Hi everyone,

I’m working on a project involving an e-bike battery system, and I need help solving a problem with the lock mechanism. Here’s the situation:

• The lock serves two purposes: it turns the battery system on/off and physically secures the battery to the bike housing.
• The issue is that the lock is rated to handle a maximum current of 15A, but the motor in the system draws about 17A, which is causing damage to the lock.
• Unfortunately, replacing the lock is not an option, so I need a workaround to protect it while still allowing the system to function properly.

To better illustrate the setup, I’ve included a diagram of the circuit.

What I’ve Tried So Far:
My initial idea was to create a PCB to split the current, redirecting 25% of the load current through an NMOS transistor and the remaining 75% through the lock. However, I realized this wouldn’t work because the NMOS transistor and the lock would be at the same voltage potential, preventing proper current splitting.

The Question:
Does anyone have suggestions or solutions to fix this issue? My main constraints are:

1. The lock must remain in the circuit and cannot be replaced.
2. The solution needs to protect the lock from overcurrent damage while allowing the motor to operate normally.

Any ideas on circuit modifications, clever current bypass methods, or other approaches would be greatly appreciated!

Thanks in advance for your help!

System diagram with the lock's locaton:

The first idea of the circuit with NMOS fets:

 

You need to limit the motor and charger currents to 15A or less.
Welcome to AAC!

 

You need to limit the motor and charger currents to 15A or less.
Welcome to AAC!

Do you have any suggestions on how to do that? Just use something like Shunt resistors maybe? Thank you for your reply

 

Use the lock contacts to control a ≥30A SPST (automotive type) relay which carries the battery current.
That way the lock contacts only carry the small relay coil current of a few hundred mA.

 

How about simply putting a 10 amp fuse directly in parallel with the lock device? The current division might not be equal, but if there was a short circuit then there would be enough current to trip the lock, and then when the lock tripped off the fuse will also fail. because all of the current would pass thru it. So the battery still will have short circuit protection, which is what you need.