# What is the recommended way to break pack voltage?



## Duncan (Dec 8, 2008)

My tuppence worth

You will normally be opening these circuits at no load,
If you do need to open under load then you should treat the "breaker" as used up and not re-use it until you have checked it

My car has
An Alderson connector to break the pack into two
A Alderson Connector to connect the pack to the car
An old forklift connector with a pull rope to an "OH SHIT" handle in the car
A contactor on the negative line
A contactor on the positive line

In normal use the contactors do all the work - but switch on and off on zero load
For servicing the Alderson connectors make the system safe
In emergency pull the OH SHIT handle

I would use something like the switch you mention with a long plastic extension so the switch would be under the bonnet and the actuation end in the cab


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## wessss77 (Jan 4, 2013)

Oh crap....I just re-read that thread again and bounced around and looked at the schematic again and a lightbulb finally went off and I got the big picture!! Wow...not that I am dense but sometimes...I couldn't figure out how the contactor was getting turned on and off by the key and finally...I saw it. So the contactor works similar to a relay but we are activating it with the key switch 12v and that is when the pack voltage begins to go live. But why does the precharge resistor not always carry current as it sits across the contactor


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## Ziggythewiz (May 16, 2010)

wessss77 said:


> So the contactor works similar to a relay
> ...
> But why does the precharge resistor not always carry current as it sits across the contactor


A contactor IS a relay...just a big fat one. If your precharge is not switched, it will always carry current when the contactor is off. When the contactor is on it bypasses the precharge resistor (path of least resistance) and carries all the current.

How much the precharge continues to carry depends on the controller. Some don't pass nearly nothing once the caps are charged. Others have bleeder resistors for safety so the precharge will have to keep passing current to keep the caps nearly full.


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## Duncan (Dec 8, 2008)

My crude way of doing this,

The Alderson connectors are manual - 
The OH SHIT connector is manual -

The 12v system goes through a race/rally type on/off switch

Switching this over energizes the 12v system (lights, indicators..) and the first contactor
and the controller's 12v side

The precharge resister (I am using a halogen bulb) is switched by the precharge switch
When it has done its stuff (filled the capacitors in the controller) the light goes out and I switch on the second contactor
I then switch off the precharge switch

Going through a start up sequence seems to impress the onlookers

When switching off I switch off the second contactor THEN I press the accelerator
This discharges the controller capacitors - the car sort of lurches a bit
Leaving the capacitors charged up leaves a potentially lethal voltage

Then I switch off the race/rally switch


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## TEV (Nov 25, 2011)

Duncan said:


> When switching off I switch off the second contactor THEN I press the accelerator
> This discharges the controller capacitors - the car sort of lurches a bit
> Leaving the capacitors charged up leaves a potentially lethal voltage
> 
> Then I switch off the race/rally switch


What is the point of doing that ? 

Your battery is still there with the same "potentially lethal voltage".

Also it's more better for the capacitors to stay charged all the time, my controller manual it's recommending to keep the controller connected to the battery all the time, unless you put the car in long time storage.

You can do whatever you feel like doing, but maybe you should look into it.


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## Duncan (Dec 8, 2008)

TEV said:


> What is the point of doing that ?
> 
> Your battery is still there with the same "potentially lethal voltage".
> 
> ...


Maybe when the car is finished I will do it that way
But probably not
I intend doing a bit of motorsport - so I expect to be continually "improving" my car

_Your battery is still there with the same "potentially lethal voltage"._

The first thing I do is to pull the battery splitter alderson connector - so the only current source would be the capacitors


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## TEV (Nov 25, 2011)

Duncan said:


> Maybe when the car is finished I will do it that way
> But probably not
> I intend doing a bit of motorsport - so I expect to be continually "improving" my car
> 
> ...


Probably you are doing that before you work on your car which is a very good and recommended procedure, from your first post it was like you are doing that every time you stop the car (like stopping at 3 stores for shopping and discharging the capacitors each time).


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## Tesseract (Sep 27, 2008)

TEV said:


> ...
> Also it's more better for the capacitors to stay charged all the time....


That is only true for aluminum electrolytic capacitors; for any other kind it doesn't matter whether you leave them charged or discharged.

More specifically, the leakage current through an "elko" increases the longer it is left discharged, and this current, if abnormally high, can result in the capacitor overheating internally. The cure for this is to bring the voltage up slowly on any caps that have been stored/unused for an extended period of time (weeks? months?), as this allows the dielectric oxide layer to reform gradually and limits heating from leakage current to a rate that the capacitor can withstand.


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## Duncan (Dec 8, 2008)

Tesseract said:


> That is only true for aluminum electrolytic capacitors; for any other kind it doesn't matter whether you leave them charged or discharged.
> 
> More specifically, the leakage current through an "elko" increases the longer it is left discharged, and this current, if abnormally high, can result in the capacitor overheating internally. The cure for this is to bring the voltage up slowly on any caps that have been stored/unused for an extended period of time (weeks? months?), as this allows the dielectric oxide layer to reform gradually and limits heating from leakage current to a rate that the capacitor can withstand.


Hi Tesseract

_The cure for this is to bring the voltage up slowly on any caps that have been stored/unused for an extended period of time_

What do you mean by slowly? - seconds - minutes - hours - longer?


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## TheSGC (Nov 15, 2007)

Duncan said:


> Hi Tesseract
> 
> _The cure for this is to bring the voltage up slowly on any caps that have been stored/unused for an extended period of time_
> 
> What do you mean by slowly? - seconds - minutes - hours - longer?


I've been working on some old school Solectria inverters that haven't been fired it in 5 or more years and one took over 3 hours to complete the precharge. They expected to always be connected to the battery pack and always charged. The manual says it takes 15 minutes PER month the inverter was unplugged from the battery pack.


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## poprock (Apr 29, 2010)

TheSGC said:


> I've . The manual says it takes 15 minutes PER month the inverter was unplugged from the battery pack.


So for a 36v 80's type cap pack left for 2yrs ; how long would you suggest? edit. How is residual voltage checked?


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## Tesseract (Sep 27, 2008)

Duncan said:


> Hi Tesseract
> 
> _The cure for this is to bring the voltage up slowly on any caps that have been stored/unused for an extended period of time_
> 
> What do you mean by slowly? - seconds - minutes - hours - longer?


The time to reform the dielectric oxide depends on how long it has been stored and on the ambient temperature, so I can't really be more specific. The Azure-Dynamics suggestion to bring the voltage up to maximum at the rate of 15 minutes per month of storage is probably overly conservative.

Of course, you need the datasheet for the capacitor to know how much leakage current is acceptable in the first place, but absent that information a good rule of thumb is that it should not exceed I = 0.01CV after rated voltage has been applied for 5 minutes. Ie - the leakage current through a 10,000uF cap rated for 450V should not exceed 45mA.

The way I evaluate a suspect elko is to first measure its capacitance and ESR with a meter specifically designed for that purpose (ie - an RCL bridge or my trusty "Peak" ESR meter). If the ESR seems high for the CV rating of the capacitor (generally more than 0.05 ohms for an elko rated more than 100uF/200V) then I don't bother measuring its leakage or reforming the dielectric, as it is "dried out" and beyond salvaging. Remember above all else that elkos have a finite life, and though that life can be measured in the tens of years if they are of high quality and treated well, every last one of them will still go bad at some point.

Assuming the ESR is reasonable then I will use a variable voltage power supply to bring the capacitor up to around 80% of its working voltage rating over the course of several seconds while monitoring the current flow through it with a DMM. If the leakage current is stable/reasonable after 30 seconds I'll continue ramping up the voltage to the full WV rating (not the surge rating). If the leakage current is modestly higher than 0.01CV then (say, up to twice that, or 0.02CV) then I will leave it at that voltage and recheck it in about an hour. If it has fallen below 0.01CV by that time then I know that the dielectric is reforming and will likely leave it at that voltage - conditions permitting - until there is little difference in leakage current from one hour to the next.

Now, all that being said, I should note that I rarely bother with doing anything besides the quick ESR measurement because the consequences of a capacitor failure far outweigh the replacement cost - if an elko is more than 10 years old I strongly suggest replacing it, especially if it has been in service all that time.



poprock1 said:


> So for a 36v 80's type cap pack left for 2yrs ; how long would you suggest? edit. How is residual voltage checked?


You don't check "residual voltage"... Instead, you measure the amount of DC current flowing through the capacitor when it has been sitting at a constant voltage for several minutes. 

Personally, if your elkos are sporting leg-warmers or parachute pants then you might want to consider relegating them to the dustbin of history... And if you think that replacing these capacitors will be costly then consider how much more it would cost to also replace any semiconductors down the line from them...


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## poprock (Apr 29, 2010)

_Personally, if your elkos are sporting leg-warmers or parachute pants then you might want to consider relegating them to the dustbin of history..._ 


Point taken. They did belong to M C Hammer


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