current limiting on Curtis and TS cells

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current limiting on Curtis and TS cells

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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current limiting on Curtis and TS cells

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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current limiting on Curtis and TS cells

Post by Gow864 » Mon, 01 Jun 2009, 20:21

So, ideally your battery box will be capable of temperature control, both cooling and heating keeping you batts in the optimum operating environment. You could call it climate control and pipe it into the cab as well. LOL

Now, how hard would that be?

Gow.

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current limiting on Curtis and TS cells

Post by Tritium_James » Mon, 01 Jun 2009, 21:13

It's exactly what Tesla have done in the roadster. The AC system is capable of regulating the temperature in the battery compartment.

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current limiting on Curtis and TS cells

Post by fuzzy-hair-man » Mon, 01 Jun 2009, 21:51

Gow864 wrote: So, ideally your battery box will be capable of temperature control, both cooling and heating keeping you batts in the optimum operating environment. You could call it climate control and pipe it into the cab as well. LOL
Well you might already have a water/fluid heater in the front to work your heater / demister, if you could find a safe non conducting liquid with the right freeze and boiling points (so that it does neither) then you might be able to circulate that through your battery boxes? if it's too hot circulate it through a radiator if it's too cold put it through the heater? now to find such a liquid huh?

Distilled water isn't supposed to conduct but I have doubts about it staying pure very long especially if you had to put antifreeze in there. Image

How do the engine block water heaters used in Canada etc work? do they leave them on all night or do they turn them on sometime before they go and anti-freeze makes sure the coolant hasn't split the block?

EDIT: Actually there's stuff people using liquid cooling for computers use Image

Non Conductive cooling formulas

Edit2 Image :
* MCT -40 and MCT-5 are not likely to cause damage to operating pc boards powered at 12 volts or less. It is not to be exposed to power supply input voltage or components in the power supply unit. Electrical component and computer board manufactures of high performance equipment go to great lengths to keep manufacturing processes clean and free of contaminants. Any exposure of foreign material to your computer system pc boards is considered a potential contaminant and care should be taken to keep your system clean and free of such impurities. Examples include water, frost, condensation, dust, and coolant leaks.
Only for up to 12 volts so maybe not... I should read more carefully Image
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current limiting on Curtis and TS cells

Post by antiscab » Mon, 01 Jun 2009, 23:14

parafin wax melts/solidifies at 25 dec C IIRC.

using a phase change increases temperature stability considerably (a phase change absorbs/releases several times more energy than simple temperature rise/fall can).

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Post by weber » Tue, 02 Jun 2009, 00:14

antiscab wrote: parafin wax melts/solidifies at 25 dec C IIRC.

using a phase change increases temperature stability considerably (a phase change absorbs/releases several times more energy than simple temperature rise/fall can).

Matt

OK. Lets put some numbers to it. The volume of the cells is about 20 times the volume of those grooves. The mass of the cell would be about 37 times the mass of wax.

Latent heat capacity of wax is about 220 kJ/kg
Specific heat capacity of cell unknown, but at least 0.7 kJ/kgK based on major components being LiFePO4 and Graphite.

So the wax melting might reduce cell temperature rise by 220 / (0.7 * 37) = 8 kelvin. I'm a convert. Image
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current limiting on Curtis and TS cells

Post by acmotor » Tue, 02 Jun 2009, 07:13

What I am planning for in Rodeo is peaks of 3C and an average of 1C on TS40s. Sorry, I should have been clearer on that. I do not plan to have spent $$$ on LiFePO4 just to flatten them in 20 minutes ! (others may !)
40A on 673V = ~27kW average. (based on 220 x TS40 at 1C)

weber, the temp rise testing you did was totally eskied at 3C to 42/52°C or so. The esky respresents an infinite boundary of adjacent cells but not real battery pack with sides, bottom, open above cells (and wiring).
You were testing worst case and to establish some heat generation properties. I understand that. I am pleased you did the tests.

At 3C (120A) gives around ~0.4V x 120A = 48W going into heat in the cell (based on internal resistance only) for your test. This is basically what you saw, having measured only external temp of cell and noticing the temp rise after load removed.

At 1C (40A) I can expect around ~0.14V x 40A = 5.6W going into heat in the cell. Big difference in the case of any potential cooling that the cells may require.

My question still stands (after some clarification for my expectations) in the hope that you or others may wish to comment (in light of the 'cold TS' posts.) Image
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current limiting on Curtis and TS cells

Post by fuzzy-hair-man » Tue, 02 Jun 2009, 18:34

weber wrote:
antiscab wrote: parafin wax melts/solidifies at 25 dec C IIRC.

using a phase change increases temperature stability considerably (a phase change absorbs/releases several times more energy than simple temperature rise/fall can).

Matt

OK. Lets put some numbers to it. The volume of the cells is about 20 times the volume of those grooves. The mass of the cell would be about 37 times the mass of wax.
This is why I figure you have to pump and then cool (via a radiator) whatever you use to cool the cells, if whatever it is is going to be solid when you want to warm up your cells then it's not going to be much good, on the other hand whatever it is could have a boiling point way less than 100 degrees of water and if boiling means it absorbs even more heat it might be good to have the boiling point around or just below the failure temperature of your cells, ie the coolant boils in a last ditch attempt to save your cells. Image

Looking at the ICE it produces an enormous amount of heat but is cooled by a comparatively small amount of water. Having liquids running around that close to so much electricity is a bit freaky though but I think that's just cause I think of water...

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Post by acmotor » Tue, 02 Jun 2009, 18:50

weber wrote: 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.


Yep, with you on that idea.Image

There have been EVs built in the past with piping for bus bars, having coolant run through the pipes (and available for cabin heating).
Coolant is preferably non conductive ! Image

Wiring with forced air flow over the top of the cells is most likely plenty of cooling or (heating) though.
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Post by Johny » Tue, 02 Jun 2009, 19:04

That's a good idea, but, keeping the coolant non-conductive may be inconsistent with not having it freeze. (At this point acmotor in WA smirks). Just one freezing event and lots of damage could be done.

Freeze proofing could be accomplished simply be having an inline heater that is always enabled (by suitable temperature control) and runs off the whole pack. The coolant circulated all the time - sometimes heated, sometimes routed through a small radiator (ex-heater core). Also solves the low temp. charge problem.

If that is done then distilled water would be the go.

Back in my dim dark youthish era I serviced Induction heaters that run around 2500 VDC on the Water cooled vacuum tube (herewith called the valve). They "isolated" the voltage with a thingy called a run-down coil. About 10 meters of industrial hose wound round a 12cm insulating bobbin (actually 2 of them). Ground potential on one end, 2500 VDC on the other. Prior to convincing them to run distilled water (they originally used tap water in a total waste system), the water fittings at the valve end had to be changed every 4-6 months as they were eaten away - stainless steel too!

With distilled water we still got erosion but a yearly service and check was enough.

Careful design as to what potential to feed water may make this is very neat improvement to prismatic battery pack systems.

This may bring up the whole floating pack issue again.Image

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Post by weber » Tue, 02 Jun 2009, 19:43

acmotor wrote: What I am planning for in Rodeo is peaks of 3C and an average of 1C on TS40s.
I agree that self-heating is not much of an issue in that case, and so if you can guarantee it won't be thrashed and you live in a cold place, then battery insulation may be a good idea.
weber, the temp rise testing you did was totally eskied at 3C to 42/52°C or so. The esky respresents an infinite boundary of adjacent cells but not real battery pack with sides, bottom, open above cells (and wiring).
You were testing worst case and to establish some heat generation properties. I understand that. I am pleased you did the tests.
Not totally "eskied". The top and bottom were not thermally insulated at all. That's only duct tape holding the side insulation together, that passes over the top and bottom.

And in fact the heat-sinking by cable was considerable (two 13 mm^2 cables with solid copper lugs on each terminal). In real life only the end cells in a block have cable that doesn't just go straight to a neighbouring cell, and they only have that cable on one terminal, not both, and they probably only have about half that cross-section of copper per terminal.

I think that what was lost by insulating the narrow edges was more than made up in cable-heatsinking, so this may not be so far from realistic. And I think the cell insulates itself quite a lot. But still, this is for 3C continuous, not what you were asking.

However, I note that you would only have to go to 3C for 1/8th of the time (and 1C the rest) for the heat dissipated in the battery to be twice what it is for 1C continuous.
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Post by Tritium_James » Tue, 02 Jun 2009, 19:45

Unfortunately you'll almost certainly have to run an additive in the water - but it should be distilled water to start with.

The problem is bacteria and algae growth. Unlike in an ICE system, the water cooling for an EV won't ever get hot enough to kill bugs, in fact normal operating temps are about perfect to grow them as fast as possible.

I've been looking at the additives for PC cooling for running in the waterblock of our new motor controller. I've been testing the stuff from Koolance, so far, with fairly limited testing, I've been quite impressed. They are similar to automotive coolant (anti-freeze, anti-foaming, corrosion inhibit properties) but with the addition of a biocide too. Yes, the solution does end up conductive.   

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Post by acmotor » Tue, 02 Jun 2009, 20:00

Johny wrote:
This may bring up the whole floating pack issue again.Image


Don't even go there. Uninformed fear by the inexperienced to cover bad electrical installation IMHO. Image
Oh sorry, you meant floating as in cooling liquid, I get it. Image

Interesting tests with coolant TJ ! Image There is so much to think about.

weber, if you still have your test rig together, can you repeat the 3C esky test with the top open to 1) natural airflow and 2) fanned airflow both with a typical BMS board and wiring mounted. I think your 'heat out of the terminals' idea will control cell temperature quite well.
(BTW, having no insulation underneath will make little difference to a heating test Image )

A suitable alarm on the battery pack could wake you at 2am and insist you go for a fang to warm the cold cells ! Image
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Post by Johny » Tue, 02 Jun 2009, 20:03

How are you suggesting they get warmed at 2AM Image

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Post by acmotor » Tue, 02 Jun 2009, 20:14

Some 3C discharge until the temp comes up to 25°C then a few bursts of 6 or 10C. All while running the ceramic cabin heater..... If it was the cells you were talking about warming.
Man, this thread is falling apart !   Image Image

edit: thus the question about insulating (thermal) the cells to make use of their heat generated in cold weather.
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Post by weber » Tue, 02 Jun 2009, 20:26

acmotor wrote:weber, if you still have your test rig together, can you repeat the 3C esky test with the top open to 1) natural airflow and 2) fanned airflow both with a typical BMS board and wiring mounted. I think your 'heat out of the terminals' idea will control cell temperature quite well.

I assume you hadn't read my earlier post where I pointed out that our test did have an open top with natural air flow and did have heatsinking of terminals by lugs and cables of about 4 times the cross-section expected in normal use. So this is neither a best nor worst case.

We would be happy to repeat the 3C test with all insulation removed except for a patch over the thermometer, and with a fan blowing, for a best case. Would that be useful?
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Post by acmotor » Tue, 02 Jun 2009, 20:58

Sorry, the pics don't show that.
I don't always read the text when I'm looking through a magazine (honesty)Image. coulomb will explain. But I thought that I had read the results.
Duct tape shown in pics is insulation both in itself and by stopping airflow.

Please, I would be interested in knowing the result, by comparison, using your test rig.
You follow my thinking ? insulated around the sides represents an infinite boundary of other cells as in a pack (worst case).
Open at the top with BMS and wiring fitted (no duct tape etc) represents the 'temperature control' zone. Hey, this was your point after all ! e.g. my BMS PCB represents 3500mmsq of copper heat fin to help couple the cell terminals to the air.

Another variation if temp rise is still an issue would be to deliberately allow air to flow through the TS cell side slots whilst still maintaining radiant and conductive heat insulation. Either natural convection or forced air flow. The heat numbers 5 to 50W are relatively low after all.    Image

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Post by weber » Tue, 02 Jun 2009, 21:44

acmotor wrote:Duct tape shown in pics is insulation both in itself and by stopping airflow.
Duct tape is way too thin to provide any significant thermal insulation, but yes, the small amount of air traped in the hollows in the top of the cell might.

We don't have a "typical BMS board" but if you send us one we will happily bolt it on and repeat the 3C test with a completely naked cell and terminals (despite the short-circuit risk) and with a fan going, and with the same heavy cables on both terminals. A better than best case. And if that turns out to be not much different because the cell is mostly self-insulating, we won't need to do the other test. But if it is wildly different then we will try whatever in-between scenario you want.

Please mail the BMS board to: Dave Keenan, 116 Bowman Parade, Bardon QLD 4065.
You follow my thinking ? insulated around the sides represents an infinite boundary of other cells as in a pack (worst case).
Sure we follow it. We wrote exactly that. But I'm sure you really do buy them for the articles. Image
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Post by coulomb » Tue, 02 Jun 2009, 23:56

acmotor wrote: 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.
...
This is where a BMS with voltage monitor comes in. (or latching <2.5V indication)

Well, I just don't know. I like Weber's "part of the battery near the terminals is likely doing destructive processes" theory.

However, here is a post that agrees with acmotor from this diyelectriccar.com thread (thanks to Weber for the pointer).
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Post by acmotor » Wed, 03 Jun 2009, 00:58

weber, I've sent a non populted BMS PCB for ease of postage. (actually I don't have any others spare). It should allow you to run some thermal tests.

coulomb, re Jack Richard's post on diy forum.

"You can see this best not at the cell level, but across the pack simply because the voltage swings are larger and easier to measure." ??
0.4V x 20 = 8V which voltage is hard to measure ??

The thoughts re 2.0Vpc under short term load I agree with.

I don't know what sort of BMS Jack has worked with but ones that involve multiple wires back to a central system from each cell surely went out with the ark (and wet lead acids !). Yes they are dangerous.
Rod Dilkes' BMS and my poor cousin for instance run one optically isolated wire around the whole pack.

Jack quite rightly points out that you should buy a few spare cells as some are likely to fail for one reason or another. My confusion comes when he discounts BMS value yet expects to find the said faulty cell in the pack ???? Do consider that without BMS you WILL KILL cells.
True, if TS cells are safe when abused then BMS may not be required to save a fire but at the same time, BMS WILL BE REQUIRED to get the max life out of a pack and diagnose problems.

I note Jack is all uptight about matching cells in a pack (as he doesn't use BMS ?). Chill out man. If you have a meaningful BMS coupled to a smart charger then you can run any size / age or chemistry cells / batteries in a pack. Try it. Ah is just Ah as long as you have enough ! The Ah of the weakest / smallest cell will determine pack Ah.
This reality unfortunately pours water on the "no BMS required" idea.
I have long said that the BMS really comes into its own as a pack ages since cells do not age equally.

The "spare cells" argument also falls apart (re matched cells) when you consider that a cell from storage with no cycles on it will of course have a different capacity to the rest of the pack that may have 1000 cycles on it !!!!

Sorry, bit of flame throwing there !
Image

edit: pack is spelt pack
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Post by Squiggles » Wed, 03 Jun 2009, 01:35

Can someone point me to a place that gives a good description of how a BMS is intended to operate?

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Post by acmotor » Wed, 03 Jun 2009, 01:43

Squiggles, you could take a wander throught this thread...BMS ideas

Basically the BMS (battery management system) is just as its name says.
There are many variations on the theme. Some for safey, some for diagnostics and some for battery longevity.

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current limiting on Curtis and TS cells

Post by Squiggles » Wed, 03 Jun 2009, 01:50

Ah, poorly defined scope...bane of the project manager!!

Thanks.

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