First build - Can someone help me confirm my calculation ?

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Andy B.

New member
Joined
Jul 18, 2021
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2
HI there,


I hope you are all doing well. Thanks for the great forum and all the information provided.

It would be my first build. I have been reading a few documents and I believe I made some progress in understanding a few aspects on how to think my battery pack. But the further I read more question are arising (it is definitely a very interesting topic). I feel I am almost there ready to buy and build. Nevertheless, I still have few questions that I did not manage to answer myself. So if someone would be kind enough to help me, I would be very grateful :)(y)



Project: Build a battery pack for a golf trolley.

A) Specs of the golf trolley:
  • The trolley comes with two engines of 120w each running on 24v.
  • They is a module linking the two engines, the control panel and a remote control (picture attached). On this module, one can read the following:
  • G7R – 24v – D 60S
  • Vol: 21-29,4V / CUR 15A
  • STM5K Rev_1.2 8BC2
B) Specs of the original battery pack:
I have attached a picture of the battery that comes originally with the trolley. On the sticker one can read the following: 24v – 13 AH – 327,6WH

C) Autonomy of the trolley:
The autonomy given by the original battery is satisfactory but why not building slightly more powerful to facto in the inevitable loss of capacity of the battery through the time and use.

D) Why build the battery pack myself?
As you know, they are a lot of restriction for plane travel when it comes to lithium battery pack. I work oversees and change residency every year or so. I wish to build this pack using the Vruz. battery kit which would allow me to disassemble the pack when traveling. I figured out it would an option for me since where I generally work and leave, lithium battery are not always available and surely it would help saving some buck not buying a new one every time I move.



As far as my understanding goes I calculate the following:
  • I should build a 7s5p battery pack with (of course) a BMS
  • I could use the LG MJ1 3500 mAh 18650 battery cells are they are well renown
  • I would get a 24v / 17.5 ah battery.

E) Questions:

  1. 240w / 24v = 10ah so why on the module linking the two engine it is written 15ah ? Which one should I consider to calculate the power needs?
  2. As you might know Vruz. offers two kits the 1.6 and the 2.1. I understand that the main difference is the current discharge one can take from one cell: 3.5A from the cheapest kit and 20a on the most expensive. Which one would be more suited for my use ?
  3. What other cheaper cell could I use staying with well know brands?
  4. How many kilometer could the trolley run on one charge ? I calculate the following but I am not sure: Using the 17.5ah battery pack mentioned above plus knowing that the maximum speed of the trolley is 8km. Theoretical autonomy would be: (17.5ah / 10ah) x 8km = 14km – Is that correct ?

Thank you very much for any help given 🙏🙏🙏
 

Attachments

  • battery 24v - 13ah.pdf
    1.6 MB · Views: 42
  • Module link the engines.pdf
    1.6 MB · Views: 44
Last edited:

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
1,805
E) Questions:

  1. 240w / 24v = 10ah so why on the module linking the two engine it is written 15ah ? Which one should I consider to calculate the power needs?
If you divide 1240watts by 24v you get 10a (amps) - e.g. a current flow on the wire. You do not get ah (amp-hours).
AH or (ah) = amp-hour. This is a measure of capacity in the context of a battery voltage. For example, 24v @ 13AH = 312wh (watt-hours). I find this link for quick calcs helpful - https://milliamps-watts.appspot.com/ Note: 1ah = 1000mah. So 15ah = 15 * 1000 = 15000mah (and vice versa). Another example - the LG MJ1 3500 mAh 18650 battery has 3.5ah (3500mAh / 1000) @ 3.7v.

  1. As you might know Vruz. offers two kits the 1.6 and the 2.1. I understand that the main difference is the current discharge one can take from one cell: 3.5A from the cheapest kit and 20a on the most expensive. Which one would be more suited for my use ?
At 24v and a 5p battery it would be 5 * 3.5a/cell = 17.5a @ 24v. One of the pictures shows 16a - implying this is the 'max' current? - so this would be cutting it close but might be worth a try if the price difference is high. On the other hand, the the 20a will work for sure and may be needed if it spikes above the 16a?.

  1. What other cheaper cell could I use staying with well know brands?
16a @ 24v for a 7s5p is 16a/5cells = 3.2a/cell. This is out of range of garden variety laptop cells (generally cheaper) and puts you squarely in 'high-drain' cell category such as the LG MJ1 that you've selected with 10a continuous.

'd stick with 10a continuous type of cell (which will be more expensive) or make the battery 6 times larger such as 7s30p so the per cell drain is 16a (max) / 30 cells = 0.5a / cell.

  1. How many kilometer could the trolley run on one charge ? I calculate the following but I am not sure: Using the 17.5ah battery pack mentioned above plus knowing that the maximum speed of the trolley is 8km. Theoretical autonomy would be: (17.5ah / 10ah) x 8km = 14km – Is that correct ?
Don't know what 17.5ah (amp-hour) / 10ah (amp-hour) means - see above.
A 7s5p with 3500mah cells will have 3400mah * 5 = 17000mah. 7s5p will be 17000mah @ 24v = 408wh. You can typically use 80% of a lithium-ion batttery so 80% * 408wh = 326wh useable.

How far can you go on 326wh?
1) Should be the same as you could with the original battery.
or
2) At 16a, that's 16a * 24v = 384w. At 384w - you should be able to go about 50minutes continuous - e.g. ~ 6.5km.


Thank you very much for any help given 🙏🙏🙏
 
Last edited:

italianuser

Member
Joined
Feb 25, 2020
Messages
377
A) Specs of the golf trolley:
  • The trolley comes with two engines of 120w each running on 24v.
Welcome to you. Sounds like a nice project!
Your 240W number I suppose comes from here (120W x2).
  • They is a module linking the two engines, the control panel and a remote control (picture attached). On this module, one can read the following:
  • ...
  • Vol: 21-29,4V / CUR 15A
15A, better to have a higher capacity, otherwise the module would be working at limit specs and would break sooner.
As far as my understanding goes I calculate the following:
  • I should build a 7s5p battery pack with (of course) a BMS
  • I could use the LG MJ1 3500 mAh 18650 battery cells are they are well renown
  • I would get a 24v / 17.5 ah battery.
Didn't do calculations, but battery cells seem ok for your application, they are INR chemistry (not ICR), correct for a high load like a motor.
E) Questions:

  1. 240w / 24v = 10ah so why on the module linking the two engine it is written 15ah ? Which one should I consider to calculate the power needs?
See above. Motor will require maximum 240W (in a given instant), so your power supply, battery, must be able to provide for that.
In one hour, at maximum power, your motor will use 240Wh. You can dimension your battery based on this number. A 480Wh battery should give you around 2 hours autonomy (maybe some percentage is lost/used by circuits).
  1. As you might know Vruz. offers two kits the 1.6 and the 2.1. I understand that the main difference is the current discharge one can take from one cell: 3.5A from the cheapest kit and 20a on the most expensive. Which one would be more suited for my use ?
oh, 20A absolutely. Again, didn't calculate anything but I know that ebikes and electric motors want a lot of power (=discharge of your battery must be able to support it), so 20A is your choice.
  1. What other cheaper cell could I use staying with well know brands?
Uhm, I don't think you'll find cheap 20A discharge INR cells. You could consider finding used 90% capacity INR cells.
  1. How many kilometer could the trolley run on one charge ? I calculate the following but I am not sure: Using the 17.5ah battery pack mentioned above plus knowing that the maximum speed of the trolley is 8km. Theoretical autonomy would be: (17.5ah / 10ah) x 8km = 14km – Is that correct ?
I don't know about kilometers, but... if your current battery gives you, lets say 10KM autonomy, then a battery with double the amperes should give you around 20KM autonomy (+/- 10%). I would calculate it like this, in respect to the current performance.
 

Andy B.

New member
Joined
Jul 18, 2021
Messages
2
Hi !!
Thanks @OffGridInTheCity and @italianuser fro taking the time to help me. Really appreciated.
Thanks for pointing out my mis use and understanding of the two units of measurement A and Ah. I now see the difference. Now I have answer to all my questions. I will go for the Vruz. 2.1 kit and will go for the LG cells mentioned above.

Now I am facing a "new challenge", finding a 7s BMS from a reputable US dealer. So far I haven't find anything. The Vruz. site was selling some but they appear out of stock and do not answer me email regarding an eventual back order.

If you have any advice.. (y)
 

asder

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Joined
Aug 21, 2021
Messages
1
I would advise you to check beforehand when you travel, what the import duty cost of lithium batteries are in that country, can it be taken along in a sea container? does it require special government permissions? Lithium cells are not as easy to transport as we think. There is sadly lots of red tape all over the world because governments do not update their laws.
 
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