18s vs 20s lipo pack build decisions

Hi there, Im planning on building a relatively cheap 20s 16ah or 18s 20ah lipo pack for this motor/controller combo;

https://www.aliexpress.com/item/33056354249.html?spm=2114.12010612.8148356.30.543563f84Jjq7c



Now initially i planned to build a 20s pack at 16ah as the controller does handle 84v charged but Im trying to make my pack as simple as possible and reduce cost as much as possible.

for the 20s I was going to use 5 of these 4s packs in series at 16ah (they also "supposedly" have 30-60c rating probably only 20c at most)

offering approx 1184wh? for around £500

https://www.banggood.com/URUAV-14_8V-16000mAh-3060C-4S-Lipo-Battery-XT90-Plug-for-RC-Quadcopter-Agriculture-Drone-Outdoor-Charger-Power-p-1497200.html?akmClientCountry=GB&rmmds=cart_middle_products&cur_warehouse=CN



But after looking for a simple pack I wondered how much difference running 18s would really make?

If the gearing for both 18s and 20s were for the exact same top speed would the higher voltage be more efficient? would I even notice any difference realistically? or am i completely backwards here?


If I go with 18s 20ah I'd only have to use 3 x of these 6s packs however they are only again supposedly 12c but would have approx 1332wh and cost around £485

https://hobbyking.com/en_us/turnigy-high-capacity-battery-20000mah-6s-12c-drone-lipo-pack-xt90.html




I'f anyone could offer any advice that would be great as this will be my first "large" lipo pack.
I'm leaning towards 18s as I'd prefer to keep it simpler unless there are significant disadvantages or I have made some calculation errors here, I pretty sure I have plenty of gear ratio options to run either 18s or 20s to meet my demands and will be balance charging packs individually each time.


Many thanks!

Less stress on the controller, best to leave some headroom.

Top speed reduction will be only significant downside.

john61ct said:

Less stress on the controller, best to leave some headroom.

Top speed reduction will be only significant downside.

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Less stress at the lower voltage? In my mind running the higher voltage would require less amps? or is it the fact of being so close to the max voltage at 20s?


Thanks!

Yes, bumping right up to the max V rating may reduce longevity or even burn quickly.

Amps will only be stressfully high with standing start, uphill, heavy loads, should be fine at 18S.

john61ct said:

Yes, bumping right up to the max V rating may reduce longevity or even burn quickly.

Amps will only be stressfully high with standing start, uphill, heavy loads, should be fine at 18S.

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Ah that makes sense, thanks again!

18s splits nicely into 6s bricks which fit with common cell monitors and balance chargers. You can do similar with 5s bricks to make 20s but it will need 25% more hardware and each will have an unused channel.

Grantmac said:

18s splits nicely into 6s bricks which fit with common cell monitors and balance chargers. You can do similar with 5s bricks to make 20s but it will need 25% more hardware and each will have an unused channel.

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yeah 3 6s packs is nice and simple, and also already have a 6s charger although it it only charges at 8 amps but will do for now..

I did also look for 5s packs but couldn't find any to suit my needs but i think i'll definitely go the 18s route for now, and if not later on ifi ever build another pack i'll try 20s then.

cheers!

3 6s would be much simpler than 5 4s. But sometimes how the bundle is shaped matters too, for carrying it.

Re the extra speed of 20s, consider the bike itself. If it starts to high speed wobble at 35 mph, then you don't exactly need 40 mph.

Depending on how you carry the motor, battery, and any cargo, bikes can high speed wobble starting at 15 mph. For this reason, I almost never ride much above 30 mph these days. Wobble is mild at 35 mph on my fastest bike, but its still making you hang on, pay attention more, etc. less fun to ride keeping that wobble down, so I cruise 25-30 mph, running 12s now. I did run 18s for a year, when I was living downtown. I needed speed to keep up on those downtown arterial streets.

dogman dan said:

3 6s would be much simpler than 5 4s. But sometimes how the bundle is shaped matters too, for carrying it.

Re the extra speed of 20s, consider the bike itself. If it starts to high speed wobble at 35 mph, then you don't exactly need 40 mph.

Depending on how you carry the motor, battery, and any cargo, bikes can high speed wobble starting at 15 mph. For this reason, I almost never ride much above 30 mph these days. Wobble is mild at 35 mph on my fastest bike, but its still making you hang on, pay attention more, etc. less fun to ride keeping that wobble down, so I cruise 25-30 mph, running 12s now. I did run 18s for a year, when I was living downtown. I needed speed to keep up on those downtown arterial streets.

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I am not sure yet but am thinking of having each 6s pack separate in their own waterproof container for easy removal from the frame, im not too fussed about removing to charge every time Id feel safer inspecting them each time anyway..

My plan is to convert a small motorbike (pit bike) and most of the time I probably wont be doing more than 20mph as it will mostly be off road on trails/woods but that said these small bikes are very stable and have ridden them up to 70mph (with gas engine and stupidly low gearing 19t front 26t rear) I think I will gear for around 35-40mph though, got plenty of sprockets to try!

Thanks

That's a very interesting conversion, I'm considering a CRF150 sized build so I'll see how yours works out.

Grantmac said:

That's a very interesting conversion, I'm considering a CRF150 sized build so I'll see how yours works out.

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Which motor are you considering? a Crf150 would make an great conversion imo I would love too myself but am on a small budget!

Here is a video of someone converting a cr80 (same size as a crf150 i think) running a qs motor 3000w - the next size up to the motor i have..

https://youtu.be/RH9slGHhV0w

I was planning a QS3000. I don't have the chassis or motor yet since I'm focusing on a better Ebike build first but that motor seems to be very popular for a reason.

Ahh, pit bike I'd be tempted to run it on 24s. Those are designed for about 40 mph, or more. I replied thinking it was a bicycle, using typical bike hub motors.

However, depending on the motor you use, it's quite possible to get 40 mph from a 12s pack. Just depends on the volts and the amps, the max rpm of the motor, and the gear ratios. But in general, more volts is more speed, if all other variables remain the same.

On bikes, 40 mph takes about 3000w, so in 12s, about 70 amps is needed, in 18s about 45 amps, so 18-20s may still be a sweet spot. 72v 40 amps controllers easy to find. The motor needs to be robust enough to handle the wattage, so you can increase volts later. 8)

dogman dan said:

Ahh, pit bike I'd be tempted to run it on 24s. Those are designed for about 40 mph, or more. I replied thinking it was a bicycle, using typical bike hub motors.

However, depending on the motor you use, it's quite possible to get 40 mph from a 12s pack. Just depends on the volts and the amps, the max rpm of the motor, and the gear ratios. But in general, more volts is more speed, if all other variables remain the same.

On bikes, 40 mph takes about 3000w, so in 12s, about 70 amps is needed, in 18s about 45 amps, so 18-20s may still be a sweet spot. 72v 40 amps controllers easy to find. The motor needs to be robust enough to handle the wattage, so you can increase volts later. 8)

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I'd definitely prefer a higher voltage but I think the simplicity of an 18s pack Vs 20s will be be better for me and sprockets for these bikes come in many sizes and are very cheap, the qsmotor 90 70h should be capable of up to around 9kw and 45nm at the shaft so im hoping it should be enough!

Only downside to a pit bike is the small inefficient wheels/tyres and heavier weight compared to a bicycle but they are so cheap (for the strength and suspension) and so much more fun in my opinion