# howto reduce inrush for vacuum pump?



## dtbaker (Jan 5, 2008)

when my vacuum pump switches on, there seems to be a pretty big spike when it starts. Not a problem in the day, but if I have headlights on at night, there is a momentary flicker. I never saw this with a lower rated Curtis dc-dc at 96v, but it is quite noticeable with the Chennic after my upgrade to 120v despite having a higher watt rating output.

I almost got rid of the headlight flicker by adding a tiny (1.5ah) battery in parallel w/ my dc-dc, which is 'always on', without a large aux battery. 

but.... the other night when I had headlights AND heater fan running full blast, the flicker when vacuum started was once again quite noticeable.

....is there something simple I can do to 'soften' the startup of the vacuum pump?


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## piotrsko (Dec 9, 2007)

well, inrush is caused by loads on the motor reflecting into the battery system, SO: in low power compressors we use a check valve and a leaky pipe to reduce the pressure in the pump exhaust pipe to ambient to reduce motor start load. Perhaps the same trick here if you can tolerate the leak. I don't know of any CHEAP vacuum unloaders. the parallel battery thing across the motor switch ought to work but it is esthetically ugly.


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## dtbaker (Jan 5, 2008)

piotrsko said:


> well, inrush is caused by loads on the motor reflecting into the battery system, SO: in low power compressors we use a check valve and a leaky pipe to reduce the pressure in the pump exhaust pipe to ambient to reduce motor start load.



if it were the initial 'load', the voltage sag would remain the same since the pump 'load' remains the same..... 

I am pretty sure its the spike that it takes to start (any) electric motor when that vacuum switch closes and pulls enough 12v amps to sag my dc-dc converter momentarily. I think there are ways to 'soften' that vac pump start, but I dunno what would be possible or work in the case, so I'm hoping one of the EE guys can tell me.


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## Rational (Nov 26, 2011)

You might try a capacitor across the line to absorb the current inrush.
If the inrush is 10A and the duration is 1 second and you can stand a 5v drop in voltage then the value of C in uF would be
IdT/dV = 10(1)/5 = 2 F. 

It might be more practical to put the lights on a capacitor and have an isolation diode that prevents the motor from pulling down the voltage to the lights. The lights would then be running on ~1v less than normal. Sizing this diode can be tricky.


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## Coulomb (Apr 22, 2009)

dtbaker said:


> ....is there something simple I can do to 'soften' the startup of the vacuum pump?


Yes. You could consider something like this:

http://shop.ata.org.au/cart.php?target=product&product_id=16512&category_id=251









It's a solar "maximizer". It's supposed to convert solar energy to low voltage, high current for starting any sort of motor, then higher voltage and lower current when it is running. Without one of these, you would need an enormous solar panel to start some electric motor, then the panel would be way oversized for keeping the motor running.

One of these might work directly, or you might have to modify one to slowly ramp up the voltage to the vacuum pump motor. Warning: they may squeal (low PWM frequency), and it may not be trivial to increase the PWM frequency. We're having this problem using one very like this on a steering pump (see these posts if you are interested). 

There are probably others out there, some of which may come with decently high PWM frequency to start with; I suspect Mouser may have them. The keywords to search for would be "solar" and "maximizer". Maybe you don't care about a little squeal from the maximizer, especially since the vacuum pump will be running any time that the maximizer is on.


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## dtbaker (Jan 5, 2008)

Rational said:


> You might try a capacitor across the line to absorb the current inrush.
> If the inrush is 10A and the duration is 1 second and you can stand a 5v drop in voltage then the value of C in uF would be
> IdT/dV = 10(1)/5 = 2 F.
> 
> It might be more practical to put the lights on a capacitor and have an isolation diode that prevents the motor from pulling down the voltage to the lights. The lights would then be running on ~1v less than normal. Sizing this diode can be tricky.



I am kindof surprised that my little tiny battery, in parallel w/ the dc-dc, isn't doing the job. Would the capacitor work significantly better? why?

something like this?:
http://www.hifisoundconnection.com/Shop/Control/Product/fp/vpid/1458708/vpcsid/0/SFV/30046


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## Rational (Nov 26, 2011)

dtbaker said:


> I am kindof surprised that my little tiny battery, in parallel w/ the dc-dc, isn't doing the job. Would the capacitor work significantly better? why?


Batteries can be optimized to maximize W/kg or W-h/kg; yours might not be able to deliver the current fast enough, but a cap can deliver infinite current instantly, at least in theory.

If you don't mind soldering many caps in parallel, try
http://www.hosfelt.com/contents/en-us/d177.html


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## Coulomb (Apr 22, 2009)

dtbaker said:


> I am kindof surprised that my little tiny battery, in parallel w/ the dc-dc, isn't doing the job. Would the capacitor work significantly better? why?


Because it would have a lower Effective Series Resistance (ESR).



> something like this?:
> http://www.hifisoundconnection.com/Shop/Control/Product/fp/vpid/1458708/vpcsid/0/SFV/30046


Yes, but it probably doesn't have to be as fancy as that.

Note that you want to put this across the key-switch line, not directly across the vacuum pump motor (putting it across the pump motor would just make the surge worse). Also, for longevity of your key-switch, which would be a pain to replace, you would want to pre-charge the big capacitor. So that's more hassle, unless maybe the fancy audio capacitors already do the pre-charge for you; I don't know.

An easier solution might be to use a slightly larger 12 V battery, say 7 Ah, and place it near the switch to the vacuum pump (where the switch connects to 12 V), or alternatively near where the lights get their 12 V (the headlight relay). I prefer to have a real 12 V auxiliary battery anyway, for hazard flashers and the like in case the DC/DC or main pack fails, or the fuse blows, etc. But that's a matter of personal taste, I suppose.


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## Rational (Nov 26, 2011)

Also, make sure that the lines that supply current to the pump do not also supply current to the lights. Otherwise the wire resistance will show itself as a varying voltage to the lamps.

One problem is that battery manufacturers do not publish or may not know the 'transient response' of their batteries.

If you post a simplified wiring diagram [it has more information than a schematic diagram] of the DC source, the lights and the pump, some nearly optimal solution may become apparent.


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## dtbaker (Jan 5, 2008)

probably not requiring a schematic... the 12v output side of my dc-dc converter currently has a little ($19) 1.2ah rechargable sealed lead-acid battery in parallel, then splits with leads to the vacuum switch, and to the main 12v vehicle fuseblock.

I was hoping that there is some simple/inexpensive way to 'soft-start' the vacuum pump as that initial spike seems to be just enough to pull down the dc-dc to the point where lights and radio 'blink' if everything is on including the heater fan when the vacuum clicks in.

I found 2F cap on sale for $32 delivered.... so I'll try that in pace of the little battery. perhaps the speedier response is all that is needed. unless anyone has a brilliant idea on how to get the vacuum switch/pump to kick on 'softer'.


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## Rational (Nov 26, 2011)

dtbaker said:


> I was hoping that there is some simple/inexpensive way to 'soft-start' the vacuum pump as that initial spike seems to be just enough to pull down the dc-dc to the point where lights and radio 'blink' if everything is on including the heater fan when the vacuum clicks in.
> 
> I found 2F cap on sale for $32 delivered.... so I'll try that in pace of the little battery. perhaps the speedier response is all that is needed. unless anyone has a brilliant idea on how to get the vacuum switch/pump to kick on 'softer'.


Try both the battery and the cap; one handles steady-state response and the other handles transient response.

If you're handy you may be able to make a soft-start circuit that ramps up the voltage or current to the pump in one second or so, for less than $32 in parts. It might take you a weekend to do assembly, test & debug. A triangle wave generator IC plus a power transistor may do it. 
There are also Positive Temperature Coefficient thermistors for power applications, if you can find one that is suitable for this.

But, this soft-start may shorten the life of your pump motor. Maybe there is some mechanical way to relieve the vacuum during startup, something like the compression relief function used with some pull-start gas engines.

Can you post a link to your DC-DC convertor? Evidently the output impedance is a bit high. You also might measure the DC resistance R of your pump, then I[startup] = V/R.


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## dtbaker (Jan 5, 2008)

Rational said:


> Try both the battery and the cap; one handles steady-state response and the other handles transient response.
> 
> Can you post a link to your DC-DC convertor? Evidently the output impedance is a bit high.


I'll try the cap before attempting electronics building. 

My dc-dc is el cheapo Chennic 400watt. I never had blinks (at lower system voltage) with my original Curtis dc-dc.


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## skooler (Mar 26, 2011)

I think you already have the answer by using the audio capacitor.

In my ICE car I have a 1200w (2400w peak) stereo. I'm not a boy racer/yob etc, I just diddn't know how powerful it was when I purchased it!

When I first had it installed whenever the music hit a baseline the lights would dim and if it hit a baseline at the traffic lights or idleing the engine would stall!

This sounds like similar behaviour to your vacuum pump. Anyway, I installed a 1 farad cap and all the problems went away. It even had a LCD display for how many watts your using!

Heres the one I purchased:
http://www.caraudiocentre.co.uk/product_m-in-phase-ipc1-2d_p-21913.htm

It simply bolts to the positive line in to the device.

Hope this helps,

Mike


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## Rational (Nov 26, 2011)

So, where to put a cap of some size and an isolating diode for max. benefit? 
This depends on how much your pump draws on start up vs. how much your headlights draw. The pump is the offender but the lights provide the user annoyance.

The diode may be a few bucks but in series with the headlights will reduce brightness somewhat but I think a useful range for headlights is something like 20,000 to 80,000 candlepower, so you have some wiggle room.


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## dtbaker (Jan 5, 2008)

Rational said:


> So, where to put a cap of some size and an isolating diode for max. benefit?



why wouldn't I put it just outside the dc-dc so that it could cover whatever causes a hit to the system? headlights, heater, giant radio, vacuum pump....

I guess I COULD put it on the line running to vacuum pump, but why not on the main line to cover everything?


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## Rational (Nov 26, 2011)

dtbaker said:


> why wouldn't I put it just outside the dc-dc so that it could cover whatever causes a hit to the system? headlights, heater, giant radio, vacuum pump....


You'll need a bigger cap and so far we don't know how big a cap you need for just the pump. You really need an oscilloscope with a current probe or current sense resistor so you know the magnitude and duration of the startup surge, or the manufacturer can provide that info.

The trouble is that actual incand. brightness depends on voltage to the 3.5 exponent, give or take, so it is a very sensitive indicator. Perceived brightness depends on actual brightness to the 2nd or 3rd power, depending on who you talk to. 
But, depending on what is turned on first you may find other undesirable effects from the vacuum pump. The heater blower draws a lot of power but it sort of starts under no load conditions, except for the inertia of the impeller.

If there are no other performance issues with other loads I'd isolate the headlights. And since you know what to look for, I'd get a fresh pair of eyes to judge how well the fix worked. Also you might be able get a cheap surplus Schottky diode so there'd be minimal voltage reduction to the headlights.

You're going to need some test instruments unless you want to guess at this. The worst outcome is that you spend many bucks to chase a goal that constantly recedes from you, for a number of reasons.


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## skooler (Mar 26, 2011)

dtbaker said:


> why wouldn't I put it just outside the dc-dc so that it could cover whatever causes a hit to the system? headlights, heater, giant radio, vacuum pump....
> 
> I guess I COULD put it on the line running to vacuum pump, but why not on the main line to cover everything?


I'm not an expert on this but I'd say by the offending device, in this case the vacuum pump.

I'd think you'd want to keep the distance as short as possible to avoid losses. It also means that there is no current present when working on other areas of the car, e.g. changing a lightbulb. you simply disconnect the battery as normal.

The lights etc work fine from the accessory battery so why wire them up differently?

Cheers,

Mike


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## dtbaker (Jan 5, 2008)

right.... I would say that since the other devices are basically steady state and haven't really caused a visible sag at start, I would be tempted to put the cap in line w/ the vacuum switch/power so that it would really only be 'used' to cover that transient sag at vacuum startup.

I don't have a monster stereo to worry about in the main block.


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## Rational (Nov 26, 2011)

If the total power draw of lights, etc., during pump start up exceeds the 400W capability of this convertor then a cap won't help.


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## skooler (Mar 26, 2011)

Rational said:


> If the total power draw of lights, etc., during pump start up exceeds the 400W capability of this convertor then a cap won't help.


Surely this would depend on how long the power draw is above 400w when the pump starts? I think you are confusing power and energy.

As the lights only flicker for a second then one would assume that this power draw is only an instantanious spike which the cap (2 farads = ~1000w at 12v) should be able to 'soak up'.

However, I agree that if the total running energy needed for accessories exceeds 400wh then you'll need a bigger dc-dc.

Its exactly the same problem seen on many car audio forums for many years. Google is your friend on that one. A 1 farad cap stopped my 2,400w amp from dimming my headlights and stalling the engine.

Cheers,

Mike


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## dtbaker (Jan 5, 2008)

skooler said:


> As the lights only flicker for a second then one would assume that this power draw is only an instantanious spike which the cap (2 farads = ~1000w at 12v) should be able to 'soak up'.
> 
> However, I agree that if the total running energy needed for accessories exceeds 400wh then you'll need a bigger dc-dc.



exactly..... its just a transient 'spike' causing voltage drop. once running everything is fine, if it weren't everything would stay out or blow fuses. anyway, we'll see, I ordered the 2F Cap as it was on a pretty good sale down from $100+ to $32.


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## skooler (Mar 26, 2011)

Wow, thats a really good price!

Please let us know how you get on.

If it works, its quite a simple and inexpensive solution to a really irritating (and potentially dangerous) problem

Cheers,

Mike


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## Rational (Nov 26, 2011)

skooler said:


> A 1 farad cap stopped my 2,400w amp from dimming my headlights and stalling the engine.
> 
> Cheers,
> 
> Mike


If 2400w/12v = 200A
and
dV/dT = i/C = 200/1 = 200
then for a dV = 12v [total shutdown of the vehicle] the dT = 12/200 = 0.06 second
and 
for a dV of 1v the dT allowed is 1/200 = 5 mS.

I'd think a dV of 1 volt would be noticeable with the lights, so
1F @ 200A for less than 5 mS would work. 

The tricky point is, for the lights, what kind of sudden voltage change is noticeable?

At these current levels a single point ground becomes important and the resistance and inductance of each wire enters into the calculation.

BTW, the load dump transient for conventional vehicles can go several hundred volts negative, so there should be a protective network somewhere in the circuitry.


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## Agust Sigurdsson (Oct 25, 2009)

dtbaker said:


> probably not requiring a schematic... the 12v output side of my dc-dc converter currently has a little ($19) 1.2ah rechargable sealed lead-acid battery in parallel, then splits with leads to the vacuum switch, and to the main 12v vehicle fuseblock.
> 
> I was hoping that there is some simple/inexpensive way to 'soft-start' the vacuum pump as that initial spike seems to be just enough to pull down the dc-dc to the point where lights and radio 'blink' if everything is on including the heater fan when the vacuum clicks in.
> 
> I found 2F cap on sale for $32 delivered.... so I'll try that in pace of the little battery. perhaps the speedier response is all that is needed. unless anyone has a brilliant idea on how to get the vacuum switch/pump to kick on 'softer'.


I suggest that you keep the battery as is but connect the 2 Farad capacitor on the end of the branch which feeds the troublesome pump motor. That way most of the inrush current will be delivered directly from the capacitor to the motor with minimum disturbance to the rest of your system.
If this helps, you can try to disconnect the battery and see whether it can be removed.

Agust


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## Rational (Nov 26, 2011)

And if you isolate the motor/capacitor with a choke/inductor that does not saturate at peak motor current it will isolate even better.

The thermal time constant for headlight filaments may be pretty long and so this works in your favor but I can see a situation where the headlight flicker will not be noticed by most people most of the time but smaller lamp filaments may visibly flicker.


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## dtbaker (Jan 5, 2008)

I am contemplating messing with the brake booster right now.... so vac pump is turned off. I did get the CAP in the mail yesterday... its BIG and HEAVY. I am hoping I don't need to deal with it, but it may well become part of the recommended vacuum pump system.....


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## dtbaker (Jan 5, 2008)

since pedal effort was more than I wanted, even with sticky pads, I turned the vacuum back on.

I also installed the 2F capacitor I got in hopes it would eliminate the 'blink' when vacuum pump kicks in. Looks like it helps in that the blink is barely perceptable when headlights are low-beam, but blink get worse if headlights are high-beam AND I turn heater/fan on full blast. Seems to be a little better in that it does not knock the power down low enough to blink radio any more, but it does show in headlights.

;(

conclusion is that the cheapo Chennic dc-dc does not seem to handle hits as well as the old Curtis did. I am somewhat surprised that the 2F capacitor did not do the trick.


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## Rational (Nov 26, 2011)

dtbaker said:


> since pedal effort was more than I wanted, even with sticky pads, I turned the vacuum back on.
> 
> I also installed the 2F capacitor I got in hopes it would eliminate the 'blink' when vacuum pump kicks in. Looks like it helps in that the blink is barely perceptable when headlights are low-beam, but blink get worse if headlights are high-beam AND I turn heater/fan on full blast. Seems to be a little better in that it does not knock the power down low enough to blink radio any more, but it does show in headlights.
> 
> ...


If you have a 'scope you can now estimate how much capacity is needed to make this dropout unnoticeable.

The Chennic seems to drop out of regulation sooner or more steeply than the Curtis. It still may meet published spec's though.


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## dtbaker (Jan 5, 2008)

Rational said:


> If you have a 'scope you can now estimate how much capacity is needed to make this dropout unnoticeable.
> 
> The Chennic seems to drop out of regulation sooner or more steeply than the Curtis. It still may meet published spec's though.



I do not have a scope.... nor would I know how to use it.


back to my original question.... softening the startup of the pump. could I use something similar to what is on my heater relay? It came with some little caps, resistors, and a diode that I was supposed to wire up.... would something like this work?


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## Rational (Nov 26, 2011)

dtbaker said:


> I do not have a scope.... nor would I know how to use it.
> 
> 
> back to my original question.... softening the startup of the pump. could I use something similar to what is on my heater relay? It came with some little caps, resistors, and a diode that I was supposed to wire up.... would something like this work?


Post all the spec's of the pump - electrical, mechanical, everything. 

I think by now even this guy
http://en.wikipedia.org/wiki/MacGyver
would have given up.


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## dtbaker (Jan 5, 2008)

Rational said:


> Post all the spec's of the pump - electrical, mechanical, everything.



I don't think I have all the specs. It is the typical Gast pump and squareD switch included in the kit from KTA-ev.com

It draws something less than 10amps at 13.8v (thats what the fuse is). When that squareD switch slams on, I get the 'blink', but it just a flicker.

The 2F cap helped I think. The blink with everything on, headlights on high beam, heater at full blast used to blink bad enough to hit the radio hard enough it blinked off, and headlights blinked almost out. Now they just flicker and radio stays on.


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## Rational (Nov 26, 2011)

So I'll assume a 60 amp surge for a half second or so.

There may be another way to do this.
Your primary battery is about 120V, so tap off approx. 12v or slightly more from this pack. 
Use a time delay relay that powers the pump off the primary pack tap for a half second or so and then switches the pump over to be powered off the Chennic output. 
The relay contacts have to be break-before-make so you don't short out the Chennic and they have to handle 10A of 'motor load'. If the Chennic has an isolated ground the contacts need to be DPDT.


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## dtbaker (Jan 5, 2008)

Rational said:


> So I'll assume a 60 amp surge for a half second or so.
> 
> There may be another way to do this.
> Your primary battery is about 120V, so tap off approx. 12v or slightly more from this pack.
> ...



I would guess the blink is far shorter than 1/2 second.... this sounds like a good way to toast the vacuum pump.


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## notailpipe (May 25, 2010)

Rational said:


> There may be another way to do this.
> Your primary battery is about 120V, so tap off approx. 12v or slightly more from this pack.


That's a good way to get your battery pack out of balance, unless you intend to count on a BMS to save you...

Dan, I was wondering your same initial question. I've got a similar setup but the ACME DC/DC that EvolveElectrics sells. It works pretty well with no headlight dimming. Could be your DC/DC but another difference is I'm using a 7.2Ah AUX backup battery from Interstate Systems (got it off Amazon, it's tiny and only $35). So that in combination with the cap you added might solve your problem completely. I'm not using a cap, but would add one if I ever saw a problem.


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## dtbaker (Jan 5, 2008)

notailpipe said:


> ... I'm using a 7.2Ah AUX backup battery from Interstate Systems (got it off Amazon, it's tiny and only $35). So that in combination with the cap you added might solve your problem completely. I'm not using a cap, but would add one if I ever saw a problem.



interesting.... The tiny battery I added before was just 1.3ah, and while it helped, did not fix completely... perhaps it just couldn't put out enough amps to cover the dip.

The 2F cap doesn't solve it completely either, and is quite large and heavy. I got it for just $32, but normally retails for closer to $100 I think.

I removed the little battery during installing, but now might go back and try a larger battery just to see if that is what was needed. I didn't want an AUX battery, but looks like one may be needed with a cheap dc-dc.


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## notailpipe (May 25, 2010)

dtbaker said:


> interesting.... The tiny battery I added before was just 1.3ah, and while it helped, did not fix completely... perhaps it just couldn't put out enough amps to cover the dip.
> 
> The 2F cap doesn't solve it completely either, and is quite large and heavy. I got it for just $32, but normally retails for closer to $100 I think.
> 
> I removed the little battery during installing, but now might go back and try a larger battery just to see if that is what was needed. I didn't want an AUX battery, but looks like one may be needed with a cheap dc-dc.


Personally, I like the peace of mind knowing if my DC/DC dies (eventually going to happen, right?) then my battery will keep things on long enough for me to get off the road (haven't installed this yet, but will have a light on the dash go on if the DC/DC dies, not sure how I will tell that yet, though it itself has a little ON LED, not sure how I could use that). Anyway, I like redundant systems on my vehicles.


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## Rational (Nov 26, 2011)

dtbaker said:


> this sounds like a good way to toast the vacuum pump.


Not to me.


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## dtbaker (Jan 5, 2008)

Rational said:


> Not to me.



ohhhh, you mean pull 12v of power from 4 cells.... not just a blink at full pack 120v. 

either way, doesn't sound good having ANY parasitic load on 4 cells would unbalance the pack pretty quickly... something I'd like to avoid.


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## Rational (Nov 26, 2011)

dtbaker said:


> ohhhh, you mean pull 12v of power from 4 cells.... not just a blink at full pack 120v.
> 
> either way, doesn't sound good having ANY parasitic load on 4 cells would unbalance the pack pretty quickly... something I'd like to avoid.


A half-second at 10A is 5 A-s = 1.4 mA-h of charge pulled out of these cells. How much cell imbalance is normal?

It all comes down to tradeoffs. Cost vs. benefit vs. risk vs. shortening the life of components vs. cosmetic flaws.


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## dtbaker (Jan 5, 2008)

Rational said:


> A half-second at 10A is 5 A-s = 1.4 mA-h of charge pulled out of these cells. How much cell imbalance is normal?
> 
> It all comes down to tradeoffs. Cost vs. benefit vs. risk vs. shortening the life of components vs. cosmetic flaws.



? reasonable alternatives are even better !

the facts are pointing toward trying a slightly bigger battery in parallel may just solve it without any extra components, switches, timers, or parasitic load on 4 cells. even if it is only 1.4mA-h per pump cycle...


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## Rational (Nov 26, 2011)

On a w/kg basis, caps come out better than batteries for quick current delivery.
http://www.google.com/imgres?imgurl...jTzdToX7JKTC0AGG3ZSlDA&ved=0CEgQ9QEwBA&dur=35

Indiana Jones had an easier time finding the lost ark.


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## notailpipe (May 25, 2010)

Rational said:


> On a w/kg basis, caps come out better than batteries for quick current delivery.


True, in general. There is certainly a supercapacitor out there that would meet dtbaker's needs. However if you plot that chart with w/kg/$, then it's an entirely different story. The battery I'm using has good-enough instantaneous power delivery and also good-enough energy capacity, whereas right now the cap has one or the other. Perhaps it is helping in his case, for the first couple milliseconds and then it runs out of capacity?


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## dtbaker (Jan 5, 2008)

notailpipe said:


> T However if you plot that chart with w/kg/$, then it's an entirely different story.



exactly.... next I guess I'll try a 5 or 7ahr battery to see if it's better than the 2F cap. $30 of battery versus $100+ of cap...

that last 5% of elegant and affordable solutions often require experimentation when it hasn't been documented before.... any engineer can come up with formulas and theories, but it takes testing to verify and come up with solutions that are reliable and affordable.


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## Rational (Nov 26, 2011)

dtbaker said:


> exactly.... next I guess I'll try a 5 or 7ahr battery to see if it's better than the 2F cap. $30 of battery versus $100+ of cap...
> 
> that last 5% of elegant and affordable solutions often require experimentation when it hasn't been documented before.... any engineer can come up with formulas and theories, but it takes testing to verify and come up with solutions that are reliable and affordable.


Sort of. . .

In my experience, if it works on paper it still may not work in the real world. If it doesn't work on paper it almost certainly will not work in the real world.
With the considerable handicap of no test instruments you're kind of flying blind, so I think the best that can be done is come up with logical conclusions based on what data and what symptoms you have. 

At least the cap had some effect on the symptom, so now you don't have a functional failure, just a displeasing symptom. 
What is really frustrating is the case where you throw everything at a problem including the kitchen sink and it does not change at all, or it even gets worse. 

If I were in your shoes I'd try to collect more clues with what you already have, perhaps by using different loads switched on and off and making a matrix of the results. There would be many cells to this matrix and by eyeballing it you may see a pattern.

BTW, Transparency International does not rate the US as all that good. . .but, what do they know anyway! 
FWIW, I do like Colorado - I was onsite at Ball Brothers for a while, at Boulder. Lift tickets were for sale in the supermarkets!


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## notailpipe (May 25, 2010)

dtbaker said:


> I don't think I have all the specs. It is the typical Gast pump and squareD switch included in the kit from KTA-ev.com


Sidenote: I don't know if other pumps are better, but I have one of those Thomas pumps (similar to Gast) and perhaps the piston-type pumps just pull too much current at draw? Sometimes I hear my brake relay doing weird things and I wonder if the sudden current to the pump dips my relay enable too low. Like I've heard a weird oscillation before.

The pump works but I'm going to go another route and try the VW/Audi pump route. Those might be designed to take less initial draw as I don't think they're piston type. I'll let you know what I find when it arrives and I get it installed.

You may still have other elements taking too much of a spike, but I think the brake pump is by far the most (based on very little data yet).


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## Rational (Nov 26, 2011)

notailpipe said:


> True, in general. There is certainly a supercapacitor out there that would meet dtbaker's needs. However if you plot that chart with w/kg/$, then it's an entirely different story. The battery I'm using has good-enough instantaneous power delivery and also good-enough energy capacity, whereas right now the cap has one or the other. Perhaps it is helping in his case, for the first couple milliseconds and then it runs out of capacity?


What battery are you using? It might be a good compromise for some applications but generally w-h/kg vs. w/kg is a zero sum game. The more room in the container to maximize one, the less room in the container to maximize the other.
I guess I'll start collecting data on w-h/$ and w/$. Since volume is somewhat important, densities for these power sources would also be useful.


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## dtbaker (Jan 5, 2008)

Rational said:


> If I were in your shoes I'd try to collect more clues with what you already have, perhaps by using different loads switched on and off and making a matrix of the results. There would be many cells to this matrix and by eyeballing it you may see a pattern.



I don't need any more testing or matrix of results to spot the pattern. The more loads I have on, the more noticable the 'blink' in headlights is when vacuum pump comes on.

using either a 1.3ah battery or a 2F cap in parallel with dc-dc reduces the symptom, but does not eliminate completely.


I have been looking at other ways to eliminate the vacuum pump completely, either by changing pedal length, master cyl bore (boosterless alternative not known for this make/model), or brake pads.... but that is not panning out to give good enough brake performance with low enough effort at reasonable cost to be a viable alternative.

so..... unless someone can put up a slick way to 'soft-start' the vacuum pump, my next attempt will be to try a slightly larger battery in parallel w/ the dc-dc; like maybe a 5ah or 7ah to see if that takes care of it.


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## Rational (Nov 26, 2011)

dtbaker said:


> I don't need


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