Battery Isolation Contactors

Technical discussion on converting internal combustion to electric
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francisco.shi
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Battery Isolation Contactors

Post by francisco.shi » Tue, 22 Jan 2019, 12:01

I am trying to find out what I need to have in terms of battery isolation and pre-charge contactors and fuses.

The list of loads are as follows:
Two motors (two separate feeds, one for front motor and one for the back motor)
Accessories (12v charger, air conditioning compressor ...)
DC fast charger connection.

As far as I understand I will need a separate fuse and contactor for each connection.
Motor output: 500A contactor with 500A fuse (the maximum motor current is 350A)
Accessories: 25A contactor with 25A fuse. (10kw output to accessories which could include a 240v AC inverter for running mains appliances)
DC fast charger: 250A contactor with 400A fuse. (to allow for 150kw DC charger) Battery is 400v 189Ah. So I am assuming I will be able to charge at 2C until about 80% SOC

So the questions are:
1) Do I need to have a contactor for the positive and negative terminals for every load?
2) Do I need to have a fuse for the positive and negative terminals for every load?
3) The battery will be completely encase in a single water tight box. I am assuming (by reading the DOT rules on conversion that when the ignition key is turned off the traction battery must be isolated by the contactors (which will be inside the battery pack). This means that I could not run the aircon with out the keys being in the ignition? or charge the battery via the DC charge port without the keys being in the ignition?

I am of the oppinion that running a 1000A 400v cable out of the battery case to some kind of emergency stop switch inside the cabin and then back again will reduce the safety of the whole system worse since you will have a live cable coming out of the battery case.

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brendon_m
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Re: Battery Isolation Contactors

Post by brendon_m » Tue, 22 Jan 2019, 12:43

From ncop14
It must not be possible to remove this key in any position that energises the drive train or makes
active driving possible.

I read that as the key only has to deactivate the motor, not necessarily high voltage to aircon etc
It also means as a minimum you only need 1 contactor. Add more for safety but you only need 1

You only "need" a fuse on the motor circuit
A battery pack over-current protection device (e.g. fuse or overload relay) must be installed in
the traction supply circuit. It should be located with the minimum practical length of cable
between the battery terminals and the device, to minimize the chance of a fault occurring in the
unprotected section of cable.
Due to the presence of explosive gases, the fuse or other device for a Class B battery pack must
be located outside of the battery enclosure unless EX (explosion proof) rated. The fuse or other
device for a Class A battery pack should be located inside the battery enclosure to help minimise
the exposed length of cable.
So more fuses are a good idea from a design point of view but you only need 1 and it goes in the battery pack
Also most high voltage/amperage fuses take a while to blow so you probably want a lower rated fuse.
For example my car has a 350a fuse and I pull up to 500a when I'm really pushing it. Doesn't blow the fuse because it's not for long enough. There are tables/charts to calculate what size you need. Bussman fuses are common for the application and I'm pretty sure it's on their data sheet... I read it somewhere...

francisco.shi
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Re: Battery Isolation Contactors

Post by francisco.shi » Tue, 22 Jan 2019, 12:53

I will need at least one contactor per motor so I can do the precharge of each inverter separately.
Also both motors together can draw over 500A which is the maximum rating of the contactors that I can get cheaply.
The 1000A version is more than double the price of the 500A.
And will also need a contactor to isolate the accessories (air con ...)

So I assume one contactor will be enough on each supply.

How about the connection to the DC fast charger (external like Chademo or the like)?

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brendon_m
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Re: Battery Isolation Contactors

Post by brendon_m » Tue, 22 Jan 2019, 13:01

The rules don't specifically cover it. Basically as long as you can stick your finger in it and not get electrocuted you are all good.
So contactors to isolate the pins until something is plugged in.

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jonescg
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Re: Battery Isolation Contactors

Post by jonescg » Tue, 22 Jan 2019, 13:07

It is possible to have the aircon running when there's nobody in the car, but it will drain the auxiliary battery in short order. So you need to either have the DC/DC converter permanently hooked up the HV battery or have a master contactor which supplies it with HVDC when commanded to.

One fuse and contactor on each of the inverter supplies is good. The onboard charger will need its own fuse too. I highly recommend a HVDC distribution box as the starting point for these circuits. You can set up a busbar with fuses and power stuff from there. Make sure it's big enough, as it's going to have contactors, fuses, and possibly even a pre-charge circuit or two.
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Re: Battery Isolation Contactors

Post by coulomb » Tue, 22 Jan 2019, 21:46

francisco.shi wrote:
Tue, 22 Jan 2019, 12:01
Motor output: 500A contactor with 500A fuse (the maximum motor current is 350A)
You don't want a 500 A fuse if the motor only ever draws 350 A. Use a 100 or 125 A fuse of the appropriate speed characteristics, so you never come close to blowing the fuse with any sort of normal use. Weber's MX-5 has 80 A fuses, which allows 100 seconds at 240 ADC; software limits this to 10 seconds. Battery, fuse and contactor layout.
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