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Battery pack for an electric motor

  1. Nov 24, 2016 #1
    Hi Guys!

    I'm new in this forum and I'm a greenkeeper, it means that you can ask me everything about grass, flowers and trees but I have ZERO knowledge in electrical systems. This is why I'm asking here if someone with good patience and really helpful can help an ignorant gardener with some basics.

    I would like to test the feasibility of a project, try to substitute a classical endothermic engine with an electric motor:

    The actual engine produce 3,2 kW at 4000 rpm and this engine has to work at least 4 hour in a row at the max speed and my question is: If we suppose that I've already a DC electric motor that develops 3,2kW, how can I calculate the dimension and the charateristics of the battery pack to have this power constantly for at least 4 hour?

    Which is the right balance between volts and ampere to obtain 3200 Watt? how can I decide this ratio and how can I know if it makes sense in the reality?

    sorry if maybe it's a too stupid question but I hope someone can give me a hand!
    Last edited by a moderator: Nov 24, 2016
  2. jcsd
  3. Nov 24, 2016 #2


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    Staff: Mentor

    Welcome to the PF. :smile:

    What's an Endothermic Engine? Just curious.

    So 3200W is about 4.26 horsepower, does that sound right?

    Battery capacity is rated in Amp*Hours at the battery voltage, so that's what you need to work toward in this project. 3200W * 4 hours with 12V batteries would be 3200W/12V = 267A for 4 hours would be about 1000 Amp*hours at 12V. The battery capacity of heavy duty SUV batteries is around 120Ahr, so that's 8 well-matched SUV batteries in parallel with a 12V motor.


    There are other configurations possible, as well as larger batteries like those used in small electric vehicles. Can you say more about your project? That may help us to come up with ideas to help you better.
    Last edited: Nov 24, 2016
  4. Nov 24, 2016 #3


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    Staff: Mentor

    You read it off the nameplate of the motor.
  5. Nov 25, 2016 #4
    That's amazing, it's always nice to see helpful people, it reminds me that the world is still good! Thank you Berkeman!
    Well now it starts to make sense! Sorry for the patience, If I would like to use for example the Tesla-Panasonic battery pack that has 233Ah, 25V and 5,3kW, should I need 2 of them to don't run out, isn't it?

    3200W / 25V = 128Ah
    128Ah * 4h = 512 Ah needed

    An endothermic engine is a normal gasoline engine, when I was at school my physic professor called it like this, I thought that it was the technical name!
    My idea was to try to build an electric mower to use in a golf course, they are a little bit more sofisticated than a normal lawn mower that people use in their garden. I was wondering why all the big companies that produce this kind of machine didn't realize an electric mower yet, there is only one company who made one model and it can work only 2 hours. How is it possible? why they don't use the new technology like Tesla is doing to build machines compatible with a normal work activity (4hours work at least)?
  6. Nov 25, 2016 #5


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    Staff: Mentor

    Not quite.

    P = V * I, so 3200W / 25V = 128A

    Then 128A * 4h = 512Ah.

    You ended up in the right place, but just had some typos in the middle. Can you get all the electric motors you want for the cart at 25V? That is a pretty high level of current, BTW, so you should get some help from folks with experience if you want to prototype your cart. It would be easy to start a fire if you don't know what you are doing when working with high currents and energies like that. Do you perhaps have a university nearby with an ME or EE club that could take this on as a project to help you out?
  7. Nov 25, 2016 #6
    Thanks for the correction, you're right!
    Well, it's a project that I've in my mind and I'm looking for some good collaborators to start it but i would like to know more about electric systems me first to understand if it's possible to do it (avoiding explosions and fires as you warned me).
    I live in Gothenburg in Sweden and we have Chalmers University that is quite known, do you suggest that I should ask their collaboration? should I write first a little project and try to present it there?
  8. Nov 25, 2016 #7


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    Staff: Mentor

    In the USA, we have many college groups that participate in a design competition involving small racing cars:


    There are other similar competitions and projects for college ME and EE groups, for different types of vehicles. You could look to see if that university has any similar projects going on in their ME or EE departments, and then contact their advisers to see if they might be interested in taking on a project like yours. It could be a fun project for all involved. :smile:
  9. Nov 26, 2016 #8
    That's nice to hear, I'll contact them soon and I hope we can start this project! In my opinion it's nice when it's not just a business but it's a develop for the entire community and maybe it can become something bigger and useful for everyone.
    Thank you so much for the inspiration Berkeman! ;-)
  10. Nov 26, 2016 #9
    Hello Diegus, I would look into heavy duty golf cart motors.

    The challenge with lawn mowing, is the load is near 100% so heavy draw, vs an actual golf cart even has regeneration when breaking.
    The up side is you can probably very well define the time, the 4 hours you mention.

    For the batteries, they may have an AnpHour rating that is not ideal for the life of the batteries. Meaning, for the best life span of the battery may be best when only discharging 30% (70% remaining), but the AmpHour rating may be to 40 or 50%. Should be easy to research, but the remedy is more batteries = more weight.
  11. Nov 26, 2016 #10


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    Good point. He should probably be looking at "deep discharge" batteries, not SUV batteries.

  12. Nov 26, 2016 #11
    Interesting.. but I was was wondering, all the golf carts runs lead-acid batteries, but do we have the same problem even with the lithium batteries?
  13. Nov 28, 2016 #12
    Pretty sure each battery type will have an "ideal" charge / discharge profile. Actually can be complex involving rates of charge and discharge in addition to depth of charge, and avoiding overcharging.

    You can get a complete Golf Cart set - including batteries, motor and controller.
  14. Nov 28, 2016 #13
    Thanks, did you already try one this golf car set Windadct?
  15. Nov 28, 2016 #14
    I had a golfcart but never repowered it - still as a source of info, and components whenever you can steal from a mature marketplece - you are better off than trying to figure it all out. Looks lie they start at ~8hp- so pretty good match for the 3200W unit, and the speed controllers are all matched.

    It looks like they all list "speed" telling me that the drive ratio of the golf carts is all the same, so you will want to figure out the drive ratio of your mower - probably not a problem. If you can get the RPM of your motor at full throttle - and measure the speed, I am sure it can get close enough.

    You may be able to get your hands on a used GC motor, and used controller to try it out, then invest in new once you have learned what you do not know.
    Last edited by a moderator: Nov 28, 2016
  16. Nov 28, 2016 #15
    TIL - they have a Frame rating based on time... looks like 60 minutes is common, and I'll bet thet this is a full power / speed rating. So full power for one hour. If you want to run continuous for 4 hours you may want to oversize a little and make sure it is not running at 100% for 4 hours. -- but I am speculating.
  17. Nov 28, 2016 #16
    that's a good help.. just a question due to my ignorance, why these 60 minutes? just because after that it can overheat or there is some other reason?
  18. Nov 28, 2016 #17
    My guess is that this is a thermal issue, yes.
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