questions about mini-hub for a child

Hi,
I've been out of the "buy new stuff" game for quite a while now. I have been riding my most recent build since 2013 and nothing has broke. I'm afraid I've lost touch with what's the latest-and-greatest.

1) I want to do another build, this time an "assist" based build, the lightest possible motor/esc for a 60lb rider and have the rider provide most of the "grunt", so I'm thinking of using a 350watt hub motor.
2) I want to do mid-drive because my riding environment often includes 1000+ foot climbs, sometimes all at once. I already know how to build a mid-hub (Build 6: The off-roader V2 mid-hub Gravity FSX)

my questions are
a) what is a cheap-ish way to prevent motor overamp/overheat on a kid's bike. I don't want to smoke this little motor. Is there some kind of over-amper prevention circuit or something out there? I currently use Cycle Analyst but don't want to spend $150 on an overamp circuit for a kid.
b) what is a cheap-ish way to prevent battery over-deplete. I currently use SOC (state of charge) as a voltage indicator and I'm just mindful of not depleting past a certain voltage but for a child I would rather not rely on them remembering thier "depleted" numbers and just have some kind of LVC (low voltage cut) circuit instead. Is there some kind of battery protection circuit out there? This will be for a 12s lithium system.

parajared said:

a) what is a cheap-ish way to prevent motor overamp/overheat on a kid's bike.

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use a controller that has a (real) current limit that along with the battery voltage prevents using any more watts from the system than the motor can handle.

as long as it's a middrive and you set it up so the gearing (presumably shiftable) prevents putting a load on the motor high enough to cause heating inside it from lack of bemf at low speeds / high loads.

if you need a temperature protection, you could use a simple (remote) bbq thermometer that beeps an alarm when whatever temperature you've set it to is reached. then make sure the rider knows they must stop at that point.

if you need something automated instead, there are cheapish thermal-control modules around (ebay, aliexpress, etc) that use your setpoints to switch a relay on or off. the thermal sensor goes inside the motor, and the relay nc contact set goes between the ksi (keyswitch/ignition) wire on the controller and battery power, so it only allows the controller to run if the temperature is below the high setpoint. if it heats up too much it cuts the controller off.

b) what is a cheap-ish way to prevent battery over-deplete. I currently use SOC (state of charge) as a voltage indicator and I'm just mindful of not depleting past a certain voltage but for a child I would rather not rely on them remembering thier "depleted" numbers and just have some kind of LVC (low voltage cut) circuit instead. Is there some kind of battery protection circuit out there? This will be for a 12s lithium system.

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the simplest is to just use a regular bms with hvc and lvc. afaicr the cheap ones are a dozen or two dollars; a number of threads recommend bestech bms iirc. if you don't want a balancing function i've been told in another thread recently that the ones unitpackpower uses are generally of that type. probably available from a lot of places.

it'll shutdown the discharge port of the pack if voltage of any cell group drops below the lvc.

if you don't use the pack for a while you can just make the pakc so you can disconnect the bms from it so it can't drain it by accident.

alternately there are simple voltage modules like the temperature ones above, that engage or disengage a relay based on the voltage they're monitoring. again, the relay goes between the ksi wire and the battery positive, to just turn the controller on/off.

you could build one that uses a voltage divider and a transistor to do that, too. voltage divider goes between battery positive and negative. transistor base goes to the center of the divider. emitter to battery negative. collector to one side of a relay coil. other side of relay coil to battery positive. divider setup so there isn't enough voltage at the base to keep the transistor on (and thus the relay) once the battery voltage drops to the lvc you want. relay nc contacts between ksi and battery positive. of controller.

Thanks amberwolf. That's got my mind steered in the right direction with thermal control modules and voltage divider.

Escs just hear "voltage" from your ebike throttle correct? Like your throttle is just a fancy potentiometer that changes the voltage from 0 volt to 5 volts or something like that right?
I recon I could port my throttle to a PWM input slot to an Arduino, and port PWM output to the esc's throttle connector. Then I could just simply use a temp sensor like you recommended to limit the the throttle once peak temps are starting to get reached. For instance 90 celcius= only 90% PWM output, 100c =75% PWM ect... As far as the voltage goes, just do a voltage resistor divider input to voltage relay output, a̶ ̶5̶v̶ ̶r̶e̶l̶a̶y̶ ̶t̶o̶ ̶m̶a̶k̶e̶ ̶a̶r̶d̶u̶i̶n̶o̶ ̶k̶i̶l̶l̶ ̶t̶h̶e̶ ̶m̶o̶t̶o̶r̶ ̶o̶n̶c̶e̶ ̶L̶V̶C̶ ̶i̶s̶ ̶r̶e̶a̶c̶h̶e̶d̶;̶ ̶m̶a̶y̶b̶e̶ ̶p̶r̶o̶g̶r̶a̶m̶ ̶i̶n̶ ̶s̶o̶m̶e̶ ̶h̶y̶s̶t̶e̶r̶e̶s̶i̶s̶ ̶t̶o̶ ̶a̶v̶o̶i̶d̶ ̶v̶o̶l̶t̶a̶g̶e̶ ̶r̶u̶b̶b̶e̶r̶-̶b̶a̶n̶d̶i̶n̶g̶.̶ ̶
Edit: Scratch that, forget the relay, just tie the throttle to LVC through programming. 0% battery = 0% throttle.

I think I found a 15-ish dollar solution. Once again, I appreciate it

parajared said:

Escs just hear "voltage" from your ebike throttle correct? Like your throttle is just a fancy potentiometer that changes the voltage from 0 volt to 5 volts or something like that right?

Click to expand...

the voltage range depends on the type of throttle the controller takes. if it's a common ebike controller, they usually use hall throttles, and usually have approximately 1.something volts to 3.something volts for an input range. usually printed on them. if not you can determine experimentally what it is.

same for the throttle itself, you can verify what any particular throttle puts out, just hook i tup to 5v and ground, and measure the signal relative to ground. it'll probably be from down around 0.8v up to around 4v for it's full motion range.

I recon I could port my throttle to a PWM input slot to an Arduino,

Click to expand...

it would have to be an analog input; or else you'd have to build an analog-in/pwm-out circuit to translate.

i iddn't suggest an mcu because for most people that's a more complex solution because it requires coding and such.

and port PWM output to the esc's throttle connector.

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for the controller's throttle output, for typical ebike controllers, you will have to use an analog output, or else use a pwm-to-analog converter circuit. something as simple as an r-c filter, low-pass type, may work, values based around the pwm frequency (which will need to be stable at a single frequency). if it doesn't you'll need something more complex, perhaps an op-amp integrator circuit.

Then I could just simply use a temp sensor like you recommended to limit the the throttle once peak temps are starting to get reached. For instance 90 celcius= only 90% PWM output, 100c =75% PWM ect...

Click to expand...

sure, that would work, though again, if it's an ebike controller it's not pwm throttle input, it's analog voltage.

As far as the voltage goes, just do a voltage resistor divider input to voltage relay output, a̶ ̶5̶v̶ ̶r̶e̶l̶a̶y̶ ̶t̶o̶ ̶m̶a̶k̶e̶ ̶a̶r̶d̶u̶i̶n̶o̶ ̶k̶i̶l̶l̶ ̶t̶h̶e̶ ̶m̶o̶t̶o̶r̶ ̶o̶n̶c̶e̶ ̶L̶V̶C̶ ̶i̶s̶ ̶r̶e̶a̶c̶h̶e̶d̶;̶ ̶m̶a̶y̶b̶e̶ ̶p̶r̶o̶g̶r̶a̶m̶ ̶i̶n̶ ̶s̶o̶m̶e̶ ̶h̶y̶s̶t̶e̶r̶e̶s̶i̶s̶ ̶t̶o̶ ̶a̶v̶o̶i̶d̶ ̶v̶o̶l̶t̶a̶g̶e̶ ̶r̶u̶b̶b̶e̶r̶-̶b̶a̶n̶d̶i̶n̶g̶.̶ ̶
Edit: Scratch that, forget the relay, just tie the throttle to LVC through programming. 0% battery = 0% throttle.

Click to expand...

right. just a voltage divider on a separate analog input of hte mcu, scaling the battery voltage itself to a range that is well within the limits of the mcu. for safety you could put a zener diode on the input pin, along with a series resistor, with the zener at a voltage below what might damage the mcu but above what the normal battery voltage is expected to be.

For a child, I'd do two things after choosing a low power mid drive.

1 remove all but the smallest front chainring. This will protect the motor, by insuring it will not be run in the wrong, very high, gear on a steep hill. Even a 350 watt will not overheat if its simply in the correct lower gears for a climb. It will also limit speed pretty effectively. As the child grows in experience, and is an age that can follow instructions reliably (5 on up) then he/she can get more speed by putting the bigger rings back on the bike. Motor won't overheat easily, unless its asked to start on a steep hill in a high gear. Even in the desert SW.

2 get a small, but good, bms protected battery pack. That bms will keep it out of over discharge, and the simple charger makes it simple enough for even a child to put on the charger, outside of course.

Audible alarm temperature probe, get the loudest alarm.

Thanks for the replies!