current limiting on Curtis and TS cells

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Peter C in Canberra
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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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current limiting on Curtis and TS cells

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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current limiting on Curtis and TS cells

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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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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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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Post by coulomb » Mon, 01 Jun 2009, 14:02

We are planning open battery rails (where possible) to lose the heat. But Brisbane rarely gets freezing weather.

So for places like Melbourne where you can get four seasons in a day, it's an interesting question. Even for Hobart where you can expect only mild summers, do you insulate or not?

I'd be tempted to put heating mats under the cells (where climate makes this necessary), and not insulate. It means you will need more heating energy that way.

Removable electric blankets custom designed for the battery pack might work, but what a pain.
gow864 wrote:Is external heating not a good idea?
No, it's just that it seems such a waste of energy. But it seems unavoidable where the climate has regular freezing, and the vehicle can't be garaged.
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Post by weber » Mon, 01 Jun 2009, 15:13

acmotor wrote: 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 ?

I expect that was a rhetorical question on your part, acmotor. Image But for the benefit of others ...

As you can see here:
viewtopic.php?t=980&p=12468#p12468
Under 3C discharge you very much need to lose heat. During the 20 minutes it takes to fully discharge at 3C, the cell case temperature rose from 20°C to 42°C. Then even after the load was removed, the case temperature continued to rise for the next 15 minutes, reaching 52°C before beginning to fall. What this means is that the cell has high thermal capacitance (thermal mass) as well as high thermal resistance and the product of the two gives a time constant of 20 minutes or so. Inside the cell, the temperature would have peaked (at the 20 minute mark) at something well above 52°C. Possibly 70°C or more. This is bad and will shorten the life of the cell.

So you see, the cells themselves keep the heat in for times similar to a typical trip. No need to make a box to do so. And no insulated box you could fit could keep the heat in overnight.

Of course, being in Brisbane or Perth or coastal places north, we don't have much of a problem with below zero temperatures, but my feeling is that unless you're a long way below zero, the quickest way to heat up the cells is to use them hard, i.e. a few rapid accellerations or a hill climb that pulls the cells down to 2.5 V and you'll soon be able to pull more and more current at that 2.5 V. In other words, high internal resistance at low temperatures should be somewhat self-correcting on discharge. However I note that, so far, this is all theory on my part.

You would not want to put a heating element under the cells as it would be damaged by their weight and vibration. I also don't think a phase change material in those skinny little grooves is going to have much effect relative to the thermal mass of the cells themselves.
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Post by weber » Mon, 01 Jun 2009, 15:35

My guess is, if you want to get heat in or out of Thunder Sky or similarly packaged cells quickly, the best way is the same way you get charge in and out, via the terminals.

And speaking of similarly packaged cells, the new Sky Energy SE cells claim better low temperature performance than Thunder Sky or their own TK cells. But they still don't want you to charge them when they are below 0°C.
http://www.evcomponents.com/SearchResults.asp?Cat=33
And charging is endothermic, so they will need heating, and temperature sensing to shut of the charger if heating fails, if you want to charge them using night off-peak electricity in places where it goes below zero.

As far as battery lifetime is concerned, the best time to charge them is while they are still warm from being driven, but of course this is usually very bad for peak load on the grid and electricity cost.
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