I am building a 20inch fat tyre ebike, I have two batteries which I will use a battery blender to join, battery one is 48v 12ah max 25 amp max, battery 2 is 48v 15ah 30amp max. I intend to buy a 48v 2000w rear dd hub motor kit for the bike, the controller (programmable) for this kit has a maximum current of 60amp and a rated current of 30 amps. My question is what would be the optimum setup using the components I have, my main concern is will I get the full potential out of the 2000w motor if I reduce the output amps on the controller so as not to damagethe batteries, or if I try to draw to much amps from the batteries will it end in disaster,
I recently posted something in another a thread that covers the question in your title.
https://endless-sphere.com/sphere/t...mps-and-lifepo4-batteries.122855/post-1793763
As for using two batteries not directly paralled, but thru electronics (blender) that you don't know *exactly* how they work and what they do in various conditions***, to support a load higher than either of them can support alone, the only way to find that out is to test it under worst-case conditions...but this can result in damage to any part of the battery or blender system that isn't designed to handle those conditions / loads. If either battery isn't able to or allowed to provide it's portion of the load, the entire load will be placed on the other one.
At that point what happens is determined by the actual load and whether and how well the protections, if any, in the battery and/or blender work. If there are no protections, the battery will see the full system load, and be unable to do anything about it. If that load is sustained, and is the max the controller could draw, then the weakest part of the system would be most stressed; that could be the BMS FETs, the cells, etc. What happens to them depends on their actual specs and how they handle that load--at minimum they'll heat up internally much more than normal.
***Blenders of various kinds work differently. Some are supposedly "smart" and "choose" which battery to use, and so they dont' actually blend them at all, they just switch between them based on whatever parameters the designers thought were good...but they don't typically say anything about how this works in any of them. Some of them are just a pair of ideal diodes, and those would always work better, because they will never disconnect a battery from the system (they leave that up to the battery's BMS like they should), and each battery would just provide however much current it could based on it's internal resistance and charge state.
Regarding max, rated, etc:
The battery specs of 30A max and 25A max are probably only for a second or few at a time for those currents, but you don't list what their continuous current is (the seller/maker may not either, it's common to only post the "best looking" numbers, and leave out anything that makes something look less valuable even though it means people will be using the parts beyond their capabilities, becuase they dont' even know them). If the batteries have a well-designed BMS (unlikely) then it will shut off the output (cutting power completely) if the limits are exceeded for whatever specified time. More likely, they have no such protections and depend on the user to not abuse them.
The controller spec of 60A max and 30A rated, doesn't say how it actually behaves. Some of these ratings mean that it could draw up to 60A for a second or few under max load and then would automatically lower the power available to limit to 30A. Some of them mean that *you* have to stop demanding more than the max after more than a second or few, or else heating and damage to the controller can happen if you don't drop the load to below the rated limit. (it also doesn't say how long or how often that max can be or will be applied).
You would have to test your controller under worst-case riding conditions, with a wattmeter or ammeter installed between the battery and the controller to find out which one of these happens. Otherwise, its' safer to assume that the limiting is NOT automatic, and must be done by the user while riding, so you would still want to put that wattmeter/ammeter on the system and keep an eye on it while riding to be sure that it does not exceed the rated limit for too long (a few seconds or less, to be safe).
my goal is to get the bike up to 40mph/60kmh will this be possible with what I have?
You can use calculators or simulators like those at ebikes.ca to see if a system generally similar to yours in type and power level can do what you want under your specific riding conditions.
You may also want to check if it's legal to operate an ebike at these speeds and/or power levels in your locality; if not you could lose it to law enforcement (or worse).
