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Any way to safely test if an AC-DC converter can accept DC input?
Posted by FyKnight on 25 Oct, 2021 14:05 -
Hi, I'm new, please correct me gently if I do something wrong. I did look to see if this question has been asked before but didn't find anything. Also I'm thrilled to find this forum!
I have a 7 kW AC-DC converter that I would like to feed DC directly to instead of AC. Yes, it's the OnBoard Charger in my EV. I'll need to charge it at an off-grid location, where I'll build a dedicated system just for this purpose. It seems silly to have a big inverter there if it can be avoided.
It could well be possible, right? If I keep to half the max current, then even though only one pair of diodes in the rectifier will be used, they shouldn't waste more power than under AC at max current. By the time we get to a capacitor it'll be no different to AC. The big question mark for me though is what will the PFC boost converter algorithm do?
So before I go any further with this thinking I'd like to test it. Is there any way I can do so without a large risk of damage to the OBC? All I can think of is to use the minimum voltage the OBC accepts (85 Vrms AC, so I guess 120 V DC?), at the lowest current (6 A, i.e. getting the EVSE to tell it to only draw 6 A) and try it for just a few seconds.
I'd also be really keen to hear if someone has done this before or even used a modified sine wave inverter to charge their EV (since that is nothing like a sine wave at all)
Thanks! -
Hard to see how it would damage it, shouldn't be too hard to use a low current DC supply to test off-load.
Might be useful to take a look at the circuitry to see if there any obvious potential issues.
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Perhaps just use the DC charge mode if there is one? It's often called "fast charging" but it most likely will also work at somewhat slower rates. https://openinverter.org/forum/viewforum.php?f=17
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Thanks very much for your replies!
Hard to see how it would damage it, shouldn't be too hard to use a low current DC supply to test off-load.
OK interesting thank you! Will using a low current DC supply get much data? The OBC minimum configurable draw is 6 A, so it would just overload a lesser supply very quickly wouldn't it?
Might be useful to take a look at the circuitry to see if there any obvious potential issues.
For the circuitry, would a photo on its own be any use or would I need to go and reverse engineer the actual circuit? I can probably manage to do that but if it isn't a massive risk perhaps I'll just try it and see.Perhaps just use the DC charge mode if there is one? It's often called "fast charging" but it most likely will also work at somewhat slower rates. https://openinverter.org/forum/viewforum.php?f=17
There is indeed a CCS fast charge mode in my EV, and that was my initial plan. I couldn't find any EVSEs under AU$50K that support DC charging on the CCS pins — they are all designed for fast charging. So I have to build one myself.
The first problem with this mode is that unlike AC charging which is controlled by a simple 1 kHz PWM signal, the DC charging uses a nightmare combination starting with powerline communications protocols over the same PWM control line and getting worse from there: (ROBO) OFDM, Ethernet, "Sounding packets", IP, TCP, TLS, HTTP and the cherry on top: XML! It's disgusting, and you need a beefy microcontroller to run it, but there are modules available and I am trying to order one (fingers crossed *).
The bigger hurdle with this approach though is that the battery is directly connected to the DC pins in the socket, and it's up to the EVSE to generate the voltage required to charge it, so that requires a DC-DC converter of similar complexity to the inverter that I'm trying to eliminate. It's still more efficient overall so worth doing, and I have a couple of these on order too.
So yeah, if I can reuse the hardware that I'm already carrying around, that would be cheapest, easiest and likely most efficient solution. I wish I'd realised this before spending so long going down the HVDC and CCS rabbit holes. Also, thanks very much for the openinverter link! Heaps of great info on that forum!
*: Probably easier than doing CCS is to just put a T junction on the HV lines from the battery and drive the contactors directly... if only the connectors didn't cost $200+ -
Perhaps just use the DC charge mode if there is one? It's often called "fast charging" but it most likely will also work at somewhat slower rates. https://openinverter.org/forum/viewforum.php?f=17
The problem with that is that you need to do the current regulation externally - the CCS port is just a pair of contactors away from the battery terminals -
Thanks very much for your replies!
The 6A is just what the EVSE advertises, you could use a dummy load to draw less for testingHard to see how it would damage it, shouldn't be too hard to use a low current DC supply to test off-load.
OK interesting thank you! Will using a low current DC supply get much data? The OBC minimum configurable draw is 6 A, so it would just overload a lesser supply very quickly wouldn't it?
Might be useful to take a look at the circuitry to see if there any obvious potential issues.QuoteFor the circuitry, would a photo on its own be any use or would I need to go and reverse engineer the actual circuit? I can probably manage to do that but if it isn't a massive risk perhaps I'll just try it and see.
You may or may not see anything conclusive, but if there is, for example an AC current transformer, or (fairly unlikely) a conventional transformer to generate an aux supply that would indicate possible issues.
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As I understand it, it's what the EVSE advertises, but it's the OBC that decides how much to pull. And it's the OBC that I'm trying to test. (According to the spec it has 5 sec to adjust its current to what the pilot signal says after a change). I mean the load (the battery) can draw hundreds (thousands?) of amps if it wanted. There's probably something I don't understand here, so please explain it to me if I'm wrong! Ah wait, should I be disconnecting it and replacing the battery with something of higher resistance? I'd be worried that would be bad for the OBC.OK interesting thank you! Will using a low current DC supply get much data? The OBC minimum configurable draw is 6 A, so it would just overload a lesser supply very quickly wouldn't it?
The 6A is just what the EVSE advertises, you could use a dummy load to draw less for testing
Hang on though, this has given me an idea, which might be what you meant all along. If I charge the battery to almost full then the OBC internally ramps down and draws less and less. I don't know if it ever gets under 6 A but maybe it could! Yeah that'd be a good and safer way to test it definitely! Sweet. And bonus, if I break it, at least I'll have a full charge to drive somewhere for a fix
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Ohh yeah good point! Given the size and power of the OBC I can pretty much rule out a main path transformer, but an auxiliary might be possible hmmm. I guess I'd better open it up and have a squizz! Thanks again.QuoteFor the circuitry, would a photo on its own be any use or would I need to go and reverse engineer the actual circuit? I can probably manage to do that but if it isn't a massive risk perhaps I'll just try it and see.
You may or may not see anything conclusive, but if there is, for example an AC current transformer, or (fairly unlikely) a conventional transformer to generate an aux supply that would indicate possible issues. -
An AC current transformer seems likely to me, the OBC has to have some way to measure input current and this would be a normal way to do it. Even if it used Hall sensors for current measurement the software might be configured to reject any DC component. If it can't measure input current it may throw an error and shutdown, or keep trying to draw more power (though you should be able to prevent damage easily enough).
Personally I would probably look at either the lowest-cost way to supply it with "good enough" AC (e.g. square wave), or move the the CCS port.
Some cars also do other tests on the AC port, e.g. a Renault Zoe measures the impedance of the line-earth loop and refuses to charge if it is over 100R.
For the CCS port you'd need at least enough communication to persuade the car to close its contactor, but perhaps as you know the model of the car and its battery type you don't actually need the car to tell you what it needs like a general purpose charging point would do? I am thinking that once the contactor is closed you could get away with a current limited voltage source of manually configured values, and that it would work to arbitrarily low charging power. -
For the CCS port you'd need at least enough communication to persuade the car to close its contactor, but perhaps as you know the model of the car and its battery type you don't actually need the car to tell you what it needs like a general purpose charging point would do? I am thinking that once the contactor is closed you could get away with a current limited voltage source of manually configured values, and that it would work to arbitrarily low charging power.
The problem with CCS is the car will think it's connected to a charger, and will probably expect the current supplied to match what it has asked for. Hard to know how it will behave in practice, and likely to vary between different cars.
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Sorry I was getting confused between the OBC and the EVSE!
As I understand it, it's what the EVSE advertises, but it's the OBC that decides how much to pull. And it's the OBC that I'm trying to test. (According to the spec it has 5 sec to adjust its current to what the pilot signal says after a change). I mean the load (the battery) can draw hundreds (thousands?) of amps if it wanted. There's probably something I don't understand here, so please explain it to me if I'm wrong! Ah wait, should I be disconnecting it and replacing the battery with something of higher resistance? I'd be worried that would be bad for the OBC.OK interesting thank you! Will using a low current DC supply get much data? The OBC minimum configurable draw is 6 A, so it would just overload a lesser supply very quickly wouldn't it?
The 6A is just what the EVSE advertises, you could use a dummy load to draw less for testingQuoteHang on though, this has given me an idea, which might be what you meant all along. If I charge the battery to almost full then the OBC internally ramps down and draws less and less. I don't know if it ever gets under 6 A but maybe it could! Yeah that'd be a good and safer way to test it definitely! Sweet. And bonus, if I break it, at least I'll have a full charge to drive somewhere for a fix
yes, that would be a way to limit the output current. Some cars also have a user-settable way to limit AC charge current
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I was thinking about this, but there has to be a method by which the car gets less than it has asked for and is happy, it is common for chargers to be rated at less than the car, or to wind down if charging several cars. The question is if the charger has to tell the car it's giving it less, or if the request from the car is just treated as a maximum.For the CCS port you'd need at least enough communication to persuade the car to close its contactor, but perhaps as you know the model of the car and its battery type you don't actually need the car to tell you what it needs like a general purpose charging point would do? I am thinking that once the contactor is closed you could get away with a current limited voltage source of manually configured values, and that it would work to arbitrarily low charging power.
The problem with CCS is the car will think it's connected to a charger, and will probably expect the current supplied to match what it has asked for. Hard to know how it will behave in practice, and likely to vary between different cars.
Plan C of course is a direct battery connection not via CCS.
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Personally I would probably look at either the lowest-cost way to supply it with "good enough" AC (e.g. square wave)...
Thinking about this a bit more, what DC source do you actually have available? There might be something straightforward you can do.
Also, note that whilst the lowest charging current in the standard is 6A, cars are not required to support it. e.g. the Zoe minimum current is 10A (was 16 A on early models prior to a software update). -
I have some experience I can share on EV charging systems.
As others have mentioned, there is a good chance your EV's OBC has safety and monitoring systems which assume AC input to the AC charging ports such as an AC current transformer and potentially an auxiliary transformer. Best, non-working case: the vehicle just refuses charge, possibly enters a locked-up fail state. Worst case: there is some auxiliary transformer which burns up and destroys your OBC when you try feeding in DC. I don't think there is a safe way to feed in a DC and validate function without running into issues. OBC systems have contactors which should disconnect all power circuitry until AC charging protocols have been completed. Once pre-charge communication and checks are done, the EV will close contactors to the OBC then it will (usually) do a "soft start" but the speed and power of this "soft start" would not give you a safe window to determine if there is a low auxiliary transformer burning up or not.
The only "safe" method I can think of would be opening up and reverse engineering the OBC. Can you share what model EV you have? or are you trying to make a general charging system?
As for suggestions on DC fast charging, as Mike mentioned, you need a current limiting DC-DC converter. DC fast charging connects the EV's battery directly to the fast charging port. Current is up to the EVSE to control. All the EV can do is try open its internal contactors or blow a fuse if current control is incorrect/not present. Furthermore, AC charging protocols are completely different to CCS (and CHAdeMO) DC charging protocols which are non-trivial to implement (you thought USB PD was a pain? HA!). CCS in particular requires a homeplug green PHY modem IC for communication which is not something you can just get. IC are all behind NDAs and dev agreements AFAIK. There are CCS (and some CHAdeMO) EVSE communication modules available off-the-shelf which mean you don't have to direct dev work on the communication subsystem but the dev kits aren't particularly cheap, also behind NDAs and they aren't "just works" out of the box either (and unlikely they'd sell to hobbyists too). They need to be integrated with control systems for the DC-DC converter and other safety and monitoring systems.
Again with any DC charging, you'll need a current regulating DC-DC converter of sufficient power capability. You're unlikely to find anything off the shelf for cheaper than an equivalent power inverter. You then also get the benefits over being able to run other mains appliances off the inverter.
My suggestion: just use an inverter, you can probably get away with a "modified sine" type.
Edit: Also definitely would not recommend bypassing safety functions and trying to connect directly to the EV battery obviously.
Edit2: If the OBC does have a DC incompatible device, trying to replace/eliminate that would be another option that would be much cheaper as well as significantly easier than implementing a DC charging system (that doesn't blow up and is safe).
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From what I've seen, the EV disconnects and stops charging (at something like a few hundred watts) before power reaches something low enough that I'd feel safe not burning up an auxiliary transformer (5W or less). No harm in trying and seeing what your EV does though.QuoteHang on though, this has given me an idea, which might be what you meant all along. If I charge the battery to almost full then the OBC internally ramps down and draws less and less. I don't know if it ever gets under 6 A but maybe it could! Yeah that'd be a good and safer way to test it definitely! Sweet. And bonus, if I break it, at least I'll have a full charge to drive somewhere for a fix
yes, that would be a way to limit the output current. Some cars also have a user-settable way to limit AC charge current -
All the EV can do is try open its internal contactors or blow a fuse if current control is incorrect/not present.
..that would be the ~600 amp pack fuse..!
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Hence why I warn against it! A DC-DC current regulating converter with properly implemented safety systems is a MUST!All the EV can do is try open its internal contactors or blow a fuse if current control is incorrect/not present.
..that would be the ~600 amp pack fuse..! -
An AC current transformer seems likely to me, the OBC has to have some way to measure input current and this would be a normal way to do it. Even if it used Hall sensors for current measurement the software might be configured to reject any DC component. If it can't measure input current it may throw an error and shutdown, or keep trying to draw more power (though you should be able to prevent damage easily enough).
It could just as well measure current later in the path, which it needs to do anyway to not charge the battery faster than it wants to. I've noticed that the car's measurement is slightly below what the EVSE says too.QuotePersonally I would probably look at either the lowest-cost way to supply it with "good enough" AC (e.g. square wave), or move the the CCS port.
Square wave is something I've considered. It'd just require a few FETs (and their drivers probably) plus a simple microcontroller (or even the trusty old 555?). That'll be safe enough for any transformers / coils then, but I don't know what the PFC algo would do with such a waveform.QuoteSome cars also do other tests on the AC port, e.g. a Renault Zoe measures the impedance of the line-earth loop and refuses to charge if it is over 100R.
Interesting, I don't think my car has gone overboard with the tests though, it seems to be pretty permissive compared to others...
QuoteFor the CCS port you'd need at least enough communication to persuade the car to close its contactor, but perhaps as you know the model of the car and its battery type you don't actually need the car to tell you what it needs like a general purpose charging point would do? I am thinking that once the contactor is closed you could get away with a current limited voltage source of manually configured values, and that it would work to arbitrarily low charging power.
The needs of the car vary over time (depending on SoC) and since I'm going to all this effort, I may as well let others in the same situation benefit like me. That said my charging power will definitely be under 10 kW, way lower than any fast DC charger. -
If you're really still interested in CCS, you can get some documentation/information packs from CharIN (the association in charge of CCS) on request for free https://www.charin.global/ The info pack has pretty complete information on the CCS protocol and processes includes some detailed slides on the charging process as well as safe guards. Not quite 100% of what you need for a from scratch implementation due to needing some cited standards but pretty good none-the-less.
Edit: If you look for "Design_Guide_Combined_Charging_System" there's an older version floating around on the internet but the contents looks the same as the version I have. Both are dated 2015-06-02. -
Edit: Also definitely would not recommend bypassing safety functions and trying to connect directly to the EV battery obviously.
Well, creating a battery connection with equivalent safety to the CSS connector but without the godawful communications protocol doesn't feel like a hugely difficult task. Yes, it requires some technical knowledge, but no more so than anything else involving DC charging. One could certainly imagine adding a moderate-current-rated connector* to the battery circuit with interlocking, fusing, etc. The remaining question being if charging without the car knowing about it is likely to upset it or the BMS.
*Something from the IEC 60309 family with a pilot contact perhaps? Or a type 2 in the uncommonly used but standardised DC mode? -
Certainly not impossible but EV batteries aren't something to be taken lightly. "If you need to ask then you shouldn't be trying" situation. More so when the impetus seems to be avoiding buying a relatively cheap inverter which would be much safer, simpler and almost guaranteed to work.Edit: Also definitely would not recommend bypassing safety functions and trying to connect directly to the EV battery obviously.
Well, creating a battery connection with equivalent safety to the CSS connector but without the godawful communications protocol doesn't feel like a hugely difficult task. Yes, it requires some technical knowledge, but no more so than anything else involving DC charging. One could certainly imagine adding a moderate-current-rated connector* to the battery circuit with interlocking, fusing, etc. The remaining question being if charging without the car knowing about it is likely to upset it or the BMS.
*Something from the IEC 60309 family with a pilot contact perhaps? Or a type 2 in the uncommonly used but standardised DC mode?
Edit: Pre-charge, pre-charge insulation checking, insulation monitoring, connector temperature monitoring and contactor welding checks would be a non-exhaustive list of safety features not mentioned. The standard IEC 61851.23:2021 Annex CC describes CCS as a 18 step process for normal startup to shut down, that's with all the communication phases lumped into single steps.
Edit2: Communication integrity checking in another one. Done in CCS as the Signal Level Attenuation Characterization (SLAC) protocol. -
Yes, hopefully it's a maximum, when I use a 50 kW fast charger my car asks for 86 kW but it doesn't get it. There is a minimum field though. I don't know what my car will do until I try it. Of course this is the same port used for V2G, i.e. negative charging currentThe problem with CCS is the car will think it's connected to a charger, and will probably expect the current supplied to match what it has asked for. Hard to know how it will behave in practice, and likely to vary between different cars.
I was thinking about this, but there has to be a method by which the car gets less than it has asked for and is happy, it is common for chargers to be rated at less than the car, or to wind down if charging several cars. The question is if the charger has to tell the car it's giving it less, or if the request from the car is just treated as a maximum. so I hope it will be okay. Here is an extract from the parsed relevant EXI request (I believe) in OpenV2G:Code: [Select]struct iso2DC_EVChargeParameterType {
uint32_t DepartureTime ;
struct iso2PhysicalValueType EVMaximumChargePower ;
struct iso2PhysicalValueType EVMinimumChargePower ;
struct iso2PhysicalValueType EVMaximumChargeCurrent ;
struct iso2PhysicalValueType EVMinimumChargeCurrent ;
struct iso2PhysicalValueType EVMaximumVoltage ;
struct iso2PhysicalValueType EVTargetEnergyRequest ;
struct iso2PhysicalValueType EVMaximumEnergyRequest ;
struct iso2PhysicalValueType EVMinimumEnergyRequest ;
int8_t CurrentSOC ;
int8_t TargetSOC ;
int8_t BulkSOC ;
};QuotePlan C of course is a direct battery connection not via CCS.
That was Plan A! But the connectors are soooo expensive, and I would still need to drive the contactors myself. So I may as well do it nicely. Also it'll be useful to more people if it uses an existing port. -
My car is an MG ZS EV and it does support 6A, I've already tested it. The stock granny charger that came with it is only 8A too!Personally I would probably look at either the lowest-cost way to supply it with "good enough" AC (e.g. square wave)...
Thinking about this a bit more, what DC source do you actually have available? There might be something straightforward you can do.
Also, note that whilst the lowest charging current in the standard is 6A, cars are not required to support it. e.g. the Zoe minimum current is 10A (was 16 A on early models prior to a software update).
The DC sources are solar or battery / supercap bank. The battery will be much smaller than the car traction battery of course.
So the big underlying idea if it can take DC or square wave is to connect the solar (of sufficient voltage and current, which effectively means ~1 kW) directly to the AC port (through an EVSE naturally), and use the car's OBC as the coarse MPPT by varying the current. Yes, if a cloud goes over and it drops out then the charging will stop. Buffering with capacitors or battery would be expensive and ruin the MPPT idea. OK you could have a battery on the side that is switched in but now we have a complicated system again. -
Maybe you could contact the maker of your car?
The reason for electrical cars are that they are environmentally friendly.
Charging them off-grid with solar, wind etc. is even more friendly, so it
should be in their interest to help you with this.
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I have some experience I can share on EV charging systems.
Awesome, thank you very much for your insight! Yeah that matches my understanding of the process and is exactly what I'm afraid of. Do you think it would be safe enough to test with a square wave input? I have even thought of having the lid to the OBC open and watching it through an IR camera
As others have mentioned, there is a good chance your EV's OBC has safety and monitoring systems which assume AC input to the AC charging ports such as an AC current transformer and potentially an auxiliary transformer. Best, non-working case: the vehicle just refuses charge, possibly enters a locked-up fail state. Worst case: there is some auxiliary transformer which burns up and destroys your OBC when you try feeding in DC. I don't think there is a safe way to feed in a DC and validate function without running into issues. OBC systems have contactors which should disconnect all power circuitry until AC charging protocols have been completed. Once pre-charge communication and checks are done, the EV will close contactors to the OBC then it will (usually) do a "soft start" but the speed and power of this "soft start" would not give you a safe window to determine if there is a low auxiliary transformer burning up or not.QuoteThe only "safe" method I can think of would be opening up and reverse engineering the OBC. Can you share what model EV you have? or are you trying to make a general charging system?
I would like it to be more general but if it is only for my model car then oh well. It's the cheapest EV in Australia, I'm sure you know the one: the MG ZS EV.QuoteAs for suggestions on DC fast charging, as Mike mentioned, you need a current limiting DC-DC converter. DC fast charging connects the EV's battery directly to the fast charging port. Current is up to the EVSE to control. All the EV can do is try open its internal contactors or blow a fuse if current control is incorrect/not present. Furthermore, AC charging protocols are completely different to CCS (and CHAdeMO) DC charging protocols which are non-trivial to implement (you thought USB PD was a pain? HA!). CCS in particular requires a homeplug green PHY modem IC for communication which is not something you can just get. IC are all behind NDAs and dev agreements AFAIK. There are CCS (and some CHAdeMO) EVSE communication modules available off-the-shelf which mean you don't have to direct dev work on the communication subsystem but the dev kits aren't particularly cheap, also behind NDAs and they aren't "just works" out of the box either (and unlikely they'd sell to hobbyists too). They need to be integrated with control systems for the DC-DC converter and other safety and monitoring systems.
Thank you for the overview. I've been researching it for months too, though it sounds like you're more familiar with the systems than that. I have two models of such DC-DC converters on the way, and hopefully a HomePlug GP EVSE module too — it's just a matter of buying in sufficient volume. I want to make this happen, and if I have to build out a company and employ EEs then I will ... but not until necessary (I'm a computer engineer, I've studied electrical engineering but have little experience on the power side. I have several failed startups under my belt too haha). However, if I don't need to go down the CCS path then you can bet I'm going to avoid it!!QuoteAgain with any DC charging, you'll need a current regulating DC-DC converter of sufficient power capability. You're unlikely to find anything off the shelf for cheaper than an equivalent power inverter. You then also get the benefits over being able to run other mains appliances off the inverter.
I appreciate the sentiment and would have said the same in general. However this is primarily about efficiency, exploring the options and hopefully making a model system, not so much up-front cost. I already have an inverter (it's huge) and battery system that I can charge with. I want to do it better!QuoteMy suggestion: just use an inverter, you can probably get away with a "modified sine" type.
That's excellent. Modified sine is usually just square wave with a gap at zero between the cycles, as I understand it. If that can work then the square wave might work and we are good to go!QuoteEdit: Also definitely would not recommend bypassing safety functions and trying to connect directly to the EV battery obviously.
I don't want to get anywhere near the 450V believe me. Ohhh interesting, yeah! Hmmm... though that might work for me, it doesn't have the benefit for many other people...
Edit2: If the OBC does have a DC incompatible device, trying to replace/eliminate that would be another option that would be much cheaper as well as significantly easier than implementing a DC charging system (that doesn't blow up and is safe). -
I want to make this happen, and if I have to build out a company and employ EEs then I will ... but not until necessary (I'm a computer engineer, I've studied electrical engineering but have little experience on the power side. I have several failed startups under my belt too haha). However, if I don't need to go down the CCS path then you can bet I'm going to avoid it!!
Maybe you should come join us then
we're based in Sydney/Canberra.