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Hello
#1
Hi!
I have an electric moped which was originally equipped with a 24V 40Ah lead -acid battery and I am planning to convert it to run on homebuilt 18650 packs.
I happen to have around 100 18650 laptop batteries. If the build turns out OK I will purchase new cells and make a proper battery. porndollz
I have some questions. The motor is of the brushed type with a nominal output of 750w (at 24v). I am planning to run it at 37V with extra cooling. This means 10S. But I am concerned with how many cells would be needed in parallel, because the motor can momentarily draw up to 140A at 24v when starting. When running in cruise mode it draws between 20-40 at 24v. I have a brushed motor controller which can handle up to 50V and 300A momentarily and 160A continous. However, it does not have a current limiting circuit. xtreamporn
I understand that LiPos are more sensitive to damage from drawing too high current, compared to lead-acid? How many should I have in parallel to avoid damage? And how do typical laptop cells compare to for example LG MJ1? I guess these are chosen for higher current capability as in e-bikes? xnxxvideo
How do LiFEPO4 batteries compare to 18650 batteries? I have seen many reasonably priced on eBay. For me, the weight and size of batteries is not as important as for an ordinary bike, because the moped was designed to carry batteries (37 Kgs !!). xnxxhub
Another question related to balancing chargers. If I decide to wire the packs to be charged by a balancing charger, how many cells would be connected in parallel? In RC packs I think the balancing wire is connected to each cell? 
Thanks for great information here!
John
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#2
Sounds like you need a proper motor controller? It should manage max current for you.
LiFePo4's are great & you might find a prismatic format gives you a good fit.
At 140A, assuming 5A/cell you'd need at least 140/5 = 28p
If overvolting re-do calcs accordingly! Some sort of parachute might be needed :-)
Re the 18650's you'd want to be testing for & using low IR cells = less heating, better performance.
Obviously high current cells will be needed for your moped.
Build your packs with vibration in mind!
Extend the chassis a bit:
https://i.pinimg.com/originals/80/3e/90/...35e466.jpg
There's also the ev forums ?
Running off solar, DIY & electronics fan :-)
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#3
I can comment on a couple of your questions to start the discussion....

>If the goal is a battery to deliver 140a @ 37v....
18650:
You indicate 10 in series. For 140a you need to divide 140a/amp-per-one-cell-in-parallel For example, if you by 10amp cells that would be 14cells per pack * 10 packs in series = 140cells. If you buy 5amp cells that would be 28cells per pack * 10 packs in series = 280cells. I typically see 5amp cells (or even 3amp cells) cheaper than 10amp cells. You don't mention run-time... so even if you build a battery that delivers the max current, you still might want to add more cells in parallel to get a longer run time. Typically cells weigh in range of 40 and 45grams each.

LifePo4
You would use 11 or 12 in series for 37v nominal
>I understand that LiPos are more sensitive to damage from drawing too high current,
Same calculations as 18650s above - just make sure you have enough in parallel or are large enough specs for what you need for the max. Then add more if needed for longer run-time and add up all the weights. And of course the overall size of battery must meet your needs.
One key aspect of LifePo4 is they have a much longer life span in terms of charge/discharge cycles that 18650.
As far as safety - in a crash? or in vibrations etc? - I don't know.



>Another question related to balancing chargers.
> If I decide to wire the packs to be charged by a balancing charger, how many cells would be connected in parallel? In RC packs I think the balancing wire is connected to each cell?
Yes, balancing involves connecting a balance lead to each cell or pack in series. So if you have 10s - then you need a charger with 11 'leads' - e.g. 1st negative one and then 10 more on the positive side of each cell in series.
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