I have a scooter in Asia. I built a 72v 60ah lithim-ion battery with Daly BMS to replace the 20ah sealed lead acid batteries. I wanted to extend the distance.
Did the system have any problems before replacing the battery?
Is the breaker part of the system, or part of the new battery?
If it's part of the new battery and is rated properly for the battery/BMS/wiring, your system is drawing more current than the battery is capable of, so you'll need a different battery that can supply more current than the system needs, instead of the one you have.
With the new battery, the 40A MCB trips when I give sudden throttle to go over a hump in the road. If I start slowly on flat ground, it doesn't trip, but even on flat ground if I increase the speed suddenly, it trips.
Do you have a wattmeter or anything in your system that displays or logs the amps or watts being drawn from the battery? If not, it would be helpful to at least borrow a clamp-on ammeter to measure the peak current your system draws in normal operation and at this trip point.
Breakers don't trip instantly at their rating, they trip well above it if it's tripping "instantly" rather than after a sustained time of that load. So either the breaker is defective, mislabelled, or the system is drawing a lot more current than the 40A breaker rating.
Without knowing what the actual current draw is, you won't know if you have properly rated stuff in the system (wiring, connections, breaker, battery, BMS, etc). (when troubleshooting this kind of problem it's not really safe to just go by the labels on things and assume currents will only be as high as they list, testing is needed to be sure).
Question is: Controllers state a AMP range on them which I think is output. My controller is run-of-the-mill from China and I suspect it has around 25AMP output - this is based on some of the searches I've done on 72v controllers. If AMPs coming into the controller are 50 how does controller output around 25?
What the amp rating on the controller means depends on the specific controller's design. Most of them list a battery current limit, and if they don't state otherwise this is probably what it refers to, so this would be input current.
If they list a phase current, or both phase and battery, the one you care about for this problem is only battery current.
Anyway, you can't just assume there's a specific amount of current based on other controllers. You must check *your actual* controller's ratings. If they are not on the label, and there aren't any markings on it to tell you, you would have to do the test with a meter to find out what it is, or else you can't reliably fix the problem you have.
Output (phase) current can be much higher than input (battery) current, because controllers are power converters--they take a voltage and current at the input and make a different voltage and current waveform on the output to make the motor do what you command via the control inputs. (if you need details on how this works, there are a few threads you can look up around here on how motor controllers work; it gets a bit complicated so is beyond the scope of this thread).
Both new and old batteries are 72v - the controller should pull same AMPs from 20ah battery as the 60Ah battery so why the MCB keeps tripping is beyond me?
If the old batteries sagged in voltage under the same load, you might not have gotten this amount of current, because the controller may ramp down it's current draw as voltage drops to prevent overload of the battery (some do this, some don't). If the new battery doesn't sag in voltage under this load, the controller would then draw more current because it doesn't see a voltage drop and thus no reason to decrease current draw.
Wire from battery to controller is labeled 2.5mm squared. That isn't very thick, but if that wire handles 72v 20Ah I think it should handle 72v 60Ah.
Ah has nothing to do with current, it is only capacity, so it has nothing to do with what the wire has to handle. A and Ah look similar because they both have an A in them, but mean completely different things.
The MCB is for a fault so I keep thinking about carefully trying to connect battery to controller without the MCB, but I'm not sure how the controller handles large AMPs in while outputting smaller amount to current to the motor.
If you remove the breaker, then whatever is causing hte overcurrent it is protecting against may damage the parts not able to handle the overcurrent (this is why the breaker is there). (assuming it's not just a faulty breaker).
Since you don't yet know there isn't a fault it's protecting against, it's not really safe to wire around it.