9KW 12HP cart motor. This is out of control.

[Tombigbee]

This is an off-road golf cart build that's gotten WAY out of hand.
I'm going LiFePo4, but past that simple statement I am lost as to which cells to buy.

It's a 9KW, 12HP permanent magnet AC motor. The inverter is contained in the 840 amp.....yep, 840.....controller. That's overkill, for sure, but I'm thinking it might draw 500-600 amps at maximum load. That's a guess on my part.
Will be running it at 72V.

My plan is 24S 3.2V prismatic cells, and would like 200ah minimum. I'm thinking I need 3C maximum discharge rating, but would like input on that.

Any recommendations as to the best cells to buy and from whom? With all this juice, are there any with heavier terminals than I'm seeing? Is bus bar size (thickness) an issue?
What about a BMS to handle all this?
What is a shunt, and do I need one?


If anyone has any advice as to what holes I'm about to fall into, please shout out.

Many thanks, and Cheers!

 

[Bud Martin]

Do you know what the surge current is going to be?

 

[Tombigbee]

Do you know what the surge current is going to be?

My guess is 600-650 amps, with maybe 175-225 at normal "cruise".
Thanks

 

[Bud Martin]

I thought that is max load, which is not the same as the surge when the power is first applied to the motor.
Did you ask the motor manufacturer for the spec? You do not want to get the surprised after building the system.

 

[Tombigbee]

I thought that is max load, which is not the same as the surge when the power is first applied to the motor.
Did you ask the motor manufacturer for the spec? You do not want to get the surprised after building the system.

Agreed. I have been trying to get in touch with their techies without luck so far.
Will make another attempt Monday.
Thanks

 

[Bud Martin]

BTW, What is the make and model of the motor and the motor driver?

 

[Tombigbee]

BTW, What is the make and model of the motor and the motor driver?

Silver Wolf Teekon

 

[RCinFLA]

Will the inverter be okay with 88vdc maximum?

I would not draw more then 1C from the 'blue' cells common to this forum. They are designed with fat cathode and anode making them good for AH capacity but not for high C current discharge. They begin to 'choke' for ion starvation at about 0.5C. This means their terminal voltage will drop a lot more above 0.5C discharge rate. 1C is okay for short periods. At sustained discharge of 1C they will get quite warm. For a 200AH blue cell, at 0.5C discharge the cells produce about 3-4 watts of internal heating. At 1C discharge the internal heating goes up to 18-22 watts.

 

[Tombigbee]

Will the inverter be okay with 88vdc maximum?

I would not draw more then 1C from the 'blue' cells common to this forum. They are designed with fat cathode and anode making them good for AH capacity but not for high C current discharge. They begin to 'choke' for ion starvation at about 0.5C. This means their terminal voltage will drop a lot more above 0.5C discharge rate. 1C is okay for short periods. At sustained discharge of 1C they will get quite warm. For a 200AH blue cell, at 0.5C discharge the cells produce about 3-4 watts of internal heating. At 1C discharge the internal heating goes up to 18-22 watts.

Even the ones specifically claiming 3C max discharge ratings?

 

[Hedges]

24s x 3.2V x 600A = 46kW

That's a lot more than 9kW, 12HP.
Is this a kart with "Ludacris" mode, putting out 60HP for brief bursts?

 

[Steve_S]

Off the top, I would say LTO not LFP simply due to the C-Rates required. NOW If you want LFP, then Winston EV Grade 5C Capable cells (but they are likely more than LTO when you build it all.)

And Gold Cart ? Seriously ??
Dude, Ultimate Go Kart !, Mini Dragster or heck, even a FIA Formula 4 Open Wheel !
PS: You can't imagine what a Blast it is to Fly Around in an F4, holy macaroni the Smiles per Miles is incredible.

 

[Sliding01]

Goto https://www.buggiesgonewild.com/forumdisplay.php?f=23
Lots of information on that site.

 

[Tombigbee]

Goto https://www.buggiesgonewild.com/forumdisplay.php?f=23
Lots of information on that site.

I'm on there. Pretty good folks, but little to no detailed info on battery builds. I'm betting the guy I need to speak to is there, but I can't find him.

 

[Tombigbee]

Off the top, I would say LTO not LFP simply due to the C-Rates required. NOW If you want LFP, then Winston EV Grade 5C Capable cells (but they are likely more than LTO when you build it all.)

And Gold Cart ? Seriously ??
Dude, Ultimate Go Kart !, Mini Dragster or heck, even a FIA Formula 4 Open Wheel !
PS: You can't imagine what a Blast it is to Fly Around in an F4, holy macaroni the Smiles per Miles is incredible.

Yeah, I may be way off base on the amperage draw at 72V. I've got to get in touch with one of their engineers ASAP.

The motor kit is pretty new to the market, but there are quite a few of them running on 48V, maybe 105Ah battery packs. Those are generally 40-45 MPH carts, which is (too) crazy fast.

I'm not after speed, but I am intent on power (hills, loads, etc) and as much range as possible.

 

[RCinFLA]

Even the ones specifically claiming 3C max discharge ratings?

You would have to investigate deeper to see how reliable or 'with conditions' the claim is., The 'blue' EVA cells spec 1C 'max discharge current' but craftfully do not state 1C for continuous discharge rate.

A true high current capable cell is built with about 20 um thick electrodes vs. 120-150 um thick for the 'blue' cells. Total amount of LiFePO4 and graphite material in electrodes determines cell capacity. Because of thinner electrode layers, high current cells require more layers to achieve capacity, the additional electrode layers also require extra layers of aluminum foil, copper foil, separator, and electrolyte so their volume and weight per AH capacity is a bit lower.

The thicker the electrode, the more AH's per volume and weight but the lower the peak current capability before ion starvation starts to occur. Ion starvation happens because the electrode material thickness is a bottle neck to ion flow out of the electrode which is proportional to cell current.

Cooler temps (<+10C) makes ion starvation occur at lower current. Once cell warms itself up, due to self heating, it can take higher peak current.

I'll give the '10,000 mAH' LiPo 18650 cells sold as my 'let the buyer beware' example.

 

[Tombigbee]

Yeah. As I said, this thing has gotten out of control.
Thanks for the explanation.
No clue as to my direction now.

 

[NwCali]

I'd be looking into used EV batteries

 

[Hedges]

Yeah, I may be way off base on the amperage draw at 72V. I've got to get in touch with one of their engineers ASAP.

If in fact you'll be drawing 120A not 600A, the battery situation gets easier.