# minimizing dc-dc voltage sag?



## rwaudio (May 22, 2008)

You could try a 1-2 Farad car audio capacitor, basically it's there to try to reduce the voltage sag when a bass note hits etc... would work the same for your application, it would be best to hook it up near the offending device. If there is more than one offender then mount it close to the output of the chennic... this would work well for an always on dc/dc converter, however if the cap was allowed to drain when you turn off the car it would be an insane load on the chennic and probably blow it up. It basically needs a pre-charge sequence just like the controller. Otherwise a small battery, since your a lithium user now, 4 series 10ah headways would be small and provide enough power.




dtbaker said:


> in my upgrade from 96v to 120v I changed from a Curtis 1400 series dc-dc to an inexpensive but higher watt rated one from Chennic. It seems to be working pretty well EXCEPT it seems to not handle in-rush loads very well.
> 
> The old Curtis seemed to show no voltage sag when the vacuum pump came on for instance, but the Chennic does.... the headlights dim for a half-second, and the radio power blinks!
> 
> ...


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## dtbaker (Jan 5, 2008)

I like the idea of a capacitor since it would last 'forever'... but I am a little bummed the Chennic dc-dc wouldn't have something like this built in already. I guess that is what makes it cheap. 

So, tell me a little more about what I need to do to manage the 'pre-charge' or initial inrush on the rare occasions I power down the system. The dc-dc is normally 'always on' as this is my daily driver, but I do occasionally power down when working on something. 

And, would I get something like this for a decent price at a local Radio Shack, or a car stereo place, or somewhere else?


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## rwaudio (May 22, 2008)

dtbaker said:


> I like the idea of a capacitor since it would last 'forever'... but I am a little bummed the Chennic dc-dc wouldn't have something like this built in already. I guess that is what makes it cheap.
> 
> So, tell me a little more about what I need to do to manage the 'pre-charge' or initial inrush on the rare occasions I power down the system. The dc-dc is normally 'always on' as this is my daily driver, but I do occasionally power down when working on something.
> 
> And, would I get something like this for a decent price at a local Radio Shack, or a car stereo place, or somewhere else?


Basically for precharge you simply need to apply 12v through a current limiting resistor (the cap should come with the resistor for initial charging).
Then you connect the power directly. It's a very simple process, just something you would need to remember to do that's all. With a couple of relays and some sort of delay you could impliment an "automatic" setup much like the precharge control in motor controllers.

You can get these capacitors all over the place, here's what a 2 second search on amazon turned up:
http://www.amazon.com/Raptor-C1-0FM...TJBO/ref=sr_1_1?ie=UTF8&qid=1299083186&sr=8-1

It has a built in volt meter, which you may or may not like, there are more basic ones without the meter.


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## Ziggythewiz (May 16, 2010)

dtbaker said:


> I like the idea of a capacitor since it would last 'forever'... but I am a little bummed the Chennic dc-dc wouldn't have something like this built in already. I guess that is what makes it cheap.


I think the capacitor approach sounds great provided the power needs aren't too much. Just keep in mind that a 1-2 farad cap may look huge but doesn't hold that much juice compared to batteries. Back in HS I was running a couple 12s at 400 watts and the headlights would flash every time the bass hit. Adding a 1F cap took care of that. ebay was by far the cheapest source at the time. IIRC 1F was < $30.

For the precharge, since you rarely power down just use a light bulb or a switched resistor. You don't need a complex timer and relay system for something you're rarely needing. However, you should fuse it in case you forget, and depending on how forgetful you are, the relay setup as rwaudio mentioned could help out.


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## Overlander23 (Jun 15, 2009)

Just an alternate solution to those mentioned above. I'm using an Odyssey PC625 AGM to power all 12V, but a Chennic DC->DC set at 14.5V to charge the Odyssey (recommended charge voltage). While the Odyssey is only about 16 AH in capacity, this setup pretty much means the Chennic does most of the work while the AGM acts like a buffer.

I've got the converter wired to a secondary contactor to come on when the ignition is switched on, but the 12V AGM is always in the 12v circuit (like the original 12V battery was)... though it's on a small cutout in case I want to kill all power everywhere.

I prefer the "battery-buffer" for its ability to act as a backup power source, as well. Say something goes wrong with the DC->DC or main pack, the AGM is there to provide headlights, hazard lights, brake pump, etc... for at least a little while.

Now, I haven't hooked up the Chennic yet, though I have it with me in CA, but I have run the beast off the AGM alone, and aside from the voltage load on the small battery dipping below the min-voltage threshold of the Soliton-1 occasionally, it was fine. When I get back to the southwest in May I'll pop in the Chennic.

I'll see how it goes, but the Chennic should keep the AGM charged while driving. It outputs 400W and I'm nowhere near pulling that much power. If it isn't enough, I'll consider not switching the DC->DC with ignition and just have it on all the time.

The AGM solution was about $110, but it's light and small.


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## dtbaker (Jan 5, 2008)

my chennic is set to output 13.8v, so that should be fine for charging battery or cap. The 1F capacitors I have been looking at are pretty big (10"x3"dia), so I am thinking now more like finding a SMALL sealed 12v battery as a buffer and emergency backup.

The caps are available mostly around $100, but a few like this one are $35-ish plus shipping
http://cgi.ebay.com/ebaymotors/1-FA...8bQQitemZ360347214987QQptZCarQ5fAudioQ5fVideo


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## dtbaker (Jan 5, 2008)

how about this little 'scooter' battery? If I just put this at dc-dc output and then from battery to my usual main 12v output to car.... ? or, would the likely amps pulled on occasion be too much for this? It would only have to 'make up' for the momentary sag of the dc-dc when the vac pump kicks in and my headlights are on as far as I can tell.....

http://www.batterymart.com/p-12v-1_3ah-sealed-lead-acid-battery.html


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## Ziggythewiz (May 16, 2010)

Wow, that's tiny. I've got a few similar that are 7.5 ah. (~$20)

It sure won't weigh anything! The only thing to watch out for is the max voltage that thing can handle. You'll essentially be applying a float charge @ 13.8 V, which might be too much for that thing over time.

You can always try it out and see what happens. Check for any leakage after a week of running. If you could find a dead UPS (the small house ones use ~7 AH ones) you could get a freebie to experiment with. 'Dead' ones will not hold a charge long, but could easily handle momentary loads.


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## dtbaker (Jan 5, 2008)

yeah, I am more concerned w/ max amps this little sucker can handle in and out. If dc-dc died completely, headlights alone are 10 amps on low beam, fan for heater is 10a on high, and I dunno what the peak inrush is on vacuum pump.... as long as the little battery can handle it for a second, it would be good and avoid the size/cost of a 1F cap.


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## Overlander23 (Jun 15, 2009)

I like the AGM because you can drain them without really killing them... and they self discharge pretty slowly. The problem with SLA is that they really need to be on a float charge constantly. If you only ever use 10% of one, they last for years, if not a couple decades. If you let them drain two or three times, they're kinda toast.

But then, my EV sits for months at a time. My needs are different.


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## Ziggythewiz (May 16, 2010)

I don't know what you size/weight constraints are, but you might consider just throwing in a real battery. Just get a small deep cycle (or starter if you need even smaller/cheaper) battery. That would ensure you could handle the amps and have adequate backup should the DC-DC go out.


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## dtbaker (Jan 5, 2008)

I REALLY don't want or need a full size battery!


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## EVfun (Mar 14, 2010)

I was considering this with my buggy as well. It has a DC to DC and no 12 volt battery. The hit from the headlights pretty much shut down the car for a fraction of a second, but the Curtis didn't care and contactor had a freewheel diode installed so it couldn't react fast enough to care either. The now installed Zilla will most likely complain and the contactor will react faster with a transzorb across it instead of a diode. The old Todd PC-20LV DC to DC converter didn't handle this much better than the reaction you describe from the Kelly. 

Pretty much any 20 to 30 amp DC to DC converter is going to cry badly when you hit the headlights. When the headlights are running they are about 1.5 ohms of resistance, so when the filaments are cold at turn on, they are briefly about 0.15 ohms of resistance. My thought is to use a audio stiffening capacitor and build a precharge system to safely handle it. To precharge it I would connect the coil of a 30-50 amp relay to the ignition switch. I would put a 1 ohm resistor across the contacts. I would hook all the cap to the one contact and the rest of the 12v system to the other contact. Anytime the vehicle was off the cap would be on precharge (unless the DC to DC was also off.) It would be switched into the system anytime the car is turned on.


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## dtbaker (Jan 5, 2008)

EVfun said:


> Pretty much any 20 to 30 amp DC to DC converter is going to cry badly when you hit the headlights.


the chennic dc-dc is supposed to handle 400 watts.... headlights plus parking lights are about 150watts on low beam. Issue arises only when vacuum pump kicks in AND headlights are on, and just for less than a second.

in my case dc-dc is ALWAYS on, so probably wouldn't need an elaborate precharge with an audio cap since it is never off..... or if a small 12v battery can handle the momentary load amps. either way.


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## EVfun (Mar 14, 2010)

A couple of minor points. 

Headlights are about 10x their running load briefly at startup. It is the nature of a cold filament, the resistance of tungsten wire increases greatly as it heats up. I don't have a vacuum pump so for me this surge is the biggest potential issue. It sounds like the startup current surge of your vacuum pump is taking the place of the headlights with my smaller DC to DC. The effect is the same, a much larger startup load than running load.

My DC to DC is always on and I don't have a 12 volt battery. The precharge system (which is really only 2 parts, a relay and a resistor) is to make sure you never power up the 12 volt system without precharging the audio stiffening cap . It will try to draw nearly infinite amps if connected directly to power when discharged. If that happens it is likely to mess up the DC to DC converter, so it must always be precharged before being dropped into the 12 volt system.


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## dtbaker (Jan 5, 2008)

EVfun said:


> The precharge system (which is really only 2 parts, a relay and a resistor) is to make sure you never power up the 12 volt system without precharging the audio stiffening cap . It will try to draw nearly infinite amps if connected directly to power when discharged. If that happens it is likely to mess up the DC to DC converter, so it must always be precharged before being dropped into the 12 volt system.


I have never dealt directly w/ caps before, so please pardon ignorant questions....

It sounds like once 'precharged', the cap can basically be used as a battery and installed directly in-line since it should not be discharging/charging 'much'. True, or do I need to BUILD IN the relay/resistor to be sure that if the cap ever gets discharged, it would pre-charge gracefully next time power is available?

Hard to believe this would not be built into the pre-packaged car stereo caps.... Just to keep the kids from blowing up their equipment? Some kind of auto-precharge, timer, relay, or delay when first powered up?


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## rwaudio (May 22, 2008)

dtbaker said:


> I have never dealt directly w/ caps before, so please pardon ignorant questions....
> 
> It sounds like once 'precharged', the cap can basically be used as a battery and installed directly in-line since it should not be discharging/charging 'much'. True, or do I need to BUILD IN the relay/resistor to be sure that if the cap ever gets discharged, it would pre-charge gracefully next time power is available?
> 
> Hard to believe this would not be built into the pre-packaged car stereo caps.... Just to keep the kids from blowing up their equipment? Some kind of auto-precharge, timer, relay, or delay when first powered up?


If your dc/dc converter is always on then once it's charged it stays charged and it will work to stabilize the voltage. You will only have to worry about it if you remove dc/dc converter power.

As for the car stereo caps, it won't actually "destroy" anything in that application because you are connecting it to a battery. What you usually get is a massive spark and some arc welding if you don't precharge first. A battery can take the abuse because the resistance of the cables/connectors will limit the current enough. The reason it could destroy your dc/dc converter is because it would act like that short circuit for a brief period on the output of your dc/dc converter. If your dc/dc converter won't tolerate a short duration short circuit it could go up in smoke.


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## michaeljayclark (Apr 3, 2008)

could go with a few 3 dollar capacitors to keep the charge and keep the voltage from sagging to much. be cheaper than a bettery and last longer than your EV.

just make sure you dont have any parasitic loads.


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## EVfun (Mar 14, 2010)

dtbaker said:


> It sounds like once 'precharged', the cap can basically be used as a battery and installed directly in-line since it should not be discharging/charging 'much'. True, or do I need to BUILD IN the relay/resistor to be sure that if the cap ever gets discharged, it would pre-charge gracefully next time power is available?
> 
> Hard to believe this would not be built into the pre-packaged car stereo caps.... Just to keep the kids from blowing up their equipment? Some kind of auto-precharge, timer, relay, or delay when first powered up?


I'm quite amazed they don't have this built in, but when you get an audio stiffening cap it comes with a light bulb or resistor to precharge it. 

A few farads of capacitor behaves a lot like a very small capacity battery with very low internal resistance (that lasts a lot longer and doesn't care about being run dead.) If the DC to DC is ever off, even for 10 minutes, it is almost certain the cap will discharge with just the small parasitic loads in a car. When the DC to DC comes back on it will "see" the discharged cap as a short circuit until it charges up (a much more painful one than my headlights or your vacuum pump.) I don't know if the inexpensive Kelly and Chennic DC to DC converters can handle a few farads of capacitive load, it is a lot more than generally present in devices. It is a lot more than is inside your controller!


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