Dracis said:
I know the battery will do 50A max continuous and 100A peak
What test did you do to determine it?
but I don't know how to check C rating, its a 16cell battery pack and the info on C rating from the battery maker is "Lifecycle of single cell: >85% capacity after 1500 cycles, > 70% capacity after 3000 cycles. (<1C discharge rate and <1C charge rate)" but not an actual number and the info i have found on determining C rating (from a different posting on this site "lifepo4 c rating" and an electrical engineering site) it seems the battery is either 1C or 2C.
1C to 2C is typical of cheap LiFePO4 packs. If the manufacturer does not brag about it then it's probably only that. So that means that the max continuous current you should pull from it is 50A-100A (50Ah x 1C =50A), but even at that it might have significant voltage sag depending on the cell type and pack construction.
Even at that current it may have serious voltage sag, which lowers the amount of power (watts) you actually get out of it to apply to the system.
I have been able to confirm no discernible voltage drop between battery and controller, the wires are short and doubled up.
Is that under maximum load, during the acceleration from a stop?
But that's not really the problematic part. The voltage sag *of the battery* is the issue usually seen.
Voltage sag is how much the voltage drops from having no load on it to having a specific load on it. The important questions here are ones you did not answer:
--what is the current drawn from the battery during acceleration?
--what is the voltage drop during that time? Meaning, what is the voltage while you are stopped? And what is the voltage during acceleration?
--What is the watts drawn from the battery during acceleration?
If you don't have a wattmeter in the system to determine these, you can use a regular voltmeter and ammeter (you'll need one of each, as you need to see both readings at the same time, or have something that logs them so you can see them later).
The minimum voltage usually occurs at the maximum current, so a wattmeter is ideal for this as it logs both of those.
Weight is simpler, I'm about 300lbs and the bike weighs about 320lbs,
That's pretty heavy for the limited power you seem to have. Without any readings from you of that, or knowing what your controller battery current limiting is set to, I'd guess that the battery is only going to give you around 2-2.5kW, 3kW if it's a 1C pack, or up to twice that if its' a 2C pack. Depends on the voltage sag of the battery.
You'll need more power (or much lower gearing) to accelerate that much mass quickly even on the flats. So the battery may have to be able to supply more power with less voltage sag than this one can, to get you twice the acceleration. (assuming the controller/motor can utilize the power). You'll need to test to find out what you're actually getting now before you can determine where all the problems lie.
SB Cruiser, while pulling 600lbs of dog food in it's cargo bed and a trailer, plus it's own weight and mine,
https://endless-sphere.com/forums/viewtopic.php?f=2&t=67833&start=550#p1314960
was about 3400W to accelerate me to 15MPh in more than several seconds (apparently I didn't note down the exact time, but it was at least twice as long as usual, so probably 12-15 seconds). I didn't ride at 20MPH with the load on there cuz it'd've been too long a braking distance, so I don't have the acceleration time for that with this load.
My battery isn't the limiting factor here, as I'm only pulling about a third of what it could do, it's the little controllers--they arent' designed to handle more power than this. When I get (or build, actually) better controllers, they'll handle enough more power to let me accelerate even with this kind of load on there, from 0-20MPH in <4 seconds (3 is my goal, but probably not with this load).
rear wheel is 24" diameter including tire, chain direct drive rear sprocket 42t motor sprocket 12t (yes i know the ratio is 3.5) i am planning on getting a larger rear sprocket of 60t or 64t but it should be possible to get a noticeable improvement in acceleration on level ground without having to change the gear ratio.
The larger rear sprocket will improve your startup torque, so it will help.
It looks like John in CR already did this above, but you could look at the motor's speed vs torque curves supplied by the manufacturer, and it's efficiency curve vs load, to determine optimal gearing to keep it at the optimal speed range while also giving you the torque and speed you need out of it.
Once gearing is optimal, then to get better acceleration you have to either reduce the weight or improve the power available.
A better battery will supply more current with less voltage drop. (assuming yours probably can't supply enough without too much voltage drop; won't know that till you test it).
A higher power controller will supply more power to the motor, as long as the motor itself is rated for it. (or you can cool it better than it's original design does).
Alternately, if the controller you have can be set for higher current limits, without destroying the motor (by overheating) or battery (by too high a current draw), that would improve acceleration.
