Temperature cut off

[Fisticuffs]

Hi

Im making an electric bike, and using batteries that have a tendancy to overheat on a full load.

Because I dont want to set myself on fire is there any way to have a temperature cut off.

I have found a circuit diagram that sounds buzzer when a thermistor reaches a certain temperature (this would be a seperate device),
Is there a way (excuse my limited knowledge) to change that buzzer to a switch that turns off a wire on my motor controller?

Thanks for reading

 

[Optikon]

Hi

Im making an electric bike, and using batteries that have a tendancy to overheat on a full load.

Because I dont want to set myself on fire is there any way to have a temperature cut off.

I have found a circuit diagram that sounds buzzer when a thermistor reaches a certain temperature (this would be a seperate device),
Is there a way (excuse my limited knowledge) to change that buzzer to a switch that turns off a wire on my motor controller?

Thanks for reading

yes, that's relatively straightforward.

1) consider using a relay for the motor cut-off. Maybe normally closed contact and overtemp energizes the relay causing it to open. When temp returns to normal, relay will close again so maybe you want to manually restart the motor to avoid unexpected motor operation.

2) Make sur ethe circuit that is driving the buzzer, can drive the relay coil that you've selected. Make sure there is sufficient pull-in voltage and drive current.

If you post a detailed schematic(including part #'s & values) you can get more help. Look over the web for relay drive circuits etc.. for some ideas about how to modify your buzzer ckt. Don't forget a flyback diode on the relay coil.

 

[Fisticuffs]

Great stuff, thanks for the information.

Over the next few days (read week) ill be getting all the parts for the bike, so ill definatley post what I have before I make anything

Thanks again

 

[mstechca]

Did you know that overheating is in relation to excessive current?

If you connected a 9V battery to a low voltage LED directly, it will burn out or at least heat up. Also the colour might change slightly.

However, if you added a resistor in the circuit, the LED won't burn out easy because the resistor is using some current for itself, and it is (supposed to be) converting it to heat that the resistor can handle.

Try doing the same with your bike. You might not get much power, but it is better than overheating batteries.

Basically you are at a trade-off. It is power VS heat.

 

[Fisticuffs]

I should be ok now, heat wise, im still going to make an alarm or just in case.

I ended up getting 3 batteris I will put in parallel,

Each one can have a constant drain of 15A and my controller can draw a
17A max so the most thats ever going to come out of each battery is
5.6A as long as I keep them ballanced, I think this was the best wat to approach my problem.


Thanks

 

[Nigel Goodwin]

I should be ok now, heat wise, im still going to make an alarm or just in case.

I ended up getting 3 batteris I will put in parallel,

Each one can have a constant drain of 15A and my controller can draw a
17A max so the most thats ever going to come out of each battery is
5.6A as long as I keep them ballanced, I think this was the best wat to approach my problem.

I'm rather puzzle over this?.

Do you mean your controller is current limited at 17A (which sounds uklikely?), or do you mean it's only rated at 17A?.

If it's only rated at 17A then I wouldn't expect it to survive long?.

 

[Fisticuffs]

The controller drives a 200W motor, the average the controller will pull from the batteries is 12A and the max the controller will pull from the batteries is 17A

The whole system works on 24V

The batteries will allow a max constant discharge of 30A

I was under the impression the controller was in charge of how much it tries to pull from the batteries, that is how motor speed and power is determined?

How would this fry the controller, or am I missing something obvious?

Thanks

 

[Nigel Goodwin]

The controller drives a 200W motor, the average the controller will pull from the batteries is 12A and the max the controller will pull from the batteries is 17A

The whole system works on 24V

The batteries will allow a max constant discharge of 30A

I was under the impression the controller was in charge of how much it tries to pull from the batteries, that is how motor speed and power is determined?

How would this fry the controller, or am I missing something obvious?

The current is generally governed by the requirements of the motor, which under heavy load, start-up, or particularly stall, will take VERY high currents. Usually all the controller does is provide PWM speed control of the motor?, adding current limiting reduces performance and adds complexity and cost.

I presume you've seen the R/C model cars?, like the Tamiya ones?, and I presume your motor is significantly larger than those?. A speed controller for a Tamiya R/C car will usually be something like 90A or 120A?.

There's generally NO real limit on the current you can draw from a lead acid battery, except for the self destruction of the battery - the only limit is it's internal resistance, which is EXTREMELY low. It's not that they will only 'allow' a constant 30A, just that that is the maximum recommended continuous current. I would expect currents well in excess of 100A from them, with a suitable load (or a stalled motor!).

Have you built the speed controller?, or is it a commercial unit?,

 

[Fisticuffs]

Sorry, I think I have caused some confusion, the motor and controller are from part of the same pack.

The controller comes with the motor, the controller has a maximum draw of 17A, (it will not take or ask for any more from the battery)

The batteries are NiMh and allow 30A continuous and 60A peak.

I have spoken to a guy in my situation, he has the same setup as me and has been using it for a long time now, I was just being over cautious about temperature.

I understand this will have quite limited power, but im not after building a rocketbike lol

(edit : Wouldnt the RC cars work on a higher Amperage because they run at a much lower voltage?)

 

[Nigel Goodwin]

Sorry, I think I have caused some confusion, the motor and controller are from part of the same pack.

The controller comes with the motor, the controller has a maximum draw of 17A, (it will not take or ask for any more from the battery)

The batteries are NiMh and allow 30A continuous and 60A peak.

Sorry, but I'm highly dubious about the values, I suggest they are all optimistically small :lol:

But as long as it's all designed to work together, then I wouldn't worry about it (or the battery temperature to be honest, presumably it's all been designed together?).

I have spoken to a guy in my situation, he has the same setup as me and has been using it for a long time now, I was just being over cautious about temperature.

I understand this will have quite limited power, but im not after building a rocketbike lol

(edit : Wouldnt the RC cars work on a higher Amperage because they run at a much lower voltage?)

Yes they would, except they are probably a far smaller motor - but a 7.2V motor taking 50A is still 360W - and they will easily take that on surges and under stall conditions.

 

[Fisticuffs]

Found out why my motor dosent pull massive amps when it starts, its brushless so it dosent have any direct contact aparantly.