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soyachips
09 September 2010, 1947
Hi there,

I've got a 12V 5AH battery for my 12V circuit (lights, horn, contactor and some relays) and it's not lasting long enough so I could either get a bigger battery or a DC-DC converter.

Of the DC-DC converters I've seen, the cheaper ones are non-isolated and the more expensive ones are isolated. If I'm using the frame to ground my 12V circuit, is it a bad idea to use a non-isolated converter?

Also is using the frame to ground the 12V circuit a bad idea? Is this mainly because of resistance? Any idea how big a difference would it make if I didn't use the frame to ground?

Thanks!
Andrew

EZwryder
09 September 2010, 2027
Soyachips, I was just wondering this myself. I was trying to think why a bike uses a grounded frame to begin with and the only thing I could think of is to provide a path to ground for the spark plugs in the motor. With that eliminated, I don't see why a floating ground wouldn't work equally as well. It might depend on how your wiring harness is configured. If the lights on your bike go to the frame to ground, you'd have to do the same with your converter. However, if each light has hot lead and ground lead out of the harness, you could use a floating ground.

This is all "thinking out loud" on my part. I'll be interested to hear an authoritative answer.

frodus
09 September 2010, 2247
Would it work? yes.

Is it safe? Not at all.

The reason we get isolated DC-DC converters is that there is no connection between the incoming ground (pack negative) and -12V output. If these were to reference the same common (non-isolated), you now have the same ground for the 12V as you do for the pack. If you ground the frame, its now sitting at pack negative. If you were working on the bike, or a wire comes loose and you touch anything with any pack voltage on it at the same time you touch the frame, you can and will get a shock. DC isn't very friendly in higher voltages. If you use a non-isolated DC-DC, then isolate the -12V for everything and you should be safe. I'd just get an isolated converter and know that its not only more robust, but its safer. I've heard of quite a few of those non-isolated versions go kaput.

BaldBruce
10 September 2010, 0544
I agree 1000% with Travis, always use an isolated converter. Safety dictates that you use one as he clearly stated. I would like clarify for our questioners though that using the frame as the return line of the output is perfectly fine IMO. As long as you are using an isolated converter, it makes little difference if you use the frame as the negative return line or not. A lot of parts built for automotive 12V use their mechanical connection as the negative lead. Some will point out that if you are re-wiring anyway, you might as well keep all voltages off the frame. If you are using the existing wiring on the bike, go right ahead and connect the negative output of your isolated converter to the frame. Just never connect the negative INPUT of the converter to the frame. As has been pointed out, this increases the odds of you doing some welding........:eek:

teddillard
10 September 2010, 0602
I'm right there too... having tried wiring the 12V off the high-V pack with a frame ground but mistakenly pulling a different negative for both the pack and the 12V (thus frying a meter I was trying to hook up, since the difference between the two negatives was 48V), and then doing the same idea, but getting it right, (but this time melting a battery due to the higher load on the 12V feed); now running a separate 12V battery- (pretty much covering all possible stupid mistakes) there's no question that the two voltage systems have to be isolated. As Bruce referenced, I ultimately rewired the 12V side of the bike anyway, so I put a wire ground on the thing, but the aftermarket turn signals are a chassis-ground, so I have a frame ground in spite of myself. There's just no percentage in not isolating the systems...

soyachips
10 September 2010, 0717
Excellent, thanks for all the great feedback! Some of the wiring is already chassis-ground and I'm going to be getting some aftermarket turn signals so I'll definitely be getting an isolated converter based on this advice. Out of interest, does anyone know how many amps their 12V circuit draws with everything on including headlight? Does 10A sound reasonable? I'm thinking I'll need to get at least 150W converter.

teddillard
10 September 2010, 0733
I'd overkill it anticipating adding more components... based on what I learned about motor cooling (http://www.elmoto.net/showthread.php?49-motor-cooling.-redux.-again.&highlight=motor+cooling), I decided to add a fan- the same one from the host bike radiator- which draws a lot more than just the lights. I'm not really sure how to do the math, and I haven't measured the actual draw, although that really wouldn't be too hard, now, would it... ;) but I have about 14aH running my 12V system and it's fine for the 1/2 hour or so that I run it...

EZwryder
10 September 2010, 0812
So....I bought my 72 - 12v converter from a Chinese company off of eBay. Is there a way I can determine if my converter is isolated, or not?

teddillard
10 September 2010, 0816
My wild guess would be to measure continuity between the connection to pack ground and your output (12V) negative, right? If they connect, you got a non-isolated converter.

EVcycle
10 September 2010, 0824
So....I bought my 72 - 12v converter from a Chinese company off of eBay. Is there a way I can determine if my converter is isolated, or not?

Ahhhhh the Chinese, no paperwork, good luck Chuck, electronic devices. I would bet Ted's money it is not. :)

frodus
10 September 2010, 0832
Baldbruce, Great point. I agree, we're not saying NOT to use the frame as return, just don't use it as return for a non-isolated DC-DC converter. Its completely safe it is completely isolated from the pack + and -.

Good choice on isolated. What DC-DC converters are you looking at? I use a Vicor and have liked it. I know sevcon makes a 300W that is pretty nice. Get something sealed (not iota). Kelly makes one but its HUGE.

Well, for me, I'll be running one 65W headlight, turn signals, running lights, BMS, a few relays, gauges/display.... so I'd estimate at least 130-140W. I'm playing it safe and using a 200W. I think anything between 15-20A for the 12V system is normal. Some motorcycles have dual headlights, which is what, 55-65W a bulb? I'd do this, add up the wattages/currents of all of the lighting, add some for anything else on the 12V system, do 25-30% more, and call it good.

One thing to also consider:
If you use a smaller DC-DC converter AND an aux battery (as I'll be doing), it'l be fine, but you have to turn the DC-DC converter ON when you're charging. This will allow the DC-DC to "charge" the aux battery while you're charging. This way, you can draw some current off the DC-DC and some off the battery while the bike is running. Its redundant too, if either fails, you're still good for a while until you get home. Its also nice because you always have 12V to switch stuff.

Another thing with using an aux battery, is that you don't bog down the DC-DC when you turn things on. If you turn a headlight on some systems, it'l dim and then go back to bright. Thats because of inrush current. The light draws a lot of current at first. If the DC-DC converter is overloaded, it should (if its a decent converter), go into current limit, which may cause the lights to dim. If you use a battery, the heavy inrush can be handled by the battery, then the DC-DC converter won't current limit, and lights don't dim. Another risk, is that if the voltage falls too much during current limit, some devices could reset (gauges, displays, BMS).

frodus
10 September 2010, 0833
how many wires?

if its 3, then its non-isolated.

If there are 4, check if Pack negative and 12V negative have continuity. If they do, non isolated. If they don't, then its likely that its isolated, but you should power up to verify.

EZwryder
10 September 2010, 0835
Yeah, I know -- I wasn't expecting much. It's a bridge to a better solution in build 2, but I realize it would be helpful to know what I'm dealing with in this cheap box.

EZwryder
10 September 2010, 0847
how many wires?

if its 3, then its non-isolated.

If there are 4, check if Pack negative and 12V negative have continuity. If they do, non isolated. If they don't, then its likely that its isolated, but you should power up to verify.


I'll do that, thanks. So if I power up how would I attach my voltmeter and what should I expect to see if it is non-isolated vs. isolated?

frodus
10 September 2010, 0920
Hook it up to the pack and measure between the 12V output and the pack negative (not the 12V negative).

If you see ~12-14V, then its not isolated. If you see something other than that, its floating and most likely isolated.

soyachips
10 September 2010, 1437
So....I bought my 72 - 12v converter from a Chinese company off of eBay. Is there a way I can determine if my converter is isolated, or not?

Out of interest, which converter did you get? I was thinking of getting this one mainly because it's so small:

http://cgi.ebay.com/Powerful-DC-48V-12V-10A-120W-Step-down-DC-Converter-/360287649724

It has 4 wires which might mean it's isolated but I've sent a question to check, otherwise maybe this one:

http://cgi.ebay.com/DC-DC-Converter-Golf-Cart-48volt-Reducer-12V-30Amp-/120552079126

soyachips
10 September 2010, 1454
Good choice on isolated. What DC-DC converters are you looking at? I use a Vicor and have liked it. I know sevcon makes a 300W that is pretty nice. Get something sealed (not iota). Kelly makes one but its HUGE.

If you use a smaller DC-DC converter AND an aux battery (as I'll be doing), it'l be fine, but you have to turn the DC-DC converter ON when you're charging. This will allow the DC-DC to "charge" the aux battery while you're charging. This way, you can draw some current off the DC-DC and some off the battery while the bike is running. Its redundant too, if either fails, you're still good for a while until you get home. Its also nice because you always have 12V to switch stuff.

Hi frodus, the Vicor and Sevcon converters look great but the budget doesn't really allow for it at the moment so I'm thinking of a cheapy however this might be false economy :confused:

I'll be using a converter and an aux battery so thanks for the tips! Have you posted a wiring diagram somewhere that I could reference?

frodus
10 September 2010, 1518
Just connect the output of the DC-DC via a fuse to the 12V battery. Connect the battery to your 12V system. Make sure you switch the DC-DC on and off on the battery side. I'd use the keyswitch with 12V from the battery to power one of these relays:
http://www.casadelgato.com/KUEP-3D55-12

That way, when you're OFF, the relay is off, and it disconnects the DC-DC from the pack. When you turn the key, you energize the relay, and turn on the DC-DC converter.

I'd also add another relay to sense the charger being plugged in. Then, if its plugged in, it bypasses the keyswitch and turns on the DC-DC converter.


I need to update my drawing, give me a few days.

EZwryder
10 September 2010, 1932
It has 4 wires which might mean it's isolated but I've sent a question to check, otherwise maybe this one:

http://cgi.ebay.com/DC-DC-Converter-Golf-Cart-48volt-Reducer-12V-30Amp-/120552079126

That's the one I got, configured for 72v. As it turns out, it IS isolated. I will eventually move up to the Vicor, but right now space is less of a problem than $$$, so the Chinnic though boxy, seems like it will work well enough.

EVcycle
10 September 2010, 2021
That's the one I got, configured for 72v. As it turns out, it IS isolated. I will eventually move up to the Vicor, but right now space is less of a problem than $$$, so the Chinnic though boxy, seems like it will work well enough.

Good to hear that is is isolated.

P.S. That is why I bet TED's money!!! LOL

teddillard
11 September 2010, 0317
<a href="http://s797.photobucket.com/albums/yy257/Gabrielle_123/?action=view&current=homer-simpson-doh.jpg" target="_blank"><img src="http://i797.photobucket.com/albums/yy257/Gabrielle_123/homer-simpson-doh.jpg" border="0" alt="Photobucket"></a>

soyachips
16 September 2010, 0505
OK so I found out that this one:

http://cgi.ebay.com/Powerful-DC-48V-12V-10A-120W-Step-down-DC-Converter-/360287649724

is not isolated which is a shame because it's so small and cheap!

EVcycle
16 September 2010, 0528
You may want to find a multi voltage step down 30 -100 to 12V. That way if you move up in voltage you do not have to purchase another unit.

I still use a single 12V battery on the motorcycle. The run time with it is longer than the drive time and was much cheaper to install. :)

http://www.powerstream.com/dc2.htm

soyachips
16 September 2010, 0631
If I stick with the Vespa there's no way I'm moving up in voltage ... there's hardly any room for a DC-DC converter let alone more batteries!!!

Thanks for the link. The PST-DC4812 model looks suitable, a bit smaller that some of the other ones on my list and cheaper too. :)

frodus
16 September 2010, 0817
$125? ripoff. We can get you something better than that. Lemme do some digging on ebay for some 48V vicor modules.

I'd recommend something sealed for anything like this.

soyachips
22 September 2010, 0649
Would love it if you can find something. From my searching $125 seemed like a good price for an isolated converter that can output 150W.

In terms of Vicor, I had a look at their product range and they have DC-DC converter modules like this:

http://cdn.vicorpower.com/documents/datasheets/ds_48vin-micro-family.pdf

and DC-DC Power Systems like this:

http://cdn.vicorpower.com/documents/datasheets/ds_megamod.pdf

Could I use either of these? Is there anything special about how either is mounted or connected that I need to be aware of? A sales person I spoke to said that some converters need additional filtering but I didn't understand what that meant. The second option above seemed like a simpler solution because it's a stand-alone unit that I can just wire up and use.

frodus
22 September 2010, 0843
Yeah, either of those are good.

Some require filtering if the DC line is noisy. Ours area LITTLE noisy, but not that bad. It just means they may need a capacitor and choke to filter out the high frequency from the controller. Nothing big.

I've found quite a few on ebay, some vicor, some lambda. The lambda looks good, its tiny and 150W/92% eff and sealed.
Can't link to it, using iphone ebay app. Search for "PAH150S"


And as with most DC-DC's, they need to be secured to a heat sink or plate so they can keep cool.

If you just want to get it running, use a couple of 75W vicor modules, wired in parallel (make sure they're trimmed to the right voltage):
Search ebay for "Vi-232-CX"

soyachips
24 September 2010, 0603
Hey frodus, thanks for pointing me in the right direction!

frodus
24 September 2010, 0842
You're very welcome. Its one of the side effects of being an EE, I know what to look for..... That and I love helping people find stuff :)

And I've been keeping my eyes open for some more 48VDC input converters.... for a BMS system I'm kinda gonna toy with.