dc-dc converter

[dillond666]

Hi,

I have purchased one of these dc-dc converters.
http://cgi.ebay.co.uk/Isolated-DC-DC-CO ... ltDomain_0
It gets connected to the pack and is enabled by connecting an extra wire to the + side of the pack. This wire doesn't seem to click a relay or anything so I am assuming that the converter stays always powered when switched off........like leaving your telly on standby. I'm not sure I like this idea (it's made in China!) and was thinking of connecting the two + wires together permanently and switching it on using a relay and series precharge resistor from the ignition switch then, after a time delay having the BMS bypass the precharge resistor with another relay. This is because I am assuming the dc-dc will have fairly large capacitors on the pack side and would otherwise wear out the relay.

Any thoughts, am I just being silly?

Derek

[jonathan jewkes]

I see that the output voltage of the converter is 13.8v. This is suitable for "float charging" a 12v battery. Once the 12v battery is charged, the converter should draw very little current from the high voltage input. (You could always check this yourself with an ammeter)
On my EV the DC-DC converter is permanently connected on both high voltage and low voltage sides. Even if left unused for a week, there does not seem to be much drain on the high voltage pack.
Fitting the relay seems to me to be a lot of fuss for little gain.
But if you are really keen to disconnect the converter when the vehicle is not in use, here is another thought. Will you be installing an isolating switch to disconnect the traction pack from the motor controller when the vehicle is not in use? (this is a useful safety feature, and can also serve as a panic emergency disconnect switch). If so, just connect the DC-DC converter to the far side of the isolating switch.
A nice big red button panic disconnect swith can be obtained from http://www.albrightinternational.com/la ... eld=&ID=63

[Night Train]

I would be interested in how you get on with this as I am looking at getting the same one in due course.

[dillond666]

Update on the Dc-Dc converter.

I have just been electrocuted as I leant against the car body while spannering the + terminal on the battery pack . The converter seems to be conducting from the battery pack - terminal to the chassis earth (chassis to + terminal shows 155v). There seems to be enough current there to pop the bulb in my light up screwdriver thingy too .
I will be removing and dismantling the converter to try and find out what's going on, the converter is supposed to be isolated.
I wonder if there is perhaps a mica insulator missing on the transistor tabs in the high voltage side of the converter? Will report anything I find.

Derek

[dillond666]

I have now cracked open the case. The thing is potted, what a pain! The problem was one of the (over long) case
screws had pierced one of the input capacitors. This caused the battery pack negative to short to the chassis. As
the PCB is difficult to remove, the fix is intricate but possible. I'm lucky the capacitor is at the edge of the
board so having pulled off the old one, I should be able to solder on a new one from the top (plated through holes yay!)
and lie the capacitor on its side. A blob of silicone and the job's a good one. I wonder if this had anything to do with
my hairballs input diode getting fried? The converter also sparks a bit when first connected, don't think they fitted a
thermistor inrush limiter thingy (boo hiss).

Derek

[Night Train]

That's not very good.

Fortunately you are able to strip, fault find and fix it, and it was a relatively easy fix. If you couldn't fix it yourself then I guess it would be a costly trip back to China and they probably would cover it if you had taken it apart.

I will think carefully before committing to a Chinese imported product.

[ChrisB]

I will think carefully before committing to a Chinese imported product.

Always the way with the unknown Chinese stuff, best always look at thinking you may need to buy at least two to make sure you a) have one that works and b) so you have spares

ChrisB

[dillond666]

Details of the repair available here.
http://www.youtube.com/watch?v=8sEOQfKpfUk
For those who have too much time on their hands.

[Night Train]

Good video there.

Were all the screws that long? If they were then it is likely that all the DC-DC converters are coming out of the factory like that and it is pot luck if the screw rips into the cap or not.

[dillond666]

Were all the screws that long?

Yes, poor show.

I've fitted the converter again and still get 150v from battery pack + to chassis. Current is <1mA now though. The zilla power section connected on its own gives me 100v from batt +ve to chassis at well below 1mA, perhaps this is to be expected on an EV? Perhaps there is some capacitance in the wiring etc and the high impedance of the multimeter allows you to read it for a while (the reading does trend down). Unfortunately I'm not a boffin so I can only do my best

D