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billmi
30 April 2011, 0538
Presently, the AC power to my charger runs through a solid state relay that's controlled by the MiniBMS. This means that I need 12v power to the MiniBMS during my charging cycle. My Vicor DC/DC converter is wired always on, I haven't setup a gate control circuit for it.

This leaves me with 2 problems.

Problem 1: With the only load on the system being the Vicor providing power for the Cycle Analyst and MiniBMS, the Cycle Analyst shows .5 amps being pulled from the batteries, so I don't want to leave the DC/DC converter constantly running.

What I do now is keep an eye on it, checking every half hour or so when charging, then once charging is completed, cut the power by switching the main cut-off switch that breaks the circuit from the batteries. It's effective, but inconvenient.

Problem 2: About 30% of the time, wen I turn on the ignition, I get a voltage fault on the Kelly controller. I assume this is from a momentary voltage drop in the 12v supply as the headlight is firing up. Cycling power off then back on once or twice gets past it, but it's undesirable.

What I want to do as my next step is use the gate control on the Vicor to turn it off when the bike is off and use a 2.5 amp 12v power supply ($5 delivered on ebay) to run the MiniBMS during the charging cycle. The attached schematic is the 12v system (rest of the 72v system and 12v load left out for clarity).

In this system, D1 is in place so that the 12v from the power supply is powering just the MiniBMS, not also going out to the headlight which would draw more amps than it can supply.

C1 is a supercapacitor, or paralleled standard capacitors. It should store and release enough of a burst to both flip the relay that will turn on the DC/DC converter and buffer against a voltage drop from the headlight causing a power fault in the controller when it turns on.

On the chance that C1 looses too much of its charge from internal leakage before I start the bike again, I've added a "jump start" switch that will break the gate control line for the DC/DC (not shown is the capacitor in the gate circuit, I know it needs to be there, but didn't have the gate circuit in front of me when I laid this out).

My questions are three-fold.

1 - Does this look on target, or have I overlooked something?

2 - How do I spec D1? I've done very little sourcing of diodes - usually I'm just going with a part number that's recommended for a purpose. Am I correct in my belief that I want a rectifier diode that is rated for 12v+ and 2 amps or so (well more than will be pulled from the DC/DC by the MiniBMS)?

3 - How do I spec C1? Ditto for caps on my experience (switch, resistor, FET and relay networks for a microcontroller to make things go, and I'm all over it). I would think I'd just want to be generous with capacitance, and go for 12v or higher. Looking at super-capacitors and ultracapacitors on ebay though, to get high F values, I find nothing in 12v, everything's around 2.5v. My gut tells me to just grab the fattest caps out of an old power supply that are rated 12v or more and parallel them.

As always, feedback welcome and appreciated.

1068

DaveAK
30 April 2011, 0923
I've had my Vicor on for a couple of weeks now at least and seen negligable drop in voltage from my pack. I'd doubt that 0.5A draw is doing anything significant.

My original plan was the same as yours, to power the MiniBMS from an external source while charging, but having left the Vicor permanently on and seeing no ill effects I'm inclined to think it's not an issue. Even for long term storage I don't see any problem as I'll be checking the pack monthly.

I did make some circuitry to do away with the external power and have it run from four cells in the pack. It still needs some work. Rob helped me iron out the bugs but I haven't got around to trying the revised version yet. My original version kinda worked in principle, but I blew up a relay - can't remember why. I use a 50V 1uF capacitor for C1 and it worked just fine, although I never calculated anything, it was just what I could get my hands on at the time. Of course, I wasn't using it in the fashion that you plan as I was getting power from the battery pack. It was just used to debounce switching the gate. The Vicor isn't designed to have high speed switching on the gate and will shut down.

I have a couple of threads on my escapades somewhere. I'll try and dig them up for you.

frodus
30 April 2011, 1431
why not a very small pack of lifepo4 or a small SLA battery (5-7AH) in parallel with the Vicor output.

Then just "enable" itself by taking 12V from the aux battery to a relay, that turns on the gate signal and also powers things. Then the DC-DC is "on" and "charges" the aux battery. Its also nice for inrush current when headlights go on. You can then have an AC relay trigger when the charger gets plugged in and it will bypass the "enable" on the vicor and turn it on, so the miniBMS is on.

billmi
01 May 2011, 0449
.5A draw is definitely signficant - over 24 hours that's 12Ah, more than 1/4 the capacity of my pack. Granted the load should be less, as I think that included the load drawn by the relay that turns on the controller, and maybe even the main contactor. In general though, just as a matter of good design I'd prefer to not be pulling power when the bike isn't running.

I had originally considered using a battery over a capacitor in the same configuration as that schematic, and SLA seems a natural choice - but the most viable location is up in the tank - and I recently had a similarly treated SLA (constant power at 12v, rather than with a regulated charging circuit) end its life by swelling and blowing out on an edge. I discovered it when I put my hand on the acid that had leaked down onto a fire extinguisher. Acid doesn't worry me in floodies where a hose routes it to the bottom of the bike, or SLAs if they're in the bottom of things, but I don't want an acid leak dripping down on my BMS boards and LiFePO4s.

It seems to me that since I only need a very short burst of power, capacitor(s) would be a simple alternative with less potential for something to go wrong, and easily backed up by the jump start button. I would expect lower mass and volume than SLA or LiFePO4, and likely less cost than either (especially if the pair of 200V 1000uF caps I just pulled from a junked power supply are up to the task).

magicsmoke
01 May 2011, 0721
You could just unplug your kill switch from the harness and wire it across the vicor gate. Run / Stop would be reversed but you could soon paint that out.
The switch would of course still kill the bike as the main contactor is fed from the Vicor.

Rob

billmi
01 May 2011, 0801
That would definitely still kill power to the motor, but I'd be in the same situation I am now where I can turn it off with the master shut-off switch, and I don't want my kill switch to cut power to the headlight. If I have to use it to cut the moter because the controller has glitched, I don't want to go dark while coasting down the street (I commute in to work before sunrise).

I just looked at the board from the scrapped power supply I have. It's got a few 3300 microfarad 15v capacitors. I'll charge one up and see if it's enough to snap over a small relay. I'm pretty sure the keyswitch is a multiple pole switch - so I could isolate the cap and relay circuit, so the cap's full charge just goes to the relay (doesn't help with drop causing a voltage fault on the controller sometimes but it should activate the DC/DC without an extra button/switch step).

DaveAK
01 May 2011, 1058
.5A draw is definitely signficant - over 24 hours that's 12Ah, more than 1/4 the capacity of my pack. Granted the load should be less, as I think that included the load drawn by the relay that turns on the controller, and maybe even the main contactor. In general though, just as a matter of good design I'd prefer to not be pulling power when the bike isn't running.
Sorry, I phrased that wrong in my haste to reply. What I really meant was that I doubt the Vicor or BMS is drawing anything significant, simply through my own experience as described. I'd doubt that it's drawing 0.5A with everything off excpet the MiniBMS and Vicor. The MiniBMS draws milliamps and I can't remember what the no load draw is on the Vicor, but like I said I'm only comparing it to my own setup.

I'm with you though and would much prefer no parasitic drain at all. I worked on achieving something that would do it but it's dropped down my priority list as I've more pressing things I need to do to get the bike on the road.

OK, I've just finished typing all of the above and something occurs to me. Is that 0.5A draw measured with the charger plugged in? I could see how the SSR might possibly be drawing some significant current - and that scenario I have not tested. My charger is unplugged and the SSR disconnected from the BMS after charging is complete as I haven't as yet left it all unattended. When I've completed my charge controller the SSR will no longer be powered by the BMS, but now I'm concerned about the current it draws.

As for capacitors, Bruce is, (or at least was), planning to take that exact route. He'd be the guy to answer those questions.

billmi
01 May 2011, 1058
I totally get you on leaving it for later - that's exactly what I did, just using the master shut-off to deal with it and be up and running. Now I'm looking to upgrade to the more elegant solution.

The .5 amp draw was before I'd even wired in the SSR and charger. The only 12v load at the time was the relay providing 72v to the controller the MiniBMS and the Cycle Analyst. I had just figured it was that load plus heat loss / inefficiency adding up to 0.5A. Could be too, that at that small of a drain, the Cycle Analyst isn't so accurate.

I pulled those capacitors. Sure enough, the 3300 uF cap held enough charge to open a relay a couple of times, but alas, it's leakage is too high. Left to sit for an hour, the charge had dissipated. :(

Hmmm, looks like either push button starting, way beefier capacitor, or a battery. I've got a few 7.5Ah SLAs, but they're more weight and bulk than I want to add to the bike, and I really don't like the idea of putting them in the most available space - above the LiFePO4s.

magicsmoke
01 May 2011, 1533
Ah, I see what you're getting at now. What about this then?
Basically it's a way of shutting down the Vicor with a break instead of a make switch.
Q1 is a P channel JFET 'switch' which is NC but opens when the sidestand is up (stand switch closed) and so activates the DC-DC.
When you put the stand down again, the FET shorts and turns the DC-DC off.
R1/R2 are a divider to provide a suitable Gate drive, R3 limits the inrush current to C1.
The capacitor is there to provide a few minutes delay in the turn off to allow you to open the garage door or whatever before the lights go out.

BaldBruce
01 May 2011, 1618
As Dave mentioned, I went the large capacitor route. The issue as you found out is that they are only available in low voltage. That is an easy one to fix though, you just put them in series. I used 2.5V capacitors 12F each with five of them in a row. That gives me the voltage stack of 12.5 I need and a capciatnce of 2.5F total. I charged them up to 12 volts and checked a week later. Still 12V......

magicsmoke
01 May 2011, 1629
Bruce, the very man, earlier I was thinking about the cap idea and was wondering whether the Vicor output had a resistor across it internally and thought you'd know.
Anyway, my concern was that the proposed schematic still has the BMS across the cap which will discharge it prematurely and if the Vicor has an output R, that'll make it worse.

BaldBruce
01 May 2011, 1629
The diode should be a shotky type for low forward voltage drop. Needs to be sized current wise for more than you mini BMS can draw under any situation. Needs to have a volatge rating of 20V at a minimum.

You shoud also think of wiring it so that the capacitors is not connected directly to the load. The way you currently have it, the 12V auxillary components will very quickly pull down your capacitor.
I used a 12V relay for this purpose. Ignition switch voltage comes from capacitor. Output of ignition switch turns on Relay feeding DC/DC. DC/DC recharges capacitor via a blocking diode. Seperate kill switch is required for this type of loop however. Elithion website has a nice white paper on the ignition conundrum.......http://liionbms.com/php/wp_ignition_conundrum.php and scoll to the bottom.

DaveAK
01 May 2011, 1654
I totally get you on leaving it for later - that's exactly what I did, just using the master shut-off to deal with it and be up and running. Now I'm looking to upgrade to the more elegant solution.
Ah well, you're one step ahead of me. :D But I'm closing the gap!

billmi
04 May 2011, 0617
You shoud also think of wiring it so that the capacitors is not connected directly to the load. The way you currently have it, the 12V auxillary components will very quickly pull down your capacitor.


Yep, but any time they are on, the DC/DC will be on. As magicsmoke noted though - the cap will still drain to the MiniBMS.

The more I think about it, the more I dig the idea of using the a sidestand switch to control the DC/DC. Seems very straightforward, and the only thing it cuts me out of doing is parking with lights on - and I can always pull the bullet connector to the sidestand switch if I want to show off the arc reactor.

Good news - after 22 charge cycles the MiniBMS has balanced the pack to the point that the charger shuts down from pack voltage level instead of cell level HVC through the BMS :-)

Talking at work today about getting an outlet or extension cord next to my parking space :-)