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Auxillary circuit help please!

Posted: Mon, 25 May 2015, 23:26
by Taxman
Hi guys,

It's been ages since I've posted about my motorbike project, I'm sure you understand when real life gets in the way of massive fantasy projects. Anyway, I've gotten back into the bike, and wondering if anyone can help me with my auxillary circuit.

I'm wiring in my DC DC converter at the moment, I'll need a fuse and a relay on one side or the other or both to turn on the DC DC converter when the ignition is on, right? I'm looking at a few diagrams and they show the DC DC connected permanently on both sides. But wouldn't that drain the battery, by inefficiencies in the DC DC?

Could I just run the high voltage feed to the DC DC from the main traction circuit, somewhere after the main contactor? So the DCDC is open circuit until the contactor closes. Would that send a dangerous spike through the DCDC?

It's just one of those times when there should be an obvious, easy way it's done, and it's right under my nose. Here's a photo of it anyway,

http://s784.photobucket.com/user/taxman ... 6.jpg.html

Cheers

Sam

Auxillary circuit help please!

Posted: Tue, 26 May 2015, 03:22
by 4Springs
Taxman wrote:I'm looking at a few diagrams and they show the DC DC connected permanently on both sides. But wouldn't that drain the battery, by inefficiencies in the DC DC?
Yep, the DC-DC will drain the traction pack. But probably slower than you'd think. You can keep the 12V side of the DC-DC converter connected all the time- it won't drain anything.
Taxman wrote:Could I just run the high voltage feed to the DC DC from the main traction circuit, somewhere after the main contactor? So the DCDC is open circuit until the contactor closes. Would that send a dangerous spike through the DCDC?
That is pretty much what I do. Do you have an auxiliary (12 or 6V) battery?
Here is the circuit diagram for my car. I don't imagine you will want yours the same but it gives me something to talk to:Image
See my mid-pack contactor? That is engaged when I turn the key to ACC. So with the key off there is no traction pack voltage (150V) and the DC-DC converter is not fed. With the key in ACC the DC-DC is fed with 150V and supplies 12V. It is not until I turn the key to START that the drive contactor is engaged.

You'll need to think about charging too. My mid-pack contactor has to be engaged when charging, or else the charger couldn't charge the pack. So I have it energised by the 240V mains. This means that my DC-DC converter is powered whenever the car is plugged in.

Hope this helps?

Auxillary circuit help please!

Posted: Tue, 26 May 2015, 05:45
by evric
One thing to remember is that if you are only using one contactor, with precharge, the only thing that is to be connected after the contactor is the controller/motor.
In my conversion I feed the main battery pack via a circuit breaker then to the DC-DC converter and the input to the contactor.
ie. The DC-DC controller must be connected BEFORE the contactor.
I see you have a Big Red Button/switch - wire the DC-DC converter after that. I presume the Big Red Button is wired directly from the main pack.
Eric

Auxillary circuit help please!

Posted: Tue, 26 May 2015, 16:01
by jonescg
From memory the precharge process can be hindered by an additional load, such as the DC/DC and it's associated 12 V loads. Provided the heater, windshield wipers and stereo are off you're probably okay.

Auxillary circuit help please!

Posted: Tue, 26 May 2015, 18:42
by zeva
Mostly reiterating what others have said - I would strongly recommend not having the DC/DC converter permanently connected to HV. Over the years I've heard about too many EVs with traction battery damage due to the DC/DC converter over-discharging them (and the BMS being unable to take action because it's permanently wired). The usual situation is someone drives the car to low state-of-charge, then leaves it idle for a few weeks without charging.

But neither can you use your main contactor to switch the DC/DC! In this situation the DC/DC fires up immediately and starts to top up the aux battery by pulling power continuously through the precharge resistors, which can damage them, or damage the main contactor due to inadequate precharge (or be detected as an error if your precharger has any smarts).

4Springs' idea of using a second contactor works well - ideal if you have a mechanism in place to switch it on automatically when charging or driving, which can be done simply with a couple of diodes from key and charge detect signals. Or as evric says, use your big red button to isolate the traction battery when not in use - but don't forget to use it if the bike sits idle for a long time!

Auxillary circuit help please!

Posted: Tue, 26 May 2015, 21:38
by Richo
Taxman wrote: So the DCDC is open circuit until the contactor closes. Would that send a dangerous spike through the DCDC?

No.
The (voltage) spike generally comes when things are powering down - especially under load.
As mentioned before (IF) you have precharge you may have to account for the extra capacitance on the line in the DC-DC.

Auxillary circuit help please!

Posted: Mon, 01 Jun 2015, 20:13
by Taxman
Awesome, thanks guys. As usual, the solution to the engineering problem is simple and complex at the same time. I'm really grateful to have feedback, half the reason the project has slowed recently is because it is getting closer to putting my balls on the line (literally) and firing it up. Daunting prospect! I'll bring it into a meeting though, when I've done so.

Cheers

Sam