Hemonster's ACIM conversion

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Post by Johny » Wed, 03 Jun 2009, 15:34

My feeling is that if you are using something like the Headway packs then having the VFD trip out (from the packs BMS) and forcing you to manually bypass a battery pack to get back on the road isn't really that big a deal.

It might be otherwise if your wife was driving the car but again I would expect some pre-warning from driving instrumentation (or charging monitoring).

If the BMS cutting out then re-engaging is going to happen, then maybe have the BMS opto linkup cause a throttle back to halve torque (or an alarm) first, then after 'n' seconds trip the VFD if throttle back didn't help. All these are a lot simpler to do and don't involve the high power side of things.

I don't like the contactor shorting system - it may well be worse than that which it is designed to prevent. (Sorry)

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Post by acmotor » Wed, 03 Jun 2009, 18:07

I'm of the same thinking as Johny.

"Normal" battery pack end of charge "flattening" is not sudden and is best dealt with by audible/visible warnings and or cutback of max (continuous) torque available.

As a matter of safety principle, a vehicle must make every effort to be driveable (even if reduced performance) when asked. e.g. you are on the railway crossing ! This should not require any technical knowledge to perform. The range and time should however be small as the problem should be addressed itself without delay, as there may be potential for fire or other damage given the nature of things high power electrical.

If you have a switch out system, the need for a module to be switched out needs to be examined promptly and switching it out does not mean you can forget about it.

There may be occasions where switching out a faulty battery module may be useful but these would be very rare. e.g. open circuit cell.
I would say you need to have more faith in your batteries and not plan to discharge them too deeply. Have a good BMS and care for the cells.
Identify a weak cell and replace it but don't build an auto bypass system. read: monster

Most reasonable voltage and current contactors for DC are not changeover type or at least not suitable for the battery shorting type changeover. You end up having to use 2 SPST contacts to achieve the changeover.

Having said that, if you have set up changeover contactors on modules (and you know I am a big fan of battery pack breakup into safe voltage modules) then planning some form of manual bypass is not a great extra effort. However, you will probably never use it.

How was that for a set of random thoughts on the matter ! Image
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Post by Hemonster » Thu, 04 Jun 2009, 02:14

Nanfeng make a contactor with two independent coils in one package which could do the job - though that is not the one that I got in the end. That could likely have done the job, though at a higher cost.

But I see what you're saying ... probably not worth the effort for the return/flexibility you get as it won't be used all that much in daily commuting. Also if they are truly balanced many of them are likely to experience the cut out at about the same time - which would render the power output of the pack probably too weak to do much at all.

A manual bypass could be just as good, if not easier and safer to implement. Thanks for the inputs y'all.

ps: I could always carry my e-bike with me, so if I do get stuck I just have to remove one 48V pack and stick it on the bike and probably go as fast, if not faster than the car Image


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Post by Johny » Thu, 25 Jun 2009, 18:08

Hi hemonster. How is the Headway battery testing going?
Have you seen weber and coulomb's results with the Thundersky cells - particularly temperature?

The reason I ask is that it would be really interesting to compare the Headway cell to the TS regarding low-temperature performance. (Hint, hint). Not direct comparison but temperature trend.
See the last entry on this page.

Are you still considering the Headway packs?

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Post by Johny » Thu, 25 Jun 2009, 18:29

I should add that the reason I ask is because of this discussion I came across about Headway cells.

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Post by Hemonster » Thu, 25 Jun 2009, 19:12

Hi Johnny,

Been a bit slack lately, but also because have just had my appendix removed ... so you can say I got a bit distracted Image

I'm setting things up still and need to get a thermistor so I can log temp - though it will all be over in 10 minutes so might just sit there and take measurements Image

I do intend to get back to it soon though, and yes the outcome of it will determine if or not I like headway or not. I'm aiming for at least 50A out of a single cell ... though Headway's charts indicate 10-15C are also possible (gets better as internal temp increases).

I need to lower my load's resistance cuz at the moment its only doing 25-30A, more curtain rails in parallel Image

I'll post when I have some results.

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Post by Johny » Thu, 25 Jun 2009, 19:27

Yes I had the impression that you had been a bit busy lately - hope all is going well.

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Post by weber » Fri, 26 Jun 2009, 01:26

Hemonster wrote: Hi Johnny,

Been a bit slack lately, but also because have just had my appendix removed ... so you can say I got a bit distracted Image

You mean you got a bit extracted. Image
Hope you're feeling better.
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Post by Hemonster » Sun, 28 Jun 2009, 03:40

The recovery is a bit slow but at least it is steady Image

However I have since been able to do some testing. Here is a picture of my latest setup:
Image

I borrowed the clamp meter from a friend of mine to check and see if my shunt was working ok or not, and to see if there was any drift as things started to get hot. I decided in the end it was accurate enough and both were sometimes within 5% of each other at the extreme ends.

However I soon learnt that my shunt was inadequate because it couldn't handle the heat and the temperature was rising rapidly (16mohms) and dissipating 50W, despite that large surface area. So I decided to use a commercial 1mohm shunt instead. This meant that the discharge current went up as a whole - but that's ok as I'm doing limit testing on these cells anyway.

Here is the load with two of the curtain rail loads:
Image

Now the battery is putting out ~77A average depending on battery voltage which is about 7-8C. I have not yet implemented a constant current discharger which would be ideal to dial up a current and just measure time. However since I'm logging both voltage and current I'm calculating and accumulating watt seconds, dividing by 3600 to get watt hours. Then from there I can get the amp hours by taking watt hours and dividing by average battery voltage over that time.

However I was still battling with heat dissipation issues, primarily the shunt. I've added a PC fan now and that seems to stabilise the temperature at 45-50 deg which I think is acceptable. The load water got up to 60 deg which was ok.

The battery itself was heading also towards 60 deg close to the end of its juice (where things get less efficient and internal resistance goes up). When the load was stopped it was at about 55 deg, but the temperature still kept rising up to 60 deg when I decided to put the fan on the cell to help it cool off. I'm thinking a bit of fan cooling in the battery boxes might be warranted especially when the cells are getting a bit low.

Unfortunately I don't have any thermistors handy and all the temperature sensing was done using non-logged probes. Also the run I did just tonight was a bit of start and stop with all the issues, but I thought I would post anyway. It's hard to believe that this little 10AHr cell was capable of spitting out ~70Amps. I'll post up a chart soon. Doing the calcs on my start/stop attempt tonight I'm getting 9.5AHrs at this rate, but I'll wait until I can do a test run without stopping in between.

Stay tuned.




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Post by Hemonster » Sun, 28 Jun 2009, 03:47

Here is the chart:
Image

Battery voltage is on the right y-axis, both power and amps are on the left y-axis.

Note that I've modified the data to take out all the periods where the load was not turned on. The spikes you see are the initial start spikes when the load is put online for that little session ... in essense you can take the period between the spikes as as the burst period. You can also use the initial start voltage of the spikes as the resting voltage of the battery when the load is not turned on.

I've calculated the stats to the right of the chart. I'll do another run tommorow. The main problem I have now is to charge up this puppy as it take a while with my 3.4A supply to pump juice into it. My friend is bringing his 12A supply tommorow which will help speed things up a tad.


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Post by coulomb » Sun, 28 Jun 2009, 03:54

Well done!

Might I suggest the simple idea of a bigger jar for the load? More water means it takes longer to heat up.

Did you notice the current going down as the load got hotter? Maybe curtain rod material, not being copper, has a lower temperature coefficient of resistance? We found just stirring the water in the container was enough to increase the load current (moving colder water to the load).

Finally, what's doing the logging? We use eyeballs, stopwatch, pen, and paper. It's not ideal.
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Post by Hemonster » Sun, 28 Jun 2009, 16:18

Thanks Coulomb, I had read your battery tests too and got some inspirations there Image
coulomb wrote: Might I suggest the simple idea of a bigger jar for the load? More water means it takes longer to heat up.
I'm not too worried about the water, after all it can't exceed 100 degrees anyway. However the shunt wasn't water cooled and I think it would have easily exceeded 100 deg, and although it itself is probably ok with that, I didn't want my slab of wood getting too hot. However with the air cooling things seems to stabilise.

coulomb wrote: Did you notice the current going down as the load got hotter? Maybe curtain rod material, not being copper, has a lower temperature coefficient of resistance? We found just stirring the water in the container was enough to increase the load current (moving colder water to the load).
It's quite hard to tell as the battery voltage was also decreasing. There appears to be a lot of agitation (a buzzing sound) when the load is turned on. I think it is expanding and contracting really quickly in the water and also bubbling the water that is in direct contact with the metal. Once it was going for a few 10s of seconds the current did seem to stabilise - but also the battery voltage stabilised too. It is irrelavant anyway as I'm only trying to ballpark the current, I'm logging both voltage and current to get watts. Accumulating that over time gives me the watt hours I'm after.
coulomb wrote: Finally, what's doing the logging? We use eyeballs, stopwatch, pen, and paper. It's not ideal.
Image

It's a labjack unit - look back on my previous current load post for the link (or google it). USB back to Pc, with free and quite powerful logging and control software. The idea is to have the Nanfeng relay controlled by the LJ unit and monitoring the battery voltage accordingly. This way I can keep things a bit more consistent between tests. I have't quite got enough bits for the relay driver, so at the moment the turn on/off is a bit manual too. I've also not got thermistors and can't log temperature direct into the PC (which would be nice) .... so its eyeballs all over the place trying to see what is getting too hot.


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Post by Johny » Sun, 28 Jun 2009, 16:29

Great work hemonster. From the raw data, do you have an initial idea of the internal resistance for the cell?
Assuming 3.2V once the surface charge has gone, and 2.65V under 77A load - I make it 7.1 mOhm. That means 2 in parallel (for 20Ah pack) would be 3.5 mOhm per twin-cell.
I guess I'm asking what the terminal voltage is for this cell, without surface charge, at a say 1 Amp.

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Post by coulomb » Sun, 28 Jun 2009, 16:43

Hemonster wrote: However the shunt wasn't water cooled and I think it would have easily exceeded 100 deg...
With our 6C (240 A) discharges, we actually dunked our shunt in water as well as the load (separate containers). I'm not sure if it's good for it or not. You seem to have it under control with air cooling, which is probably way better for it.

Re the LabJack: ah, right, one of these:

Image.

I can see it in the corner of some pictures, now that I know what it looks like. Nice.
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Post by Hemonster » Sun, 28 Jun 2009, 17:30

There are more detailed pictures of my setup in the previous page:
viewtopic.php?start=4&t=1113


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Post by Hemonster » Sun, 28 Jun 2009, 17:34

Johny wrote: Great work hemonster. From the raw data, do you have an initial idea of the internal resistance for the cell?
Assuming 3.2V once the surface charge has gone, and 2.65V under 77A load - I make it 7.1 mOhm. That means 2 in parallel (for 20Ah pack) would be 3.5 mOhm per twin-cell.
I guess I'm asking what the terminal voltage is for this cell, without surface charge, at a say 1 Amp.


Thanks Johny, but what is a surface charge? Also, is the internal resistance a dynamic thing? or constant over the discharge cycle?

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Post by weber » Sun, 28 Jun 2009, 18:07

Hemonster wrote:Thanks Johny, but what is a surface charge?
For some time after charging there is a small amount of charge that is stored at a high voltage. You just need to put a small load on for a few minutes to bleed that off. Or a big load for a few seconds and then wait a few minutes for the voltage to recover. There is no real definition of when that is done. It's all very vague. Basically when the voltage "levels off" somewhat.
Also, is the internal resistance a dynamic thing? or constant over the discharge cycle?

According to what I've read, it changes rapidly at the start and end of discharge but seems to increase only slightly between say 80% and 20% SoC. We have tried to do our internal resistance tests around 50% SoC. And we've used the voltage after 5 seconds on load as our low voltage.

Internal resistance is however _very_ dependent on cell temperature.
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Post by Johny » Mon, 29 Jun 2009, 01:13

Well stated weber. hemonster, notice that voltage change over the first 50 seconds then it seems to stabilize at about 2.63V - that's when the "surface charge" has been removed. For comparison's sake if you do internal resistance at the same point as weber/coulomb did it would be good - about 50% SOC. I must admit I'm not sure what technique is best for internal resistance check.

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Post by Hemonster » Mon, 29 Jun 2009, 02:54

Just another update. My friend did bring over his PSU and we tried to do a 20A 2C charge and the battery didn't seem to mind. There was only a marginal temperature rise no higher than 35 deg. It was quite difficult to maintain that charge current without hitting the 3.65V voltage limit however ... does anybody know how much over voltage is ok short term (say for regen)? At most my wall charger will be a 3 hour charger, so this isn't an issue for me.

After another charge cycle we decided to repeat the 7C test as I did before, but this time monitoring the battery temperature both at the body AND the terminals. Unfortunately I was probably doing bad things to my cell when I allowed it to go up to 60 deg on the body last night I think this might have translated to >80 deg internally. The test we performed today seemed to indicate that the battery would handle 7C ok as long as there is adequate cooling. I had to stop the test part way through and restart it when the battery was cooler. The second attempt was using a fan to help with the cooling. The 2nd half of the test was also longer however as the battery voltage started to get lower and less power was being dessipated - hence the temperature started to stabilise at 64 deg max at the terminal. The specs on the datasheet say working temp up to 85 deg.

Here is the chart:
Image

As you can see that because there were fewer starts and stops in there, the overall battery W/AHrs is less. ie. the battery's efficiency has dropped a little - though I'll have to redo the test at least another two times to be sure.

I'm not entirely convinced that internal resistance is the best means for testing a battery's performance. This is because internal resistance depends on state of charge, temperature and surely cycle life of the cell too?? all this in addition to chemical variances in the production itself (maybe?). Should the test not just be if the battery can do the job or not?? ... (totally open myself to flaming here Image)

Anyway, at the middle of the test here are the battery values:

Load disconnected:
3.3V nominal resting voltage (left for a while after the first discharge - hopefully eliminating surface charge)

When the load was re-attached:
Secs   Volt   Curr
0      2.84   78.4
60     2.63   73.0
120    2.59   71.9
...

What can you guys gleem from this?

I plan to next reduce the discharge down to the 5C rating to see if I get a full run without overheating and without added cooling. This translates to about 60kw peak, and for me I think it should be ok. Then I'll run the battery at 20kw to see if I can do this sustained without over heating and that should give me the answer if this battery is good enough for the job.

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Post by coulomb » Mon, 29 Jun 2009, 03:34

Hemonster wrote:Anyway, at the middle of the test here are the battery values:

Load disconnected:
3.3V nominal resting voltage (left for a while after the first discharge - hopefully eliminating surface charge)

When the load was re-attached:
Secs   Volt   Curr
0      2.84   78.4
What can you guys gleem from this?

We can infer the internal resistance: (3.3-2.84)/78.4 = 5.9 milliohms.

To put this into perspective, with this being a 10 Ah cell, that would be 5.9/4 = 1.5 mR at 40 Ah (i.e. four of these cells in parallel). That's better than half the internal resistance of a Thunder Sky cell.

In other words, the sorts of battery pack sagging that we see with a Thunder Sky pack at 3C are about the same as you are getting for your cells at 7C. (That is, just barely good enough).

Put another way, however many amp-hours of Thunder Sky battery you need for a given peak motor current, with Headway cells, you have the option of getting away with half that number of amp-hours. That makes the pack have less range, of course, but it turns out what with Thunder Sky cells, you are more or less forced to have a rather good range (and rather high cost) just to meet the peak power requirement. With Headway cells, you can go for the lower cost, lower weight, lower space, and lower range option, or the full range option, or anything in between.

That said, Headway costs more per amp-hour than Thunder Sky, but from memory it isn't as much as twice; more like 60% from quick research.
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Post by antiscab » Mon, 29 Jun 2009, 09:51

coulomb wrote:
That said, Headway costs more per amp-hour than Thunder Sky, but from memory it isn't as much as twice; more like 60% from quick research.


Best price for Headways ive seen is US$2.30/Ah (admittedly i havent looked all that hard)
Best Price for TS/Sky is US$1.10/Ah

remember to add shipping (and port charges which are AUS$700 + 15% not having them sent by a freight company)

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Post by coulomb » Mon, 29 Jun 2009, 14:01

I was going on Zeva's price here:

http://www.zeva.com.au/tech/headway/

That might have been a theoretical or very old price, I don't know.

Of course, the shipping, GST, import duty etc etc would apply equally to both. Though I didn't realise that there was a substantial fixed port charge that would favour a larger shipment. Ouch.
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Post by Hemonster » Mon, 29 Jun 2009, 14:17

coulomb wrote: I was going on Zeva's price here:

http://www.zeva.com.au/tech/headway/

That might have been a theoretical or very old price, I don't know.

Of course, the shipping, GST, import duty etc etc would apply equally to both. Though I didn't realise that there was a substantial fixed port charge that would favour a larger shipment. Ouch.


My most recent quote from Headway was $560USD per 48V 20AHr module (with BMS and 6A charger), and $95USD to ship 14 of these to NZ. Note this is not their standard shipping forwarder however, but instead the one that Greensaver were using - that's after I asked them to investigate the difference. The standard Headway forwarder was $500USD !!! ... by sea! just be weary of that.

They knocked the price down to $518 without the BMS, and I think the charger was quoted to Johny? for $45? (I was quoted a separate 3A charger to be $30). Excluding duty, this means that the complete pack with BMS and charger is $1.70USD/AHr and without the BMS and charger $1.50/AHr. Note the BMS needs modification to support just being an indicator rather than breaking the stack.

As these cells have a lower AHrs per packaging, I can see why they are more expensive, however not by much.




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Post by Johny » Mon, 29 Jun 2009, 15:40

antiscab wrote:remember to add shipping (and port charges which are AUS$700 + 15% not having them sent by a freight company)
Sorry Matt. What do you mean by "not having them sent by a freight company"?

Can we summarise the costs of importing batteries.
1. purchase price
2. shipping cost
3. port charge
4. import duty (are batteries 15%)

Is that about it?

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Post by coulomb » Mon, 29 Jun 2009, 15:47

Hemonster wrote: The standard Headway forwarder was $500USD !!! ... by sea! just be weary of that.

Could this US$500 be the AU$700 that Matt spoke of?
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