Battery isolated. One button press does precharge+delay+go !

[mikehains]

I have created the circuit below, but as I am an electronics beginner, I could certainly do with some helpful comments, suggestions, or criticism.

I'm hoping the circuit will work like this:

1. At the start, the battery is physically totally disconnected.
2. Press the "start" button energizes the relay, which latches "on" (the start button only sees 12Volts).
3. This relay then opens the "pre-charge" circuit (getting ready to go).
4. The relay powers up an "RC" delay.
5. After the RC delay, the transistor switches on a regulator, which powers the Contactor.
-later-
6. If a tether switch is activated, or emergency "stop" button pressed, the batteries are completely isolated (these switches only see 12 volts)

Thanks in advance for your help.

Regards,
Mike

[ EDIT: First circuit deleted ... to fix errors with delay circuit and transistor ]

 

[mikehains]

Corrected circuit ... fixing problems with delay for the pre-charge ... and adding in working values for resistors, and transistor.

 

[liveforphysics]

I think the circuit could be simplifed a lot by using this to take the place of your final relay and fet to toggle it, and voltage reg for the fet.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRFP4468PBF-ND


Just a 5.1v zener with a pull down resistor that gets pulled up on a cap charging delay to switch on the IRFP4468 (logic level gate). You could eliminate the first relay as well by using another zener with a small fet, (like the BD681 you used to drive the coil of your output relay), and set it up to energize the gate from the voltage difference (again with a zener and a pull up/down setup to protect the gate), then once voltage gets close between the controller cap voltage and the pack voltage, the BD681 would turn off, which could be used to trigger the connection for the IRFP4468 to switch.

Then you've got no mechanical parts to fail, the circuit works from all voltages between about 10v to 100v, and it could take up very minimal space.

 

[mikehains]

... and that is why I defer to people who know what they're talking about. Thank you.

The thing that worries me ... is leaving the Bike/Go-Kart switched on - with batteries connected. One of my sons is bound to do this. I'm assuming the worst - perhaps it is even left for a week or even two or three weeks, switched on, batteries in circuit. If there is any real current drain, the LIPO's will be destroyed. This could even be dangerous. This led me to the cave-man suggestion of a relay Contactor.

As I read the datasheet on the Mosfet, it will only leak 20uA. This is magnificently small. Can the rest of the circuit crafted to leak such a small amount ?

Now I think about this a bit, it is obvious that there must be a solution. Electronics manufacturers use Li-Ion batteries, and must have created "shut down" circuits to protect them when they are not being used. And obviously (and this is your point) they don't use any physical relays to achieve this.

Thanks again,
Mike

 

[liveforphysics]

Thank you for your kind words.

Your diagram looked good, and would also have worked fine. I've just become partial to FETs as switches after having relays either fuse shut, or get some carbon between the contacts from arcs, become a high resistance point, and melt down when you are counting on them. There are some very good vacuum relays on the market that would work well, but I'm just partial towards FETs.

As far as leaving it alone for weeks or months, I think you will find the self internal discharge of the batteries to greatly exceed the rate a proper FET switch would be bleeding. It is a valid concern though, as running a pack of LiPo to 0 volts is a very sad thing. I left my first 20Ah LiPo pack on my ghetto loadbank (a bundle of 300w lightbulbs), fell asleep and forgot about it... Boy did I feel like a retard! And what a waste of cells! This was just break-in cycles for the new pack!

 

[NeilP]

Just reviving an old thread here, but did a circuit ever get drawn up with your FET suggestion LFP?

This seems to neat idea...how come this thread died? Just too complicated when a resistor and switch does the same job? or some other reason.

I'd like something like this but would need a schematic and values for me to build it

Edit:
Thanks for update on schematic

 

[NeilP]

HI thanks for updating the schematic
another question I am afraid

The relay with the diode across it, A the moment I cant get my head around it...is it double pole ...i just can't figure it...could you maybe draw the switch inside showing which connects to which when the relay is energised.

Thanks

 

[NeilP]

Just noticed this..maybe my lack of electronics knowledge...but I have never seen a diode given a forward resistance before.

What is that about?

 

[auraslip]

This is timely. I just finished making a schematic for the same thing. http://i.imgur.com/tsidV.png

sorry for poor picture. Ctrl+mousewheel up!

It's based around this little timer http://www.ics-timers.com/khtimers1.html

It might be easier or cheaper to make an RC delay to control the relay. http://www.electronics-tutorials.ws/rc/rc_1.html