How do you guys size your main fuse/breaker?

[atarijedi]

I'm talking about the fuse/breaker between the battery and the controller.

I've read a whole bunch of different methods online. One of the mathematical methods was to determine max battery current, and divide by 5, +-20%. So if my battery pack can put out 480A, than the breaker should be 96A +-20% to find a breaker that will fit (Probably 100A). Also taking into account the maximum current the controller can handle, which in my case is 120A.

So how do you guys figure out what size breaker/fuse to use?

 

[Drunkskunk]

The closer the fuse size is to the maximum current you will actually use, the faster it will blow if there is a problem. You'll want the lowest rated fuse you can get that won't blow in normal use. So for a 120 amp controller, get something like a 125 amp fuse.

 

[ecycler]

This is probably not the best safety advise to share, but I have stopped using fuses on my ebikes. The few times I have experienced a short, it has caused one of the connectors to turn into a self proclaimed fuse before the actual fuse does its job, LOL.

 

[TristanP]

It depends a bit on the kit used. Personally I would use a fuse rated at 150% of the full load current of the motor. Quite a lot kits are advertised with their peak current however, which makes something close to that a valid choice. (Of course it's also advisable to use the appropriate wiring and connectors.)

 

[atarijedi]

It depends a bit on the kit used. Personally I would use a fuse rated at 150% of the full load current of the motor. Quite a lot kits are advertised with their peak current however, which makes something close to that a valid choice. (Of course it's also advisable to use the appropriate wiring and connectors.)

The amount of current being used by the motor isn't necessarily the same as the amount of current being pulled by the controller from the battery pack.

At full torque the motor could be pulling 300 phase amps while the battery is only putting out 100 amps. So if the fuse, or breaker, is going between the controller and the battery, you want something rated for that.

 

[motomech]

This is probably not the best safety advise to share, but I have stopped using fuses on my ebikes. The few times I have experienced a short, it has caused one of the connectors to turn into a self proclaimed fuse before the actual fuse does its job, LOL.

Exactly! I have determined that 4 m/m bullet connectors have an appropriate INTERRUPTING RATING for 12S LiPoly (nom. 46 Volts).
Testing is a little nerve-racking though

 

[TristanP]

The amount of current being used by the motor isn't necessarily the same as the amount of current being pulled by the controller from the battery pack.

That's a correct observation, but it's still well within the specifications. (Not even mentioning the controller's specifications probably are corresponding with the motor.) If there's a big discrepancy between the current drawn by the controller and that applied to the motor your controller is just very inefficient. Also it's noteworthy controllers for e-bikes actually do limit current. Stall current of the motor for instance (i.e. the maximum current drawn when applying it's maximum torque) could easily blow controllers (e.g. the RC ones) without it.

At full torque the motor could be pulling 300 phase amps while the battery is only putting out 100 amps. So if the fuse, or breaker, is going between the controller and the battery, you want something rated for that.

No, you don't want that. One does not choose a fuse corresponding with how much current a power source may offer. That's a ridiculously dangerous approach. Especially given the high current most power sources, including batteries, may offer. A fuse should match the circuit, being it the source circuit or the load circuit. As you mentioned a motor I assume the latter.

When a pack doesn't output more then 100 amps max, you can't exceed that by drawing more current. A properly build battery pack will have a BMS, and all the decent ones offer over current protection. Typically one which doesn't exceed the capabilities of the cells. If you somehow decided to build your own pack and recycled a BMS, or am confused by how the specs are stated, you should know which fuse you want to use. After all you chose for certain cells, connections, wiring and connectors to a certain specification. Any fuse within that specification will do, in regards to source circuit protection.

That being said you mentioned your motor and the fuse to be used between the battery and the motor. For that my previous statement is still valid. If in typical applications your motor may draw more current then your battery can offer the battery simply isn't suitable. The fact that the source circuit needs a lower amperage fuse, than the load circuit, just makes that more apparent.

 

[atarijedi]

If there's a big discrepancy between the current drawn by the controller and that applied to the motor your controller is just very inefficient.

That's just not true, at all. Controllers are PWM devices, so the input current and output current won't always match. Current drawn by the motor depends on applied voltage, BEMF, Kv, and phase current limits. The same amount of power might flow from the battery to the controller, and the controller to the motor, but that doesn't mean the current will match, nor does it mean there is any large inefficiencies.

The Adaptto controller and the Cromotor hub motor are a common combination which can, and does, pull 100A from the battery pack into the controller while the motor pulls 300A from the controller, and it's one of the best on the market.

No, you don't want that. One does not choose a fuse corresponding with how much current a power source may offer. That's a ridiculously dangerous approach. Especially given the high current most power sources, including batteries, may offer. A fuse should match the circuit, being it the source circuit or the load circuit. As you mentioned a motor I assume the latter.

How is that a dangerous approach? If the controller pulls 100A from the battery, and the CPD is going between the controller and the battery. Than you want a CPD rated for that circuit. You want a CPD that allows a continuous 100A but if it goes much higher, it will trip. My question is all about how much higher, what kind of CPD, fast blow fuse, slow blow fuse, same for breakers, obviously it needs to be a DC breaker, but beyond that I am at a loss.

When a pack doesn't output more then 100 amps max, you can't exceed that by drawing more current. A properly build battery pack will have a BMS, and all the decent ones offer over current protection. Typically one which doesn't exceed the capabilities of the cells.

The purpose of the CPD is if the BMS fails, or if the controller fails, or if the motor fails, to protect all the other devices in the circuit.

That being said you mentioned your motor and the fuse to be used between the battery and the motor.

Actually I didn't say that. You said that. I said that the CPD will be between the battery and the controller. Controllers are PWM or VFD devices, so the voltage and current coming out of them will be different than the voltage and current going into them. Your motor can be pulling 300A from the controller, and the controller can be pulling 100A from the battery pack. So obviously, you would need to use different breakers/fuses for different points within the circuit.

 

[Punx0r]

Most controllers use current limiting based on battery current, so size fuse/breaker to 150% of maximum continuous controller rating and call it done...

 

[atarijedi]

Most controllers use current limiting based on battery current, so size fuse/breaker to 150% of maximum continuous controller rating and call it done...

I'm assuming you mean for a fast blow device?

 

[Punx0r]

Not really, but not reason to choose a slow-blow device?

For the average ebike the fault current available from the battery tends to be large, as does the wiring. The intended role of the fuse is to protect against shorts (rather than protect the motor from overload), so I can't see any reason to run a fuse/breaker rating close to the normal load rating and unnecessarily risk nuisance tripping. When a wire comes loose or chaffs through the resulting short-circuit current is going to be huge and will blow/trip most devices very quickly.

 

[ecycler]

^^That has been exactly my point and experience. Basically the shorted wires to your battery become a fusable link if you do not have a BMS that will trip on overload!