20s Lipo for cyc x1 - external BMS?

[jacobm906]

Hi guys.

I'm wanting to do a 5kW+ cyc x1 pro gen 4 build on a downhill bike. Most of the bikes I'm looking at won't fit an off the shelf 72v 18650/21700 battery (especially when looking for ones available in Australia). I've looked into building my own 21700 battery, but with the cost of a good spot welder, it gets pretty expensive.

Therefore, I'm planning on using some Turnigy high capacity Lipos. Something like five 4s 12Ah lipos in series to give me 72v 12Ah (potentially could go 16Ah ones instead, or maybe three 6s ones, need to mock up the layout once I buy a bike).
The series connection of the batteries should be pretty simple, and to simplify charging, I'm wanting to use a BMS. The plan is to make up a harness that goes between the JST-XH plugs on the lipos and the BMS, and just make sure I wire it properly wrt how I have series'd the batteries.

However, it seems like the cyc app lets me set a low voltage limit, so I'm wondering if I can get away with not having a BMS on the bike (i.e. discharge straight from the battery and rely on the cyc low voltage limit). Then for charging, I would have a BMS attached to my charger, and plug into a charging port and a 21 pin balance connector (if I run a 20s battery). So essentially just avoiding packaging and wiring of the BMS on the bike.

I am planning on building protective cases for the lipos, from the reading I've done, this should alleviate most of the safety concerns with Lipos on ebikes (?).


Any help greatly appreciate. Cheers

 

[amberwolf]

A safer and longer lasting battery option (especialy without a BMS) would be used EV modules from places like batteryhookup and greentecauto. They come in a wide variety of capacities, cell types, capabilities, and voltages. Some are easily reconfigurable, and most come in a fairly sturdy framework if not a complete enclosure.

 

[jacobm906]

A safer and longer lasting battery option (especialy without a BMS) would be used EV modules from places like batteryhookup and greentecauto. They come in a wide variety of capacities, cell types, capabilities, and voltages. Some are easily reconfigurable, and most come in a fairly sturdy framework if not a complete enclosure.

Thanks, but looking at Australian suppliers, I can't find any less than 50Ah 1s cells, so to get enough in series for the cyc would be massive.

 

[bananu7]

if I can get away with not having a BMS on the bike (i.e. discharge straight from the battery and rely on the cyc low voltage limit)

This won't protect you from a short or overcurrent and will require extra steps when charging instead of simply plugging in. You'll also lose the temperature monitoring during discharge.

LiPos without a BMS are, IMHO, battery chemistry ideal for drones - not just because they offer insane power to weight ratio, but because when they catch on fire, they're far away from you.

 

[jacobm906]

This won't protect you from a short or overcurrent and will require extra steps when charging instead of simply plugging in. You'll also lose the temperature monitoring during discharge.

LiPos without a BMS are, IMHO, battery chemistry ideal for drones - not just because they offer insane power to weight ratio, but because when they catch on fire, they're far away from you.

Ok thanks. Running a bms on board seems like the go then.

So does having a bms negate your second comment? I.e. is your safety concern the lack of bms, not inherently the lipo chemistry?

 

[bananu7]

So does having a bms negate your second comment? I.e. is your safety concern the lack of bms, not inherently the lipo chemistry?

I think it really depends on what you have available. For both of my ebikes and my CRF, I ordered very affordable pre-welded packs which I enclosed. In that case, adding the BMS to it was a breeze. Right now I'm evaluating using large EV pouch cells like amberwolf mentioned, because they are much simpler to combine, offer good parameters and most importantly I can source them cheaply. If neither of those options works for you, then I guess a well designed lipo system could be the only remaining alternative that fits your volume/weight/cost/capacity budget.

 

[scianiac]

One of my bikes has exactly that, 5x 4S 8ah lipo packs currently with no BMS and using the controller voltage limit. Honestly if they are new packs and you test them to make sure their capacity is all in spec and charge them with a balancing charger and have a suitable fuse, it's fine (not ideal but fine) but a BMS is better. I'm installing a BMS now mostly because it's easier to charge. You probably won't have any issues with temperature as that much battery you will cook that motor long before you even warm up high discharge lipo bricks.

And yes if you can get industrial batteries that fit instead they are safer, there is probably some company down under that sells surplus EV cells. Also using EV cells or cylindrical cells will result in a much higher energy density battery (because they are lower discharge) which can be easier to fit and good for weight reduction. Remember many of those RC lipos are made to run for like 30min or less so that's the kind of discharge they are optimized for but I assume you intend to ride for considerably longer than that.

Also you should consider the size of the battery vs what your use intention is, a smaller battery is way easier to fit and with lipo bricks you can make a smaller battery with high power output. My bike is around 5kw, DH bike, ride on trails and I've never got even close to using a full battery. It really depends a lot on where and how fast you are riding. If you want to use that much power continuously like on the street going fast or on fast dirt bike trails (so using up that battery quickly) honestly that motor is a bad choice. If you are riding on trails at slower speeds (because you can't go that fast safely) and you aren't intending to do super long distance rides a smaller battery may be better and that is more suited for that motor.