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Charger Precharge Circuit

Posted: Sun, 10 Jul 2016, 18:44
by TooQik
I've put together a simple circuit diagram to allow precharging of two Brusa chargers connected in series as follows:

* I've removed my image because it's just plain wrong *

Anyone see any potential issues with this design?

Thanks in advance.

Charger Precharge Circuit

Posted: Sun, 10 Jul 2016, 19:02
by TooQik
Actually, I've just realised that I've forgotten to put a third contactor on the lower charger between traction pack and charger HV - terminal.   Image

Charger Precharge Circuit

Posted: Sun, 10 Jul 2016, 19:38
by TooQik
Third time's a charm, as they say:

Image

Anyone see any issues with this proposed precharge circuit?

Charger Precharge Circuit

Posted: Sun, 10 Jul 2016, 20:23
by coulomb
Much better than the first one Image

I have an uneasiness about what happens if main charger contactor 1 closes a millisecond before main charger contactor 2. In that millisecond, if the chargers draw different current for whatever reason, then the voltages across the chargers could unbalance, making the connection more straining than it needs be. Similarly, if main charger contactor 2 disconnects first, and now you could be under load, the charger voltages could go a bit wild.

Finally, you're relying on the capacitance of the two chargers being identical (they could easily vary by 20%, perhaps more), and that their startup currents are similar or small compared to the precharge current.

Back when Minitactors were much cheaper than main contactors (such as EV200s and their successors EVC500s), then I'd say avoid all these issues with a second precharge Minitactor. But if it's a fourth EVC500-like contactor, you'd be very tempted to go with what you have drawn above.

Charger Precharge Circuit

Posted: Mon, 11 Jul 2016, 02:11
by Johny
I hadn't seen anyone pre-charge the output caps in chargers before. Usually the chargers are powered up then connected to the Pack when they are ready. This might be a ready signal or a delay.

Charger Precharge Circuit

Posted: Mon, 11 Jul 2016, 04:35
by coulomb
Johny,

An excellent point that I'm quite embarrassed to admit didn't occur to me. I don't know what's in those chargers, but Elcon / TCCH chargers have a relay that isn't connected until there is less than a small voltage exists across it. So there is no need to pre-charge the output capacitors; the charger does that. I'd say that all chargers would have something similar.

Each charger should connect to its half of the pack. I'd strongly suggest that there should be a mid-pack contactor, and that should be open while charging. That way, the chargers don't ever see full pack voltage with respect to earth, even if there is a worst-case leakage to earth. That's how we do it in the MX-5.

Charger Precharge Circuit

Posted: Mon, 11 Jul 2016, 05:01
by TooQik
Thanks for the replies coulomb and Johny.

The reason I'm looking to precharge the Brusa chargers is due to it being a requirement according to their documentation. I'm also trying to build the precharge circuit to fit in with their recommended configuration for series charging as seen here:

http://www.brusa.biz/_files/drive/02_En ... 2xNLG5.gif

It might just be easier to precharge the chargers during the installation phase and not worry about a permanent precharge circuit.

Charger Precharge Circuit

Posted: Mon, 11 Jul 2016, 14:40
by coulomb
That's certainly unusual to pre-charge the outputs. I would also not like to see the charger outputs always connected to the pack. Are you sure the pre-charge is needed at the output? It would make a little more sense if it was needed only at the input, and then only if charging from DC (an unusual case).

Charger Precharge Circuit

Posted: Mon, 11 Jul 2016, 17:31
by TooQik
The Brusa manual definitely states the charger DC outputs needs to be precharged. I can't copy the section unfortunately, but here's a link to the requirements from the Metricmind website:

http://www.metricmind.com/data/mmc_006.pdf

Charger Precharge Circuit

Posted: Mon, 11 Jul 2016, 17:48
by Johny
Fair enough.
In that case I'd implement two pre-charge circuits - one for each charger. The contactors that close the precharge circuits only need to be modest units as they will never open under load and only handle a couple of Amps.

A minor bonus of keeping the circuits separate is to allow for a charger failure at a time that you really need the car but can afford the time to charge, then swap the good charger from one pack half to the other.

Charger Precharge Circuit

Posted: Mon, 11 Jul 2016, 20:55
by Richo
TooQik wrote: The Brusa manual definitely states the charger DC outputs needs to be precharged.
It actually says that you only need to have the precharge if it's not permanently connected to the pack.
If it's permanently connected you should be able to install it like the high F caps used in audio.


http://www.metricmind.com/data/mmc_006.pdf
There is an error in this doc.
It should read NTC not PTC.
Putting in a PTC will blow it. Image

Image It amuses me the whole document is to prevent damage but a little typo will cause it Image

Charger Precharge Circuit

Posted: Tue, 12 Jul 2016, 21:17
by Richo
coulomb wrote:Back when Minitactors were much cheaper than main contactors...


It wont be long before a solid state SiC FET Relay will beat a contactor on price for the 10A region for precharge circuits.


Charger Precharge Circuit

Posted: Tue, 12 Jul 2016, 22:41
by TooQik
I'm kind of in two minds whether to have a permanent precharge circuit for the chargers or not. I don't particularly like the idea of having full voltage at components unless they are being used at the time, but on the other hand the chargers will be located in the same area as the traction battery pack so accessing them would also mean you'd have access to the battery circuits anyway.

I like your idea Johny of having the ability to swap the chargers if needed, so that's a plus one for individual circuits for each charger.

On the other hand no permanent precharge means less components required and slightly less cost.

Charger Precharge Circuit

Posted: Wed, 13 Jul 2016, 21:06
by Richo
Seems a lot of effort for 2 x 6.5uF.
What's the cost of the precharge? $200+
Relay's that wear out, extra weight, extra parts to fail and extra cost.
The charger claims to be galvanically isolated.

I wouldn't bother - there is no benefit.

Manually precharge them once and permanently connect them.

Charger Precharge Circuit

Posted: Thu, 14 Jul 2016, 20:31
by coulomb
Richo wrote: Seems a lot of effort for 2 x 6.5uF.

And the battery is effectively a much larger capacitor than that.

I note that while 13 uF at 12 V stores a tiny amount of energy (assuming that they aren't in series), at 324 V (27x the voltage), it stores 27² = 729 times as much energy. Very low ESR capacitors can cause prodigious currents to flow from EV sized batteries.

I think that they may have a very aggressive battery voltage control loop that could oscillate if there is significant inductance in the cabling to the battery, so they need this capacitance to keep the voltage stable. Other chargers don't seem to need this imposition, which as Richo has pointed out, is not insignificant.

I don't think I'd want to take the chance on completely ignoring this advice, though. Maybe I'd do a manual pre-charge and leave the output across the battery all the time, trusting the Brusa engineers to know what they are doing. I'd say there are many such installations in the world, and it doesn't seem to be an issue.

[ Edit: "are in series" -> "aren't in series" ]

Charger Precharge Circuit

Posted: Thu, 14 Jul 2016, 21:02
by Richo
A 6.5uF Film cap for this application typically would have less than 20mR impedance.
350V/20mR = 17,500A Image
So you can understand how this can damage the output section of the charger.

I believe the point of the document was:
IF you have an ON-board charger (ie plugging in mains to car to input of charger) manually precharge once and then leave charger connected to pack.
IF you have an OFF-board charger (ie plugging in output of charger to car) you will need the precharge circuit.

The crap part is they could have implemented the precharge on the cap line in the charger using an NTC (NOT PTC...) and SiC FET for less than $10.
That's a LOT less than the $200+ required by a user to implement on the external charge line.

20/20 hindsight I guess.
Brusa engineers must have been sleeping that day Image

Charger Precharge Circuit

Posted: Fri, 15 Jul 2016, 00:24
by TooQik
Thanks all.

Guess I'll go the manual precharge route then and save myself some cash, complexity and weight. Image

Re: Charger Precharge Circuit

Posted: Thu, 07 Jun 2018, 00:55
by BurchSung
Hi...i am a new user here. As per my knowledge i would like to say if the chargers draw different current for whatever reason, then the voltages across the chargers could unbalance, making the connection more straining than it needs be. Also, if main charger contactor 2 disconnects first, and now you could be under load, the charger voltages could go a bit wild.

low cost pcb