simple two mechanical/reed relay combined power and precharge/anti spark ignition switch.

fruitman

100 µW
Joined
Jun 7, 2019
Messages
7
i designed this simple ignition switch/anti spark circuit that precharges and connects the motor controller to the battery using one switch and a minimum of two relays. there is no timing circuit required and no solid state relays.

this setup use relays that with coils which require 27 volts to actuate (marketed as 36v relays), though you could use a voltage divider on the control relay and limiting resistors on the power delivery relays and use relays with 12v coils, it would be kind of inefficient though.

they are 5 pin SPDT relays.

the minimum number of relays is two, one relay is used as a control relay and it can turn on one or more other relays. depending on how many amps you want to run, i would use one relay for every 10-15 amps. i have one control relay and three power delivery relays on my current setup.

this is for a 36v system. you can buy 48v versions of the same relays which should work for 48/52v setups.

i bought the relays on am@z0n, they are 36v 40a relays. i measured the resistance on the relay coil at 400ohms which is quite high so they will use up to 4 watts of power each (maybe more i need to connect a meter up and measure the amps across the coils still)

i tested the relays and they turn on at 27 volts, im using this to my advantage to reduce inrush current and prevent contact welding.

there is no spark when connecting the battery because there is no direct connection to the capacitors.

i am using a vesc which makes it easy to connect a wire to the positive lead on the controller AFTER the capacitors, but you should be able to do the same on any other controller though, if you open it up.

this is the most basic setup. on my setup i have modified it to use a step down voltage buck converter to power the three power delivery relays at 27.5 volts to reduce their power consumption. i could also put one on the control relay (i believe) but i only had one voltage converter to work with.

it does not require a switch and it does not require a high wattage resistor

i havent had much practice drawing schematics and i still need to learn how to use circuit design software, so all i have is this picture i drew by hand on a whiteboard.

i will add a parts list later.

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As if I'm five years old, please explain why this exists, what real life problem it prevents?
 
hi john thanks for using your valuable time to comment on my post. the title pretty much says it all, did you have a specific question? out of curiosity how do you turn your ebike off and on? is there a spark when you connect the battery?
 
It is a genuine question, asked because I have no clue in the topic, I'm just learning hence my ELI5 request. e.g. What's the problem with the spark?

If you prefer not to answer then maybe someone else will?
 
Running 72V on my eTrike. Both of my batteries have a switch to the BMS that splits/drops the voltage to 36V. I use this when connecting and disconnecting my batteries.
 
the spark happens because the capacitors on your motor controller are empty and hungry when disconnected from the battery, so when you first connect the battery the capacitors pull in as much power as they can very quickly until they fill up and reach the same voltage as your battery pack, this is referred to as inrush current and it can be many hundreds of amps. if you dont find a way to prevent the spark it burns the connector and adds more and more resistance over time, shortening its useful life span and eventually ruining the connector. if you allow it to spark you might only get a hundred or so connects before you have to replace the connector. there are many methods people use to prevent sparking and they all utilize a "precharge resistor". the xt90-s connector has a built in resistor at the front of the connector for the positive connection on the female side, as you slide the connector in it first contacts the resistor and the current is limited as the capacitors charge up, once the connector is fully inserted it no longer has to travel through the resistor and you have a full unrestricted connection from the battery to the motor controller. but i wanted a switch that can connect and disconnect the battery from the motor controller and has a built in precharge resistor.

another option to prevent the spark is to use a switch so that when you connect your battery leads up and the switch is open the connection from your battery is still broken. but now you have the issue that when you close your switch by flipping it on the spark happens inside the switch, which again eventually ruins the switch. you can add a resistor inline with your switch to prevent the spark but now you have a resistor constantly connected in series between your battery and motor controller that consumes a lot of power and heats up, you can add a second switch that bypasses the precharge resistor. so now you have two switches to connect your battery and you have to flip them in the correct order.

this circuit which i designed only has one switch, uses a precharge resistor, and once your capacitors are full it automatically bypasses the resistor for you, and it does all of that for you with the bare minimum of parts-- no fancy timing circuits or other "intelligent" components. its cheap, easy to build, simple, efficient, and durable.

i read a lot of threads about anti-spark switches on this forum and nobody ever proposed this design as a solution. this circuit only requires two reed relays(at minimum), a resistor, a diode, and a switch (only if you want to have a switch, it still works as an anti-spark connector without the switch).

i wanted a circuit that could connect a vesc motor controller, which are not made to be ebike specific, and uses an ebike throttle mounted ignition switch.
 
Thanks so much for taking the time, very clear explanation, and I am sure will be helpful for others reading this in future
 
fruitman said:
this circuit which i designed only has one switch, uses a precharge resistor, and once your capacitors are full it automatically bypasses the resistor for you, and it does all of that for you with the bare minimum of parts-- no fancy timing circuits or other "intelligent" components. its cheap, easy to build, simple, efficient, and durable.

I use a breaker/switch between my controller and battery so I never get a spark when connecting the battery. So I guess you could say my circuit only uses one switch and a bare minimum of parts and no fancy timing circuits as well. ~4000+ actuations, so I guess it meets the durable criteria too.
 
E-HP said:
I use a breaker/switch between my controller and battery

~4000+ actuations, so I guess it meets the durable criteria too.
Link please?

 
john61ct said:
E-HP said:
I use a breaker/switch between my controller and battery

~4000+ actuations, so I guess it meets the durable criteria too.
Link please?
It is shown in this thread (along with a few other switches):
https://endless-sphere.com/forums/viewtopic.php?f=2&t=110347

11th message from the top.
 
the controller i have on this bike doesnt have a sense wire for turning it on and off, it turns on when the battery is connected. i wanted my handlebar switch to turn on and off the bike, so thats why i was unable to use a breaker switch. i was thinking of wiring in a breaker switch and using it as a master switch so that the bike cant be turned on by strangers or on accident while transporting it in a car or parking it at work in the backroom.
 
Some Marine Disconnect switches have a removable key.
https://www.amazon.com/dp/B07H7ZW4L1
Those are usually very large though.

Another alternative is a male female set of XT90 or XT60 connectors.
The female goes inline and the male gets a hardwire jumper.
Pull the male plug out and stick it in your pocket.
 
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