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Unread 12-06-2017, 11:24 AM   #1
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Default Motor amp draw chart/curves for Lithium battery conversion

I plan to convert a 2002 EZGO TXT to Lithium battery packs. My cart has dead batteries and I have decided to start with the cart and get up to speed on Lithium batteries for this and some other larger projects. Am I correct that the motor in the cart is 2 HP? 36VDC. Are there any published motor amp draw curves or charts. Is the cart controller constant voltage and regulates amps, inrush amps?

I would prefer to start with the smallest pack that will give me a couple of hours run time to keep experimental cost to a minimum. The pack voltage can be assembled plus / minus 36V. Any comments are welcome.
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Unread 12-06-2017, 12:01 PM   #2
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

Assuming you have a sepex system; the motor field mapping is usually not published by the mfg, though you could attempt to request it. Though unless your planning to swap the controller itself you shouldn't need to worry about this info.

The field and armature voltages do vary during operation depending on the throttle percentage and mapping, but all of this is regulated by the controller.

All you really need with Lithiums is a BMS to regulate charging/discharging and you should be all set.
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Unread 12-06-2017, 12:03 PM   #3
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

Quote:
Originally Posted by mikeswheels View Post
I plan to convert a 2002 EZGO TXT to Lithium battery packs. My cart has dead batteries and I have decided to start with the cart and get up to speed on Lithium batteries for this and some other larger projects. Am I correct that the motor in the cart is 2 HP? 36VDC. Are there any published motor amp draw curves or charts. Is the cart controller constant voltage and regulates amps, inrush amps?

I would prefer to start with the smallest pack that will give me a couple of hours run time to keep experimental cost to a minimum. The pack voltage can be assembled plus / minus 36V. Any comments are welcome.
Modern controllers use PWM (Pulse Width Modulation) techniques to limit average voltage/current to the motor. The biggest limitation with older stock controllers is the ability to handle higher voltages. Lithium packs have a wider voltage swing from the minimum SOC (State of Charge) to maximum SOC than most older golf cart controllers are designed for. By sticking to an older stock controller, you limit your available range to what the controller can accept for maximum or minimum voltages. You'll probably want to upgrade to an Alltrax controller. I just bought 48 LiFePO4 100 A/H cells for my project. 32 will be connected 2P16S for a 48V 200 A/H nominal pack. 16 will be for another project.

Is your TXT a series cart or a SepEx cart?

Visit the Carts Unlimited site

http://www.cartsunlimited.net

and learn. That is the best source I know of for Alltrax controllers, chargers, and other goodies. If your cart is series, consider going 48V nominal. If it is SepEx, consider going 42V nominal. Whichever you go with, I suggest a true BMS (Battery Management System) instead of a balancer. A balancer wastes power in load resistors to bring the higher cells down to match the lowest cell. A true BMS uses active balancing (uses power from higher cells or the charger to bring the SOC up in lower cells. With a cart, you're limited in A/H capacity by your budget, so you don't want to waste power in your pack to load resistors.

There are members here that have done lithium conversions that can provide advice.

Bob
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Unread 12-06-2017, 03:12 PM   #4
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

Remember, your 36v lead acid battery pack fully charged was around 38.2v. I do not remember what the 'on charge' voltages were for a 36v pack but I *think* during the absorption phase the charger would hold 43 or 44 volts. So you don't have to limit your pack to a max of 36v.

Most Likely, your controller was built to handle the max voltage you would normally see during a charge cycle without letting all the magic smoke out... Depending on what lithium battery you go with that gives you quite a range of options. If you get cells with 3.6v fully charged then 11 of them would have you at 39.6v max and should be fine.

I just picked up some 4.2vpc max Chevy Volt cells for my 48v cart. I'm slowly getting it set up for a lithium pack. I've set the charger and balance boards for 4.18vpc so my 14s pack would be 58.52v.

I'm both nervous and excited about the conversion...

Good luck with your conversion!
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Unread 12-06-2017, 03:44 PM   #5
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

Here is a picture of a 42V Leaf pack I installed in a stock 2006 PDS Cart. Stock motor and controller. The guy was not looking for all out speed, just no more LA batteries. Cart is very quick off the line and tops out at about 22-23 on 18" tires. This is a double pack connected 5S2P (10 modules) Great range. No solenoid change either. Just added a DC-DC converter for accessories. For what you mentioned, you would only need 5 modules for the setup.
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File Type: jpg Gamez - Lithium Upgrade Complete.jpg (513.2 KB, 0 views)
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Unread 12-06-2017, 05:21 PM   #6
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

Thanks for the reply's. I will look them over tonight and respond. My original thinking was to fabricate the battery packs with (10) 18650 cells in series that are nominal 3.7 volts, max 4.2 volts. That would make the pack 37V to 42 volts max. The cells are available in 2200 mAh or 3500 mAh so I was trying to determine how many rows I need in parallel and a rough idea of the run time I would get.
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Unread 12-06-2017, 05:36 PM   #7
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

The lithium Amp Hours are a little different than the Lead Acid as with the Lithium you get about 80% of available Ah whereas with lead acid you get at most 50%. But having said that the lead acid packs tend to run around 225 Ah for a new 36v pack of Trojan batteries.

So fifteen of the 3500 mAh strings would be 52.5Ah, twenty would be 70Ah. So twenty strings of ten 3500 mAh batteries would be OK probably. You are increasing voltage and decreasing weight so that may do what you want. But you'll get less range than a brand new set of Trojan batteries.

You also need to know what the discharge rate on those batteries are. If they are 1C then that's no good as many stock controllers can pull 250 amps. At 1C discharge that's a 70 amp limit. So you want cells with a 5C discharge rate or higher. Or more strings in parallel.

Last thing to think about, you may want to consider a different type of battery than the 18650 as the BMS you'll use could be rather complex. Some people do run without BMS boards, but you need to consider how you want to control your charge and if you want to top-balance or bottom-balance.
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Unread 12-07-2017, 01:48 AM   #8
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

A 20P10S configuration of 18650s could be built that would work with a 10 cell BMS. Protected cells could be used as well to simplify the project. Cell maximum current limits would still have to be observed of course.
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Unread 12-09-2017, 11:02 AM   #9
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

So here is my current thinking.

* 36 volt, 40 amps for 2 HP, 268 amp hours for 6 to 7 hours run time at full speed.
* 250 amp controller? Do I need the battery to have a continuous discharge current rating of at least 250 amps or Max 250 amps?
* LGEAMF11865 size:18650 li-ion cells, 2200 mAh batteries. 10 amp continious, 20A max.
* Use 20 cell 10 series, 2 parallel with BMS rated at 40A for each pack, 1 hour runtime per pack
* Use (6) 2p packs in parallel for a total of 12p, 120 cells, 268 amp hours, 120A continious, 240A max
* Operation range 30V to 42v
* Series busbar wire needs to be sized for 250+ amps
* Pack in clear plastic housing?
* Charger? Greentime from AliExpress

I think I would want to limit max and min volt/amps so this pack would last 15 to 20 years.

Thoughts?
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Unread 12-09-2017, 12:02 PM   #10
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Default Re: Motor amp draw chart/curves for Lithium battery conversion

Quote:
Originally Posted by mikeswheels View Post
So here is my current thinking.

* 36 volt, 40 amps for 2 HP, 268 amp hours for 6 to 7 hours run time at full speed.
----------------------
HP is the product of RPM times Torque divided by 5252 and amp draw is proportional to torque produced, so the motor could be drawing over 100A at low RPM or less than 10A at a high RPM and still produce 2HP.

Setting that technicality aside, most carts with stock DC motors draw from about 50A to 75A or more when traveling at top speed.
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