72 volt motor choice?

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CometBoy
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72 volt motor choice?

Post by CometBoy » Thu, 26 Jun 2008, 03:49

Any Advice?

I was just reviewing my choice of DC motors and perhaps I have under rated my need. My conversion project is a small light 2 seater Suzuki “Mightyboy” vehicle. It weights 600kg and has a load capacity of 350kg giving a gross weight of 950kg. I was planning on using a heavy duty version of the 72 volt D &D SepEx separately excited DC motor kit with the Sevcon SEM PPAK controller (see http://www.thunderstruck-ev.com/ddmotor.htm ) but I am now concerned it might be under rated compared to the original petrol motor (550cc developing 19.2 kilowatts@ 6000rpm). Would this choice be reasonable or is there a better choice. The current battery choice is six standard Trojan (or close equivalent) 12V 115Ah batteries. I would like to end up with a short range (50km) urban EV with a top speed of 85-100 kph. The clutch and 4 speed transmission will be kept. I liked the concept of the separately excited DC motor kit and the fact that I get regen braking as well...

Image Any technical view as to the selection greatly appreciated!

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Richo
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72 volt motor choice?

Post by Richo » Thu, 26 Jun 2008, 05:21

Umm I'd be more concerned about buying a motor where they don't even publish the full specs.
Are you buying it just because it has regen?
I haven't done the calcs but 72V with that motor maybe pushing to get 100km/hr. The torque seems on par. You would probably need more than 72V for the speed what you want.

Rod in Perth has already done a mighty boy conversion.
Looks like he used an Advanced DC 8" motor.
http://www.ev-power.com.au/IMG/pdf/elec ... _renew.pdf
He also sells the TS lithium Batteries.
Your batteries maybe cutting it fine for the 50km range.
(would be close though)

In general the bigger the motor "tends" to have better running efficiency as they use less current for the same given torque.
So find the biggest that fits.
And by the looks of Rods car the ADC8" just fits.
So I'd go with that one.
On the plus side he might be able to help with a motor adpator plate.
And you don't have to import the D&D from the US.
http://www.evmotors.com.au/products/8inch.html
This might be more than you originally budgeted for so maybe just a smaller ADC motor might be better for you.

Here are the specs of the mighyboy as well.
http://www.icbm.com.au/mightyboy/specs.htm

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72 volt motor choice?

Post by CometBoy » Thu, 26 Jun 2008, 14:31

Thanks for the useful advice Richo…

The ADC 8” model 203-06-4001 (72 volt version) was my original choice but the $ made me look at the D&D product. I had thought the efficiency curve would have been better for the SepEx motor in the comparison. So I will forgo the regeneration feature in favour of better performance. It’s the old story; you get what you pay for. I will team it up with the stock standard Curtis speed controller (model 1231C).

I will still start off with the Trojan batteries but design the battery box to accommodate future Lifepo4 batteries (should I ever win Lotto!).

I am not to concerned about the top end speed 90 km/hr is fine as 99.99% of my use is 50 to 60 km/hr anyway. I would like at least a 40 km range and I think I should get close to that with this combination as well.

I have been in contact with Rod Dilkes and he has kindly given a few tips. I have the full specs on the Mightboy and workshop manuals but thanks for the links.

Cheers

Bruce

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72 volt motor choice?

Post by antiscab » Fri, 27 Jun 2008, 07:37

Gday Bruce,

im not sure 115AH of floodeds at 72v will give you the range your looking for. If you are using the Trojan 27TMH batteries, at the 75A rate (only 5.2kw@72v), their capacity is only 63AH. at max 80% DOD, this reduces it to 51AH. this is only 3.6kwh. At 0.2kwh/100km this will only give you 18km of range. All floodeds suffer this reduced capacity at high discharges.

To get your 40km range, you are going to need a pack more than twice the size, closer to triple.

Out of curiosity, what are you looking at spending on the trojan batteries?

Matt

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72 volt motor choice?

Post by CometBoy » Fri, 27 Jun 2008, 13:37

Thanks Matt,

Yes, I think you are “spot on” with your calculations, given the light weight of the vehicle (kerb weight of 600kg) and only 162kg of battery weight. I was hoping for 20km. Based on its power to weight ratio I think this should be the case. The battery box will allow for twice the number of batteries in the future (ie only half the area will be used now). I am viewing this as a cheap starting point as the batteries will cost me $260 each ($1560 total for the bank). The other advantage is a quicker recharge time from my Zivan NG3 charger.

Hard too know the right choice in battery selection for a given application and I guess we all hope Lifepo4 batteries will be affordable in the next few years; they cost about $2 an Ah per cell in the US at the moment. I am lucky as my use is only short trips on the flat Adelaide plains.

Bruce

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72 volt motor choice?

Post by antiscab » Sat, 28 Jun 2008, 04:11

gday Bruce,
LiFePO4 batteries are alot cheaper than US$2AH. in the last australian buy, we got the price done to $1.62AUD/AH delivered.(the price we paid was $1.10US/AH, the rest was shipping and duties)

As far as adding batteries later, you shouldn't mix new and old batteries, so to upgrade you would need to replace the whole pack.

Also don't count on discharging to 80%DOD everyday, your pack will have a very short life if you do, (280 cycles) Its best to size your pack so your not discharging below 50%DOD on a daily basis when using lead acid. You also have to take into account loss of capacity due to cold weather (i cant remember how cold it gets in SA).

Youve probably already thought about all of this and have accepted the limitations, but wanted to make sure u knew neways:)

good to see another conversion,

Matt

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72 volt motor choice?

Post by CometBoy » Sat, 28 Jun 2008, 21:39

Thanks for that Matt,

Yes I am familiar with the no no of mixing of batteries and the other lead acid issues.

But I am trying to get my head around the current LiFePO4 products that are actual on the market. I have looked at http://www.zeva.com.au and data like http://www.zeva.com.au/tech/headway/ and the battery forum http://www.zeva.com.au/forum.php?category=2&thread=40 that is on that site. After all that I am more confused!

So I guess I am asking are there a 72v LiFePO4 pack (or simple connectable modules) and associated chargers currently available off the shelf and if so any agents within Australia that might be worth looking at closer? Doesn’t seem it be an issue for the small EV bike market, I can find a number of packs for them?

As mentioned above I am only starting to research the LiFePO4 battery area so any suggestions welcomed.

Cheers

Bruce

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72 volt motor choice?

Post by antiscab » Sat, 28 Jun 2008, 23:07

no worries Bruce,

I bought my 48v40AH pack and bms for my scooter from www.evpower.com.au
They were thundersky cells aswell. These are large format, and they sell suitable chargers and bms.

If your willing to wait for a group buy to happen, you could get a cheaper price, but as too when that wil be, who knows.

The small format cells mentioned on the zeva site are great if you need very high power in low weight. The cost per kw is on par with the thunderskys, but the cost per kwh is more than twice. So if you need twice the power, buy twice the thundersk cells, and enjoy the greater range, unless weight issues are stopping you.

For your setup, id recommend large format, such as thundersky. There are other manufacturers aswell, but i havent had any direct experience with their products.

Going with a lower voltage higher capacity cells is the cheapest way, as far as bms is concerned.

Matt

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72 volt motor choice?

Post by CometBoy » Sun, 29 Jun 2008, 01:07

I will talk to Rod early next week...

I have checked the everspring.net site and looked at the data on the LFP90AHA cells - they seem best suited but think 18xcells@4.0v might be over of the budget this year.

Looks like they would give me longer range (40-50km min maybe??) and at least 5 times the life but hard for me to do the comparison against lead acid technology.

Cheers

Bruce

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72 volt motor choice?

Post by antiscab » Sun, 29 Jun 2008, 03:12

If you want 72v, you need 24 cells, the nominal voltage for LiFePO4 is 3.2vpc, or 3v at 1C load. Your cells shouldnt get to 4v often even under charge. under discharge, the highest you will ever see is 3.4v.

Matt

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72 volt motor choice?

Post by CometBoy » Sun, 29 Jun 2008, 19:00

Stupid question now Matt....

The spec sheet at http://www.everspring.net/product-battery-LFP90AHA.htm states:

For the Model: TS-LFP90AHA an Operating Voltage: 2.5-4.2V and a Nominal Voltage: 3.2V

In addition the following is given:

- CHARGING
For best charging, use special Solid State Lithium ion Power Battery charger, charging in constant current and limited voltage. Connect their polarities(+ and -) according to the marking inside the battery.
Charging should be separated into two stages with constant current, starting charging is less than 1C electric current (the best charging current is 0.3 times of rated capacity)
When the battery is fully charged (the highest setting voltage is 4.25V),change to constant voltage charging. The charging will stop when the current is less than 1000mA.
When multiple battery cells are connected in series or parallel, each cell should be monitored against voltage overcharging

- DISCHARGING
Single battery discharge is strictly controlled, please cut off when measured to 2.85V (the best constant discharge current is 0.3 times for rated capacity of LCP model and 3 times for LFP model).

So the stupid question is what does the 4.2V (or in fact 4.25V) mean? Do you simply charge them to 3.2V in practice?? And how do you get the 3V at 1C value? Would it not be 3.2V at 1C?

Also how long have you been using these on your project?

Cheers

Bruce
            

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72 volt motor choice?

Post by acmotor » Sun, 29 Jun 2008, 21:27

I'd like to hear the answer to this one too !
Go for it Matt.
iMiEV MY12     110,230km in pure Electric and loving it !

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72 volt motor choice?

Post by antiscab » Sun, 29 Jun 2008, 22:56

Hi Bruce, Tuarn,

4.25v is the absolute maximum your cells should ever go. Beyond this you start plating lithium, and significant cell damage occurs (loss of active material and rise in resistance). admittedly i havent destroyed any cells yet, though i know Rod as at least one.

The evpower BMS disconnects all cells if any cell goes above 4.2v. You can charge up to 4.2v every time, however for best service life, im told, you should charge to 3.8-3.9v. the difference between 3.8v and 4.2v represents about 1% of capacity, he majority of which is a surface charge anyway. On the evpower BMS, the BMS brings every cell down to 3.6v, so the highest soc your pack is ever at for any length of time is 98%.

On discharge, it is far more important to watch your AH counter than the battery voltage. the cut-off at 2.85vpc is assuming the 1C load (i think). The ev power BMS disconnects the pack if any cell falls below 2.5v ( i think, i never connected that side up).

under no load the thundersky cells sit slightly above 3.3vpc. the voltage sags about 0.2vpc for every C of load. My 40AH do it, ive seen the 200AH cells in rob masons mustang and ute do it. So far it has appeared to be pretty consistent.

On my bike i have a "cycle analyst" it shows AH, volts, amps and kw. when im cruising im pulling 40-50A continuous, and the pack voltage is slightly above 57v (i have 19 cells, 15 are thundersky, 4 are yesa). I have a second guage which is the one that came with the bike that shows around 45vpc (it measures the 15 thundersky cells only). The guage on the bike doesnt have voltages mark on it, just a succesion of different colours, so the voltage reading from that isnt super accurate, however i have compared its measurement against my cycle analyst.

The thundersky cells have been in my bike since january, ive done 5000km on them (more than the two previous AGM lead acid packs combined) and done approx 300 cycles to 25% dod, and a dozen or so to 80%, and a couple to almost 100% (such as when i arrived at the last meeting, i had pulled 36AH out of the pack even though at 1C its only rated to 32AH :s, but nothings changed, looks like i got away with it:p).

I actually bought them originally in october and arrived in november, they sat on my garage floor for 3 months. accidentally ran them to 100%dod so bad they wouldnt power the scooter first time i used them (hooked up the original charger wrong).

dw, its not a stupid question, its probably something a few people would have liked to know (especially those who just bought a bunch in the last group buy:p)

Matt



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72 volt motor choice?

Post by antiscab » Sun, 29 Jun 2008, 23:20

a few more thing to add now that i think about it:
The amount of voltage sag is very temperature dependant. My scooter is parked in the garage over night which gets rather cold. on my ride to work the cells for the past fornight or so have been at about 10 celcius or less. at this temperature the voltage sag is more like 0.3v per C of load.

on the way home (after a full charge) the voltage sag is back to its normal 0.2vpc per C of load. Back in summer when they originally went in , and temps were 30+, the combined pack voltage wouldn't go below 56v even at 75A load.

The pack on my scooter is small, spreadout and well ventilated, so it is very susceptible to temperature variations. In a car, this is less of a problem, so cold weather is unlikey to have as greater effect.

I finally got around to testing 4 of the 15 at 6C. I bought myself a carbon pile load tester 500A model (man these are useful). the fully charged 12.8v nominal pack of 4 gave 250A at 8v (2vpc). one day when i get around to it ill actually take a group of 4 cells out of the scooter so i can test them at their max power point of 320A (8C) @ 1.7vpc. the clamps on the carbon pile tester were a bit awkward in the tight space.

After a couple of cycles, the BMS brought the 4 tested back in balance with the rest of the pack, before i could get around to charging them individually with my power supply (i seem to have a habit of leaving things half done:p).

Based on the performance, (and the fact the pack survived the various things i did to them), i felt confident enough to borrow the money for the pack that goes in my car. boyo that was an expensive fortnight (bought my donor, the batteries, and paid the deposit on my zilla), ive never spent more money in my life.

I originally bought my scooter so i could test out different batteries before i built a car (this was before i discovered the association existed). If i had known then that others in Australia were already using thunderskys, i may have jumped straight to converting a car.

Matt

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72 volt motor choice?

Post by CometBoy » Mon, 30 Jun 2008, 01:55

Good stuff Matt!

So in looking at the data, it would seem that with my project I might actual get 45km with 24 cells on the 72Volt motor. I will have a chat to Rod and get his view on this.

I see Rod has designed his BMS, I think I should also discuss this as I was going to design one....

This forum has been very useful. I had involvement with the original Flinders University Electric Car Project 30 years ago that was scrapped by the South Australian Government. I have buckets of dead MOSfets to prove it! We didn’t have these lovely well protected Curtis controllers back then.

Historical, here is a reference to an Australian conference back then:

http://search.informit.com.au/documentS ... res=IELENG

Or check out this link, even Modem Motor was into it:

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... %26ps%3D42

Wish I could find some of the original stuff from the project but I think it was all tossed. Both the little Fiat and van where nice to drive.

Amazing to think 30 years ago we talked (in some ways more seriously) about the same issues and we have ended up with little progress.

Anyway this current project is just something to play within the shed and have some fun building it.

Re these cells again,

I assume we use 3V at 1C value? And not 3.2V at 1C??

Any concerns about the max temp on the cells? Just thinking, Adelaide, full hot blazing sun, 40 degree summer day and black tonneau cover over the poor cells. Certainly going to be up there near the max temp spec (BTW not sure to view max as 75 or 100 degrees from the info I have).

On a completely different topic anyone know what happened to the Reva import issue? Can only find this older reference:

http://www.ccsa.asn.au/files/electric%2 ... 0nov06.pdf

After seeing all the little electric cars running around the bigger cities in Italy and the fact they could plug in a certain parking meters one must question why we don’t look into a new registration class in our various states.

Love this one (it was just before Johnny left us)...

Image

Image

Thanks again Matt for helping with some of this basic stuff on the newer battery technology.

Bruce

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72 volt motor choice?

Post by antiscab » Mon, 30 Jun 2008, 02:21

Hi Bruce,

for doing power calculations for acceleration and such, always use the actual battery voltage not the nominal voltage. for your application i would say 1C (3vpc) is about right, though for acceleration you would use 5C (or 2.3vpc) as this is what the cells will sag to at 450A.

For my car i use 3.1vpc for calculating range and cruising voltage, as my avergage draw (in theory) should be about 45A. (im using 90x90AH cells).

As far as max cell temp, i haven't really looked into it. I have ridden my scooter in 45 degree weather (never thought i would feel hot cruising along at 70 in shorts and a t shirt) and the batteries were fine (actually more than fine, they were giving more power). My scooter is black, and when i have to park in a car park, it does get a bit hot in summer. But as i say, i havent had any temperature problems.

They might have a reduced service life if operated above 60 degrees continuously, but if its only once in a blue moon, don't worry about it. I think on the data sheet it says something like thermally stable at 250 degrees.

Matt

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