor sufficient to make much difference would be big in price and/or size. Thoughts?
The other idea I had was to put a string of cheap, small, used, salvaged lead/acid cells parallel to the main traction pack. It might add a few extra kWhrs that might come in handy but might also reduce the impedance of the battery pack as seen by the controller. Any obvious problems?
NB. I'm not about to rush out and try either of these. This is more about kicking some ideas around and then storing them away for the future when I feel like tinkering again.
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current limiting on Curtis and TS cells

Post by Johny » Fri, 29 May 2009, 18:20

Someone suggested temporarily installing a volt meter (just a handheld) to check battery volts during this 270 Amp event. It would be helpful to know absolutely whether it's the BMS (caused by noise) or a real voltage drop. Monitoring the entire battery pack initially would help. Not sure if you already do this.

If noise IS the cause then ultracaps are not necessary - just some reasonable size electros with low ESR or maybe even ceramic caps (1 or 2uF) may do the trick - but that then gets into how to precharge them (if electros) etc. Best to find out first what the actual problem really is.

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Post by Gow864 » Sat, 30 May 2009, 00:53

I run 16 TS040's and I have noticed the when it is 35deg or above, I can yank 170+ amps from them and no little red light on the BMS (same as yours), below 25deg, and I can turn the little red light on at about 100amps. TS cells (in my experience) seems to suffer from the cold.

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Post by Peter C in Canberra » Sat, 30 May 2009, 01:13

Thanks for that info. It would make sense that more chemistry will happen when it is hotter. I'll have to look forward to hooning in summer! In the dead of winter perhaps I'll have to make a point of setting the timer on the power point to finish the charging just before I want to use the car in the morning to get them warmed up a bit.
I have the voltage reading on the TBS meter. On the way home today, whenever I got the alarm to go off the volts were just a bit over 120; when I was coasting the volts were over 145V. The pack should have been at least 2/3 full. I guess I have one cell at 2.5V when the average is 2.7-2.8. Perhaps that is a normal scatter. I have yet to have a deep discharge so I don't know if I will seem to run out before getting the proper amp hours out of the pack.
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current limiting on Curtis and TS cells

Post by Peter C in Canberra » Sat, 30 May 2009, 23:02

Gow864 wrote: I run 16 TS040's and I have noticed the when it is 35deg or above, I can yank 170+ amps from them and no little red light on the BMS (same as yours), below 25deg, and I can turn the little red light on at about 100amps. TS cells (in my experience) seems to suffer from the cold.

Gow.


I think this could be it. This morning I took the car for its longest run so far, just over 50km, with stops for various shops. I was occasionally tripped the alarm right on 3C as in previous days. However, after getting home and running the charger continuously for a few hours, I went out again for a short trip in which I could only get the alarm to go off once at about 330A or 3.7C. I suspect the longer driving followed by continuous charging may have warmed the battery a bit so the chemistry was a bit quicker.
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Post by antiscab » Sat, 30 May 2009, 23:39

Peter C in Canberra wrote: I suspect the longer driving followed by continuous charging may have warmed the battery a bit so the chemistry was a bit quicker.


Yep, this is consistent with what i have observed aswell.
i remember one day it was 40deg C and i was on my 3rd discharge for the day (had 2 full charge and discharge cycles previous) and while pulling 120A from my 40AH cells, the average cell voltage was 3.2v!!

much less impressive when they're cold :(

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Post by Johny » Sun, 31 May 2009, 00:15

Hey Peter - maybe you could throw an electric blanket over the batteries at night or figure SOME way to keep them j_u_s_t safely warm. Won't help the trip home but the morning run must be your worst time.

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Post by acmotor » Sun, 31 May 2009, 03:07

Or move to Darwin ! Image
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Post by Peter C in Canberra » Sun, 31 May 2009, 15:51

OK, it looks like I am just learning something useful about how these cells vary in performance with temperature. Remind me to report back in a few months time. I have no other option than for the car to be out overnight under a tiled roof carport with no side walls at some distance from any other building. We'll see how the performance is after a heavy frost and -5oC overnight. I deliberately put a timer on the power point so the cells can be charged when demand on the grid is at its lowest. Whatever the cell's internal resistance is times 15A charging current squared may work out to be not so different from the power rating of an electric blanket, so I'll report with whether the performance is better after a cold night with charging in the early AM (hopefully a bit warmed) or the previous evening (stone cold).
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Post by acmotor » Sun, 31 May 2009, 15:58

A timer on the charger that sets when the charge occurs and limits the duration of the charge to the required kWh is a good idea.
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Post by coulomb » Sun, 31 May 2009, 16:20

Be aware that charging LiFePO4 batteries is endothermic for the bulk part of the charge. So you won't get much warming in that part of the charge.

At the end of the charge, however, the cells do warm up, as most of the power isn't going into storage, but into various lossy processes. This is the "inefficient" part of the charge, although for warming purposes, this "waste" heat is actually useful heat.

Also those BMS boards that are bypassing will be warming the area near the cells a little near the end of the charge. So your charge-initiated warming happens right at the end, and not much happens before that.

This is pretty much ideal, unless your timer is off a bit (e.g. there is an hour blackout during the night and it's a mechanical timer). Or if you suddenly get called in to work an hour early.

That's also assuming that the last part of the charge does enough warming for freezing Canberra mornings. I guess that will have to be found out by experiment. My guess is that you may need more heating than that.

The other thing is that LiFePO4 cells aren't supposed to be charged at less than 0°C. That may have been part of the cause of the charging disaster. There may be something in the cells that freezes around this temperature, and if it's partly aqueous, it would expand when freezing, possibly allowing shorts or partial shorts. Well, that's totally speculation on my part, but they seem to be pretty adamant that you should not charge at freezing temperatures. With your EV's position, and the fact that bulk charging will not do much warming, may necessitate separate warming, like the warming mats they use in the colder parts of the US.

Maybe you should consider charging earlier (before it gets freezing) until the heating situation is sorted out. I'd hate to see Australia's first charging disaster.
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Post by acmotor » Sun, 31 May 2009, 16:35

If you charge early i.e. probably not at off peak rate, to charge while cells are warmer (good point coulomb), then have the charger turn off once kWh is replaced in battery or after a set time in case there is a fault.
If the mains go off in the night ? Well all these possibilities need to be catered for in the 'ultra intelligent charging system'. Image

My thoughts wander to the Zebra (nickel / molten salt) batteries that the Solartaxi drove around the world. They have proved themselves in many ways ! Ambient temperature tollerant ! Image
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Post by Gow864 » Sun, 31 May 2009, 17:12

Johny wrote: Hey Peter - maybe you could throw an electric blanket over the batteries at night or figure SOME way to keep them j_u_s_t safely warm. Won't help the trip home but the morning run must be your worst time.


Is external heating not a good idea? Home-brewers use wrap around and base warmers during winter they keep the brew at 25dec. Why not have heater pads under the batts? Either only on during charging or running off of the pack itself (probably a bit energy wasteful). Install a switch to turn them of when you don't need them? Is this a bit dangerous?
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current limiting on Curtis and TS cells

Post by antiscab » Sun, 31 May 2009, 22:13

insulation and a phase change material down those nice grooves in the pack is another (more energy efficient?) idea.

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current limiting on Curtis and TS cells

Post by acmotor » Mon, 01 Jun 2009, 06:17

So in terms of battery box design for TS, is self heating an issue at 3C ? I mean, do you design to keep what heat is generated in or plan to lose heat ?
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