Increasing the power of cheap eBay BLDC-controller

After installing sensors in the Turnigy 80-100 motor I needed a high current sensored BLDC controller. Since I’ve decided to use a 12 S LiPo battery the maximum voltage of the newly charged battery is 50,4 V with a nominal voltage of 44,4 V. Most high power e-bike controllers are designed to operate on >72 V and are quite large.

When i find the time I will build my own controller but for now, I want to modify a small 48 V 350 W controller, that I bought for $25 from eBay, into something that is a lot more powerful. The key to increase power handling capability is to decrease the heat losses under high power. As a side effect, more of the energy in the battery will be used to move the bike and less to heat the controller.

the modification is done in a couple of steps described below.

Replace transistors

The controller originally contained six STP75NF75 MOSFET which can handle a voltage of 75 V and (according to the datasheet) 75 A. The typical resistance when turned on is 10 mΩ which is quite high. Realistically I think six of these is capable of handling ~15 A continuously with decent cooling. I’m not even sure if they are genuine and 48 V 350 W will be ~7 A  so the original controller isn’t really pushing them.

Close up shot of controller motherboard. You could see that I've replaced the MOSFETs with IRFB3006 and that a STM8 processor powers the controller. SOMETHING WENT TERRIBLY WRONG WITH THIS IMAGE! I WILL FIX THIS.

Instead i will use six IRFB3006 which can handle 60 V and up to 195 A (again, according to the datasheet). The silicon could actually handle up to 270 A but the wire bonds between the silicon and the case limits this to 195 A. The typical on-resistnance on these are 2 mΩ, five times lower than the original FETs! Another popular transistor to use when modding e-bike controllers is the IRFB4110 which is capable of handling 100 V but not as much current as the IRFB3006.

STP75NF75 Datasheet
IRFB3006 Datasheet


The controller

Beef up the PCB traces and wiring

The original high current PCB traces of the controller had some extra solder on them to increase the current capabilities. To increase this even further i added 3×1.5 mm copper wire to these traces. Compared to copper, solder is a pretty bad conductor so this will decrease losses and heating under high currents considerably.

Upgraded power traces
The PCB traces carrying high current are upgraded with 3 x 1.5 mm copper wire.

There was one problem with this, copper and PCB laminate have different Coefficients of Linear Thermal Expansion, a view from the side reveals that the board got a little curved when soldering. I hope this doesn’t break anything.

Curved PCB
The copper wires shrink when they cool down after soldering 🙁

The wimpy phase and battery wires on the original controller is replaced with 6 mm² wire instead to handle he increased current. And a lot of the special function wires on the controller are removed. I only need the throttle and brake inputs.

Modify the current shunt

When I ordered this controller I was pretty sure that it were based on the Infineon XC846 ship as most china-made e-bike controllers are. These controllers can have the current limit and many other properties changed in software by connecting your computer ti the controller. Instead this controller is based on a STM8 microcontroller, maybe this is programmable but I haven’t found any information on how.

Instead of programming I can increase the current limit by decreasing the resistance of the current shunt. The processor measures the voltage drop across a short bit of wire with a known resistance to determine how much current the motor uses. If I for example decrease the resistance of this wire to half, the current will be twice of what the processor thinks.

Current Shunt
The ordinary current shunt (the curved silver wire) are paralleled with a thicker and shorter copper wire to decrease resistance.

Today I recorded two videos running the motor. The first one is just running the motor in sensorless mode. I actually got it running once in sensored mode but as soon as I started adjusting the sensor angle the controller fell back into sensorless mode. The throttle in this video is a tired 10k trimpot hence the uneven throttle signal.

I also made a small load test just holding the motor. In this video the motor is run on the lowest speed possible in sensorless mode. The battery used in this clip had a voltage of 46 V. Since the currentmeter maxes out at 5,5 A the load power I created is somewhere around 250 W

20 thoughts on “Increasing the power of cheap eBay BLDC-controller”

  1. I was wanting to use IRFB4110 on a controller I bought. The problem is, where is the linear regulator which powers the microcontroller and other electronics and what is it’s voltage limit?

    1. I’m not sure I understand your question.

      The most common voltage regulator used is a LM317 with a resistor in series to increase maximum operating voltage.

      1. The problem is, If I start putting more voltage into the bike controller than it was designed for, would the voltage regulator be damaged or could I fit a different one? Maybe even use a small switching regulator instead.

        1. Yes, you are correct. Most controllers I’ve seen already have a higher input voltage than the voltage regulator could handle. This is achieved by having a resistor in series before your voltage regulator. For this to work you need to know the controllers current consumption for the low-voltage circuit.

          The figures below is just made up to show you how this could work:
          Lets say your controller right now has a maximum input voltage of 50V, if you look at the circuit board you have a large 220Ω (around 5 W capable) resistor between the input and the voltage regulator. If you measure the voltage over this when the controller is powered up you will have 26.4V which will leave 23.6V for your regulator (which is within spec). Ohms law gives you that the current consumption of the controller is 120mA. If you instead would like the controller to handle 100V you need to replace this resistor with one that drop the voltage an additional 50V. Ohms law gives you that this resistor should have the value (26.4+50)/0.12=637Ω. The resistor will need to be rated for a loss of (26.4+50)*0.12=9.2W.

          1. hey i have a infineon speed controller 72v but i cant run 48v would the resistor help me if i changed it and if so where is the resistor?

        2. A small switching regulator could also work, but those usually require a few extra components like capacitors and inductors. It will probably not be just to replace it.

  2. Wondering if I open my controller(48v 1000w got it a while back as a kit with my 48v 1000w hub motor-wheel), can you help me beef it up?

    I can make connections stronger(I’m good with soldering) puting copper wire etc but replacing chips isn’t my think, I mean I can google for them but I’d like a more experienced person into this 😀

    1. The easiest way to increase the current capabilities of your controller is to modify the current shunt. In almost all cheap e-bike controllers you can find a short (1.5-2 cm) piece of copper wire protruding from the circuit board. This is used as a low value resistor to measure the current going through the controller. The main chip measures the voltage over this “resistor” and use that to calculate and limit the current. By changing the resistance of this shunt you can fool the main controller to allow a higher current before limiting. If you for example double the copper cross sectional area of the shunt the controller will believe that half of the actual current is passing through the controller.

      Doing this will of course put a higher thermal stress on the high current parts of the controller. If you would like to make a significant decrease of shunt resistance you should as well add some copper wire to the high current traces and perhaps even replace the MOSFETs for lower RDSon versions.

  3. i like to ask what the 3 capacitor doing for the board where they are connected to the motor output
    the 3 capacitor of mine is 47uf can i change it to other value to make my motor run faster?
    i already circle them with red colour
    thanks alot
    my main motive is to increase the maximum speed of my controller
    i am using the 48v 350watt controller
    while my motor is 500 watt
    i already did the shunt modification but it does not increase the max speed
    but i would like to make it even faster at higher top speed
    and there is a resistor what if i change it to be a lower value ?
    am i able to increase my max speed ?


    1. Hi, changing these capacitors will not affect the maximum speed of your bike, these are to counter the inductance of the power traces between the large input capacitor and the MOSFETs.

      Modifying the shunt will increase the maximum torque (acceleration) of your bike. To some point also increase your maximum speed, mainly if the controller is to weak to reach the maximum speed under certain circumstances (i.e uphill).

      To increase your maximum speed you need to increase the battery voltage, you’ve probably noticed that the bike is faster fully charged at 50 V than with the battery discharged o 35 V. Beforee starting to search for a higher voltage battery you have to verify that your controller can handle it. The parts sensitive to higher voltage could be capacitors, MOSFETs and voltage regulators for example.

      1. Can I build a circuit with many capacitor to temporarily increase the voltage when needed while it’s moving
        Ex: a turbo switch , once I press it will release the charged and make my voltage higher for a while when it’s already moving

        1. That’s probably possible with some smart circuitry your controller must however still be able to handle the higher voltage.

          You could also calculate how much turbo boost you theoretically could get from a capacitor. The energy in a capacitor is (C * U^2) / 2 joules. The large one in your picture for example is a 1000 uF, if you charge it to 48 V it will contain ~1,2 joules. Assuming you, and your bike, weight 100 kg this is enough energy to accelerate you from 25.00 km/h to 25.01 km/h Hence you need quite a large capacitor. I would guess that it’s much easier getting a higher voltage battery.

          1. The main problem is the battery pack is limited in the deck and I want to design something that is hang on to the handle bar
            Or is it better that I use a pack of 18650 that connects to my battery and once I press the switch it will link up with my battery pack costing it to have high voltage for a short time ?

          2. That sounds like a better idea, you should be able to connect them in series with your main battery. Just make sure that they can handle the current and that your controller can handle the voltage.

          3. After I add in wire for the shunt the moment I full throttle my power will cut and I need to dismantle battery cable and put back then it’s ok but if I full throttle again it will cut again
            Is there anyway out ?

          4. I am using the 18650 48v pack 12ah

            Btw can I use the power capacitor theory for the car audio system to maintain my voltage so that my power will not cut ?

  4. Thanks for the writeup!

    i am currently doing the same, to my 48V 1000w kit

    I have thickened the shunt and beefed up all the traces, and have about 20% more torque and rideablity but i am not sure about a few things, firstly my board has 3 IRFB4410Z and 6 60NF06 (pictures in the thread on ES if you have a second?)

    i want to replace my FETs with some IRFB3006’s would i replace all 9 of them?

    i would ideally like to go from 22A rated to 48V 30-40A (about the limit of my battery)

    also what do the resistors need to be changed for? do i need to change them if i am running the same voltage just wanting more amps?

    thanks for your time! ill check back asap!

    1. My guess is that the manufacturer of your controller used two of the cheaper 60NF06 in parallell instead of the more expensive IRFB4410Z to keep costs down. If you replace your current MOSFETs with IRFB3006 it’s probably enough with only 6, you don’t need to populate one of the parallell transistors.

      What you need to watch out for is that the transistor drivers is powerful enough to drive the IRFB3006, the total gate charge for one IRFB4410 and two STP60NF06 is around 100 nC while one IRFB3006 has over 200 nC hence the switching time of the MOSFETs will be more than twice as long. This may or may not be a problem. A reduction of RDSon from ~8 mOhm to ~2 mOhm will reduce conduction losses by a factor of 4 while the switching losses will double. In theory your controller should be able to handle the dubble current. This is however very much a simplification and you probably need to try it out to be sure. Take it easy at first and check if the controller gets hot.

      The only resistor you need to change if you want to run more amps at the same current is the shunt.

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