Aloha!
I have a not bad offer for a 12s7p batty build on LG MJ1, which gives approx 70A max amp.
And I’m wondering - is it enough for these two motors? I’m a very light guy (like 65kg max), this is gonna be my first esk8 so road-ripping going to be quite moderate at first, I guess. Aimed for 16/62 gearing on 175mm pneumatics.
What do you think? It is like 30-40% cheaper than my primary plan (12s5p VCT4), where maybe fewer amps, but double the range.
I don’t know the cells but surely the burst is better than 10a. With 7p sounds good enough. And then motor amps settings can be much higher and not limited by the cells
The sag on ultra-high capacity cells like the MJ1 gets pretty bad past 7A or so.
If run at more than 5A-7A or so per cell the MJ1 has a lot of extra voltage sag that will rob you of a lot of that extra capacity. All ultra-high capacity 18650’s (over 3000mAh) are best run at about 50% of their rating, 75% at the most.
It’s not a safety issue, it’s all about performance.
Could you explain to me what the sag exactly is, like to a person who never experienced it? I’ve read it’s bad and unwanted, but I really can’t imagine how it influences the ride. It’s my first build 
I can handle that. Actually, the range is not my priority at all, at first I have assumed 20-30 km tops. Any extra km is just a bonus for me.
So performance-safe rating would be somewhere between 35-49A, right? That sounds a bit low - do I understand correctly that this will limit my motors’ performance?
It’s tough to explain without taking you through a bunch of graphs but “voltage sag” is the extra voltage drop that occurs inside the cell due to its “internal resistance” (IR).
This voltage drop, the voltage sag, is different and in addition to the drop in voltage that steadily happens as the cell is discharged. When current is pulled from the cell the cell’s IR causes the cell’s voltage to instantly drop, sag down. The higher the current level the more the voltage sags down.
This sag only happens when current is being drawn. As soon as the current stops the cell’s voltage pops back up.
When using a cell this sag causes the cell to reach your low voltage cutoff point sooner (since the cell is running at a lower voltage). If you run at a low current level there is only a little sag and your running time will not be shortened by much. If you run at high current levels (for the cells you are using) then your running time will be shorter. If you are running the cells very, very hard your running time could be very short.
You might have bumped into this voltage sag effect before. If you ever used a battery powered device with the battery almost empty and it worked at a low speed but immediately shut off when you tried a higher speed then you saw the effect of sag. The higher speed drew more current which caused more sag which caused the battery to instantly drop to its low voltage cutoff point.
Or maybe a power tool could be run with no problem when just holding it up and pressing the trigger but it stops immediately when you try to drive a big screw in.
Sounds a bit low? What do you feel it should be and why?
It depends on your preferences and how much riding time you are willing to give up. Sure, you could run your pack at 75A. But every time you draw that much current, even for a second, the pack’s voltage will sag way down and the ESC might throttle down because of that.
Less current can limit what the motor(s) can do but you don’t set current limits based on what you want to do. You set your limits based on what your pack can do.
Go ahead and experiment. Compare your riding time with low and high battery current limits. See where you want to balance performance with riding time.
All this theory stuff is fine to discuss but if you haven’t played with these tradeoffs before then it’s better to just go out and (safely) see how this all works. 
Great piece of explanation, many thanks!
It would be the best thing to do. But, I do not have this pack yet, I’m up to decide if buy it (a really good offer I think, ~250$) or add some money and make 12s5p with vtc4. I’m kinda budgeting here 
The choices are then 7P of MJ1 or 5P of VTC4?
A couple of thoughts…
The thing is: vtc4 are 1,85€ /cell (reclaimed). Lowest price for p26a… 4,45€ 
(nkon)
Price is the last thing to consider when choosing cells. You only have to pay for the cells once but you have to live with the performance of the pack you end up with every single day for a couple of years.
Any extra cost disappears over that amount of time and you get to enjoy the extra performance you bought every day.
Having said that though, if VTC4’s are the cells you want to get then those are the cells we will work with (and MJ1’s).
Have you worked out what your current draw from the pack will be? When just cruising and when accelerating hard or going uphill?
I have no idea - like I said, the first build. Did not have any chance to verify that so far. I’m not going to ride a hardcore off-road MTB, rather cruising and carving. My flatland is not so flat, so there will be some (moderate) ups and downs. And - I’m complete noob in esk8 riding (but pushin the analogue longboard for 4-5 y), and I value my health and bones deeply, so it will take a long time before I will hit 100% motors capability.
And that is exactly the issue - I do not know how to estimate desired pack amp draw, I lack any reference point.
Unfortunately I’m not able to help with that, hopefully someone else will chime in here soon though (or you can ask in the noob question thread). Then I can help with the best cell choice for that level of current.
Thank you very much anyway, a lot of useful info. Would be great to buy you a beer or something
Personally I think the MJ1 pack is the better option, even though it would be the limiting component in terms of power.
If you plan just to cruise, and not to hit speeds far over 50 km/h, I think the 50A (7A per cell) or so that you could pull from the 12S7P is more than enough, not considering the motors. Yes, the motors can handle more power than that, but I’d personally take a longer range, better cells, and a cheaper battery over the slight increase in power youd get from the 12S4P VTC4 pack.
Just to put amp draw into perspective, I am about 75 kg and run a 10S3P Molicel P42A urethane build at 50A (thats less power than your 12S at 50A) with dual 6355 motors, and it has a top speed of 55 kmh and has more than enough torque for me. I havent found a hill yet that i couldnt get up at less than 35 kmh. I use it for commuting though, so i dont really care about power or speed so much, but moreso just getting from A to B.
It really all depends on how you plan to ride it though. If you see yourself moving away from slower carving and getting a bit more reckless with more power (higher speed or crazy acceleration) then the 12S7P battery will limit you.