EV Conversion battery requirements

[Briswolf]

Hello,

I am new here, but have been dreaming of doing an EV conversion for a long time.
I am a professional electronic engineer, but do not have extensive HV experience, but am happy to earn that badge.

I am starting my research with the battery, as I consider it to be the most important part of any EV conversion. Please educate me if I am wrong!

As such, I want to know if there is some kind of list of requirements. I've already found Vehicle standards bulletin 14 which contains NCOP14 Guidelines Electric Drive V2.
As I am planning to use lithium ion cells, they are class A cells and do not require venting or anything like that.
The minimum specifications seem to be

- ingress protection IP2X (against fingers), and also some level of water resistance (presumably IP22)
- fire cannot move from one battery case to the next (I am unsure if this applies to class B batteries only)
- battery restraints must withstand 20g, 15g, 10g, and 10g impacts for front, side, rear, and vertical (rollover) impacts respectively
- an impact sensing switch should (must?) be used so the traction circuit is opened in case of an impact
- labelling and marking requirements (warning labels for chemistries etc, orange wire used for HAZV)
- protection against shock (appropriate covers and ingress protection for HAZV circuits and connections)
- Hazardous voltage isolation (high voltage battery is floating with reference to the chassis and ELV, even during operation?)
- Hazardous voltage disconnect (traction circuit is controlled by a relay powered from ignition switch)
- Overcurrent protection device (fuse or relay) inside the battery enclosure for class A battery

Is NCOP14 a fairly exhaustive list of requirements for the battery pack(s) in an EV conversion?

I saw on another thread
https://forums.aeva.asn.au/viewtopic.ph ... elv#p72377
A discussion asking "Are you wanting to retain the `max ELV between any two points with the power off` criterion as well? That means a contactor every 29 cells"...

From this I can only infer that one possible method of ensuring the HAZV isolation is to break up the battery pack into smaller packs, each isolated from each other, when the car is turned off. Is this an absolute requirement? Are there other methods of ensuring the isolation (eg by simply isolating all drive electronics and using say optical isolators for communications between the drive electronics and the user interface/computer/pedals/instrument cluster etc?)

Is there any essential resource (other than NCOP14) which I should really sink my teeth into before I do any more planning?

I am in QLD, if that makes any difference

Thanks!
- Wayne

[jonescg]

Well, hard to say if it's the most important, but it won't work without it
Welcome to the community BTW

My advice with the battery is to work out your maximum volume available and plan to make it fit within that. You can sort of chip away at the battery as you do other jobs, but you will reach point where some things need to happen in the right order, and that may necessitate putting the battery pack in at that point. As a general guide I'd say get your motor and inverter / drive train sorted as soon as you can, because that will dictate what space and mass budget is left. Ideally the battery will be a single monolithic unit, but we all have to make a compromise at some point - so if you do have to split it up, make sure they are self contained and isolatable. The battery doesn't need to be isolated down to ELV levels, but it must be isolated from fingers at all times. So features like interlocks on the main connectors are a good idea.

Always always always make sure the battery is waterproof. If not waterproof, at least out of the way of direct spray. If you were to drive full speed into a giant puddle, would your battery internals get wet? If the answer is yes, then you need to work on the design so that it can survive it.

There's a few high voltage build on this forum so have a read and see what's involved. It's not that much extra work, indeed the standard you'd build to for a 700 V DC pack should be the same standard you'd build a 100 V pack to.

[Briswolf]

Thanks for the advice. Am I correct that it was you doing the conversion on the CRX? I am still trying to settle on a car, and the CRX is a nice little thing (I was looking at CRX before I saw you had started a conversion), but maybe too small (I have some devout car guys recommending all manner of cars for me to convert, I don’t really consider myself a car guy, but definitely an EV guy!).

How did you go with the conversion? Or, in hindsight, is it madness to convert a CRX, given the size?

[jonescg]

Yeah I have a CRX build thread I started here a few years ago, but I'm still driving it as a petrol car for now. As soon as we buy a used OEM EV I will start work on the conversion.
Is it madness? Well, yeah, converting any car is madness. But a CRX is a good choice because of its light weight and superior aerodynamics. You can't fit a lot of battery in there, but the efficiency should make up for that to some extent. I think 20 kWh is entirely feasible, giving 120-130 km range. I had originally aimed for almost 40 kWh, but I now realise that was perhaps a little too ambitious and needlessly large given we wouldn't go more than 100 km in a day anyway.

[Richo]

From this I can only infer that one possible method of ensuring the HAZV isolation is to break up the battery pack into smaller packs, each isolated from each other, when the car is turned off. Is this an absolute requirement?

No.
Many of the production cars don't.
Some of the hybrids only have one contactor in the middle of the pack which electrically splits it in 2.
The other reason is if you have something like a 700V pack a charger is hard to come by.
Electrically splitting the pack up into smaller segments means you can use of the shelf chargers.
Physically they can still be all in the same box.

So in most cases the breakup of the pack comes down to your overall pack voltage and charger charging voltage.

Personally I would still like to see one contactor on + and one on - so that when the car is off there is no voltage coming out of the battery box.

[Briswolf]

Alright thanks for that advice! Makes sense to me now. Also yes sounds like a good idea so isolate the pack when the car is off.
Just to be sure, is a contactor also the same as a relay, or is there a crucial difference? Obviously the break voltage and current must be sufficient so as to avoid arc over upon opening the relay/contactor?

[Briswolf]

Is it madness? Well, yeah, converting any car is madness. But a CRX is a good choice because of its light weight and superior aerodynamics.

Haha alright! A little madness makes life worth living IMO I hope you get to do that conversion soon. I have been talking to my car friends a lot (I am not traditionally a car guy, but definitely an EV guy) and I am now looking at converting a blown up RX8, since they are rotaries and blown up cars in otherwise good nick are not too rare.
My goals for the conversion have probably changed a little, I am thinking it has to get me 100km without anxiety, and I would also like this thing to be really exciting to drive. I'd like to impress my petrol friends haha. Maybe that is just pride getting in my way. Time will tell! I might start another thread with musings about my motor torque and RPM requirements.

[coulomb]

is a contactor also the same as a relay, or is there a crucial difference?

There is no crucial difference in use, just in construction. A relay has a hinge; a contactor does not.

For EV purposes, contactors are the larger ones, capable of higher currents. We might use a relay for 12 V circuits, usually not much else.

[Richo]

The high voltage parts are usually filled with random inert gasses.
Arcing and all that..

blown up cars in otherwise good nick are not too rare.

Ha ha I'm waiting for a blown up 86