# Why do you need 2 contactors



## kevinjclancy (Sep 25, 2017)

Can anyone explain the logic or better yet the science behind the use of 2 contactors - see attached wiring plan - I see that all of the wiring digrams that you can download from ev build suppliers and from controller manufacturers, show one contactor. However I have learned that manufacturers of cars such as the Leaf, Imiev and Tesla all use 2 or more contractors - Also it is required by law if I do a conversion to EV here in New Zealand to have 2 (one for neg other for pos) 

https://1drv.ms/b/s!AmbWDB9AS-tJhZgbaX2RmuJFHXmUFA


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## Duncan (Dec 8, 2008)

Use two contactors

Contactors can and do hang up - especially if they are breaking current 
By having two contactors one of them is always used with no current and that massively reduces the chance that a contactor will hang up and fail to open

You only need one braking system - but all cars since the 80's have had dual braking circuits


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## Gigawatts (Aug 8, 2013)

Redundancy = safety


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## Immo1282 (Jul 12, 2019)

2 Contactors is good for redundancy as other have said. I'll be doing something very similar in my build with a split pack front and back so a fuse for the front pack is essential, and two contactors is a good idea.

Why include a separate voltage regulator? The DC-DC Converter should output a stable well-regulated output by itself if you get a good one!


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## kevinjclancy (Sep 25, 2017)

perhaps you are right about the voltage regulator - I have the old one from when it was an ICE car. Do you know how it is the DC to DC converter does not overcharge the 12 volt lead acid battery ???


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## major (Apr 4, 2008)

Often two contactors are used to facilitate pre-charge circuitry. BTW, your diagram doesn't show any such pre-charge. You need to study the controller recommendation for battery and contactors. Also that of the charger and DC converter. I think this forum's wiki has some info. Certainly there have been more than a few threads on the subject.

Regards,

major


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## kevinjclancy (Sep 25, 2017)

As for the pre-charge my wire diagram is not at all complete - I just drew it up to show the 2 contactors - but I do appreciate the input - I am doing my build at a snails pace and getting all of the advice and info I can


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## Immo1282 (Jul 12, 2019)

kevinjclancy said:


> Do you know how it is the DC to DC converter does not overcharge the 12 volt lead acid battery ???


Same as how a properly working ICE car alternator outputting 14V will not overcharge a battery. Set the output of the DC-DC to a safe float voltage (Something like 2.25V-2.27V/cell, so 13.5 ish), and the DC-DC output current will decrease to almost nothing as the battery voltage rises to match it.


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## kevinjclancy (Sep 25, 2017)

sounds like great advise but I really don't think my DC to Dc converter has any sort of settings - it just puts out 13.5 volts - I suppose if it come down to it I might have to purchase a different DC to DC converter? - I think I need to talk to a well qualified auto-electrician maybe they can help???


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## Immo1282 (Jul 12, 2019)

13.5V is fine - there should be no issues just hooking that up as is to a 12V Lead acid battery. 

If it's outputting 13.5V by default and there is not any adjustment screw etc. then it's most likely been designed to float charge a 12V battery as its output is right for the job.


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## kevinjclancy (Sep 25, 2017)

Sounds like a plan that might just work - I was planning on using a "sealed" lead acid battery., I don't know if that makes any difference but I read somewhere that a sealed battery becomes a bit more problematic if it gets overcharged - at any rate I will cross this bridge when I come to it and in the mean time try to get the best advice.


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## Immo1282 (Jul 12, 2019)

My previous job was working for a company that designed and sold Lead acid battery chargers into industry - Our chargers were configured to float charge all Lead Acid types (flooded, AGM VRLA) at 2.25v/cell @ 20C with appropriate temperature compensation. New Zealand has a very mild climate, so you won't run into issues with temp comp anyway.

"Sealed" batteries don't like to be overcharged as the electrolyte will release more gas than can be recombined - and eventually dry out. You won't be overcharging it with a 13.5V DC supply that's only switched on when the car's running! It's a much gentler scenario than a normal car alternator, so it's well worth focusing your concerns onto other areas of your project - this bit's dead easy 

Have you got plans as to what you're doing for a battery management system? If not, look for a solution that can be implemented with the split pack - My project is the same way with around 1/3 of the pack over the front axle, and the other 2/3 behind the passengers, so I'm looking to do a similar implementation as and when I get to that point...


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## kevinjclancy (Sep 25, 2017)

I have a very simple low cost battery management system - it is 2 separate devices - I have a balancing system that actively balances the batteries - it has an app that you log into on a smartphone - it shows total voltage of the pack and voltage for each battery - I then have a Mypin coulometer ZW9E-RN2F 
it is a 100 mm cube that sits on the dash board - it will shut off the charger at what ever KWH or Voltage for the pack - its totally adjustable - I have a full pack of 18.5 KWH i will charge it up to 16 90% of the time and only wind it up to the max if I need to max out my range


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