Choosing a Relay

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smithinparis
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Choosing a Relay

Hi,
I'm new to the forum (been reading for some time, but never posted), and to ebikes as well (converted my trek last year). This year I'll be modifying my setup to fix a couple of issues I found.

A little background on my setup:
26" Trek Mountain Bike
Crystalyte 406 Rear hub motor
36V 8Ah NiCd battery pack

The two main modifications I'll be making is:
Add a second battery pack (36V 8Ah NiCd again) and replace the throttle with my PDA (Palm OS).

The PDA will be interfaced to a microcontroller that will be controlling the throttle, monitoring the battery packs and (the reason for the post) selecting the active battery pack.

I need some advice on selecting a relay to switch between the packs. There are lots of different ones available, and I just can't settle on one (either because of cost, availability or odd specs from the supplier), perhaps somebody here has already gone through the pros/cons/cost of different ones and has something to share.

I would like to have (but can compromise):
1) SPDT
2) 5 volt coil (same voltage as the microcontroller)
3) Capable of switching 36V 30A
4) Easily available (through Digikey, Jameco, Mouser, Canadian Tire, Radioshack ...)
5) Not terribly expensive (less than $10 would be great)
6) Later I'll likely be modifying it so that the I have a couple of relays that will switch between the battery packs OR tie them in series to give me 72 volts on the fly - has anyone looked at 72 volt high current relays?

Any pointers would be fantastic!

LinkOfHyrule
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Re: Choosing a Relay

I'm not sure if there are any suitable ones that have a 5V coil, but I could be wrong. You'll probably have to have the ┬Áchip drive a transistor, unless it can deliver enough current to switch on the coil.

No idea if you can get a relay that size for less than $10, but I haven't really searched that hard. Most EV contactors are like $30 minimum. An automotive relay might work, though.

Any reason you don't just want to put the packs in parallel? That would be easier on them, since they would only have to deliver half the amperage.

The author of this post isn't responsible for any injury, disability or dismemberment, death, financial loss, illness, addiction, hereditary disease, or any other undesirable consequence or general misfortune resulting from use of the "information" contai

smithinparis
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Re: Choosing a Relay

LinkOfHyrule,
Thanks for the reply. I agree about the 5 volt coil, that is one of the hardest parameters that I'm trying to meet. In the end, you're right, I could just use a transitor and a 12 volt supply, but I'd like to keep the design as clean as possible - if I can avoid needing 2 voltage busses (5 and 12) then I'd like to.
I've looked at various automotive relays, and that's probably the way I'll be going, but I'm just hoping somebody has already found something.
Finally as for running the packs in parallel - technically it can be done, but the packs I bought get tested to make sure the cells are all matched (for cell reversal), and if I run the two packs in parallel, I won't get the full capacity out of them because some cells will reverse prematurely. To run them in parallel, I would consider using a diode joiner, but I'd like to keep some active flexibility in my design, because next year I'll likely be updating the relay configuration to switch between 36 and 72 volts.
Again, thanks for the feedback!

LinkOfHyrule
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Re: Choosing a Relay

I see. I was actually going to do the same thing with a lithium pack some time in the future. Let us know what you turn up.

The author of this post isn't responsible for any injury, disability or dismemberment, death, financial loss, illness, addiction, hereditary disease, or any other undesirable consequence or general misfortune resulting from use of the "information" contai

smithinparis
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Re: Choosing a Relay

Well, after searching high and low, I've settled on a relay. However if anybody has any suggestions later on, please post as I'm always looking for alternatives!

The relay I've chosen is a Panasonic, and it will do for the 36 V setup (I still need to find one that will be suitable for the 72 V setup I'll be working on next year). The only real disadvantage to the relay that I've choosen is that it is using a 12 volt coil, so I'm going to need to include 2 voltage busses on my PCB (not a big deal, but still something I had wanted to avoid).

So the details:
Manufacturer: Panasonic
Switch type: SPDT
Coil: 12 V
Price: $3.63 (from Digikey)
Contact rating: 30 VDC 35 A
Dimensions (mmxmmxmm): 15x20x22
Digikey PN: 255-2183
Panasonic PN: CM1-12V

frodus
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Re: Choosing a Relay

well, you could just use a simple voltage coubler circuit to get 10V out of the 5V supply.... it should be enough to drive the relay... there's also a voltage tripler circuit out there...

I've got a quantity (18) of 30A relays with 12V coils, I'll check on voltage. They're 3PDT.

____________

Travis Gintz
1986 Honda VFR Conversion
www.evfr.net

smithinparis
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Re: Choosing a Relay

Travis - thanks for that. Though I have no problem putting in the 12 volt supply, I'm kicking myself for not having even thought of the voltage doubler - as it's a more elegant solution in my mind than the brute force 'if if needs it put it in' approach. I'd rather the 'use what you've got' approach.
Tom

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