Powering a 6KW system: 60V or 72V?

Hey guys,

I'm in the process of converting an old Vespa over to electric, and I'm building a battery pack. My motor is a Kelly hub rated for 6KW at 72V, and the literature says it'll push over 8KW pulling 125A at 66V. The controller will handle 8KW at 24-84V, and up to 300A I think.

My battery pack, on the other hand, is small. I've just bought 80 Samsung 25Rs that are good for 20A. My initial plan was to build a 20s4p pack, and have 72V at 80A. However, after looking at the world of BMS options, and finding none at 72V that I have much faith in, I'm considering switching to a 15s or 16s setup.

I'm not planning on this scooter going fast - 40mph tops would be fine. What I am interested in is torque - I live in San Francisco and I'd like to be able to punch it up steep hills and not feel like the bike is struggling.

So my question is, if I'm delivering the same amount of power to the motor, will I see a difference running at 60v instead of 72v? Are there advantages to either (other than top speed)? And do you have any recommendations for a smart BMS to use?

Also, are there any other threads that tackle this, so I don't go asking a million previously-answered questions?

Thanks in advance!

That's a tiny battery -why so small?
That pack will be flat in a flash pulling 200 amps after a few hills or accelerations. Not to mention the sag and the very short life of that pack- you will be absolutely trashing those cells at that draw. 4p is low power bicycle territory. My midi-e is on a bike, its peak is around 6kw and I'm running 10p of 30q. 4p of low capacity cells on a Vespa won't last long either- like a few mins. 10ah/200amps=3mins. 100 amps =6mins. Cruising on the flat at say 50 amps gives you 12 mins run time. The lvc will have you walking home on the long (short for you) walk of shame
Beef her up 12p min, I'd say more like 16p for decent performance. I'd ditch the 25rs and get some 30qs
Sorry to rain on your parade but better fix it now before you get on the rd.

No, that's good to hear, and thanks for the advice. I also think the pack is small. My initial goal was to match the stealth fighter (https://www.stealthelectricbikes.com/stealth-f-37-fighter/) in specs, as a local SF reviewer got around 50 miles in range, and the little vespa will weigh about the same. That's with a 1500wh pack, and if I go with a 20s4p configuration I should have about 1/2 that at 740wh. My goal was 72V 20-25AH. Hopefully a 72V 10AH pack will go for more than a few minutes?

I'm also initially limiting this to whatever I can fit into the gas tank, so I'm limited initially to an awkward 7.5L of space. Here's a quick sketch of what I'm working with:



Either way yes I will most likely strip out my Vruzend kit and build something higher density, but initially my hope is that 740wh will at least get me 10 miles of range. Am I kidding myself?

And if I'm going for more density, I could probably fit another few cells in. If I could go with 30qs and a 15-16s6p that would give 90-96 cells and over 1kWH in the pack.

Also, you mention adding more in P. I could pretty easily swap things around and end up with a 36V 10s9-10p system. At that point I'd be cranking 200A. But is there a benefit to 200A at 36v over 100A at 72?

It more depends on what your target top speed is.... Upping the voltage is great for getting a slow motor spinning up faster, but if you're already ok with the top speed, then focus on upping the amps for hill climb.

Hey thanks. That’s good advice as this motor is already set up to go way faster than I am interested in going. Here’s the published power curve:

http://kellycontroller.com/mot/downloads/72V_6KW_10inch_curve.xls

I’m not sure exactly how to get all these numbers to line up, but if I’m reading it right it does suggest that at 48V the motor would be happy to hang out at 35mph. And if I want to burst to 8kw, that would require 10kw on the battery side, which is 200A.

200A means 10P cells. So I guess I’m looking at 13sP10, or 130 cells. I’m pretty sure that’s too big, but I might be able to get 104, which would be a 13s8p. Does this sound like a better route?

Batteries are expensive and your pack will be a major part of your build in time and cost. I rate my pack size (as an absolute minimum)at peak draw of controller =continuous max draw of cells. At 8p and 200a you could be drawing 25amps from your cells which at high frequency or for long periods will still be stressing them. Although it's way better than 4p cause now you're within spec, be aware that the pack will have a shorter life as a result. Heat will be the byproduct and it will only get worse as IR increases.
Bigger pack equals less stress and longer life.
At 25amps per cell you will need to pay special attention to the series connects, current sharing and thermal sensing otherwise you may still have a rapid failure of the pack. At 25amps an ordinary piece of 8x0.2mm strip of nickel will burn you if you touch it.
Your pack design will need to be careful considered in many aspects.

That’s really good to know - thank you. I’ve been considering the series connections as well, and am considering just getting a spot welder and using nickel plates instead of strips. That seems far more stable.

If I went that route I’d also be able to pack a lot more cells in there - the Vruzend kits take up roughly 2x the space. And not getting so hot things melt seems like a plus.

On higher current draw stay away from the strips....too small in most cases and then by adding a double or triple layer just increases the resistance. Go with wider strips that can carry more amperage and takes care of both parallel and series connections. Then add copper to the top of the strips for proper current sharing on the positive and negative strips. Also if you want to go up hills you HAVE to find more room for batteries. A good rule of thumb is twice what you actually want as far as range or current draw. You will soon find the batteries limit and send it to an early death with such high amp draw. You will also get wicked voltage drop if you don’t severely raise the P count. The cost is there up front but it will live a long life it built and cycled properly or at least within its limits. I was good with a 25 amp pack as far as range but wanted plenty of amperage for big torque. My latest pack is a 20s15p 30Q which is 45 amps but capable of 225 amps draw. I pull 200 amps from 0 till right up to 65 mph or until my man card batteries go dead....haven’t tried to go faster than that yet. Actually doesn’t pull 200 until about 10-15 mph on my MaxE. Most I’ve used so far is about 25 amps on the pack. You really don’t need cell holders as they take up valuable room....

Anyway build as big a battery as you can and higher voltage is always better with 20s being the sweet spot for the majority.

Tom

That looks excellent, and really more like what I should be doing. Yeah, I should just bite the bullet and invest in a spot welder, and commit to doing this the right way. At least the Vruzend kit will let me get my controller set up, and perhaps the 25R / 30Q difference won't be all that noticeable.

Where do you get your nickel?

Hey what are your thoughts on BMS? I'd like to run without one in between the battery and controller. My fear is that it could kick in with protection during regen and cut braking. I'd like to have one manage everything else - charging / balancing / etc - and my guess is that I could just add a BMS on top of the battery. Does this sound workable? Any inssues you can think of?

Also, thank you so much for all the advice so far. I'm learning a ton and would not feel as confident without the support of this group.

Ya go higher voltage if you can it will make it easier to max out your power with smaller wire guage. I run my kelly on 18s because lipo is easy to get is 6s format but i already want to increase to 20s to get a bit more top end and overall power.

The only down side to higher voltage is expensive chargers and needing a pre charge circuit

18650’s never perform like the spec sheet says they do. You would be crazy to waste your time on a 4p pack and expect over 100a all the time..go big or go home(pushing a dead bike)

I got the nickel with the batteries from nkon https://ru.nkon.nl/

I have a MaxE and the controller limits battery power to 200 amps.

Most do the BMS for charging and balancing but I think discharge protection is a must but I’m not for sure. The one that “CyklonEbike” uses looks to be a good one with up to 300 amp current draw. One example, https://www.aliexpress.com/item/Free-shipping-1pcs-smart-display-3-2V-3-7V-300A-lithium-battery-protection-board-balanced-management/32741659733.html?spm=2114.search0104.3.36.76792612JIzGpd&ws_ab_test=searchweb0_0,searchweb201602_4_10065_10068_10130_10890_5730315_318_10547_10546_319_10548_317_10545_10696_450_10084_10083_10618_452_535_534_5729215_10307_533_5025915_532_204_10059_10884_323_325_10887_100031_5728715_320_321_322_10103_448_449_5728415,searchweb201603_55,ppcSwitch_0&algo_expid=9bdca9f3-09cc-4a93-8b9e-36eb184e747b-5&algo_pvid=9bdca9f3-09cc-4a93-8b9e-36eb184e747b&transAbTest=ae803_4

Tom