DC-DC converter sending high voltage back to battery?

[ralphius]

I use my ebike for commuting and wanted to add 12v accessories (motorbike lights/horn etc). I bought this 60W DC-DC converter off eBay so I could run accessories off my 48V battery (13S).
https://www.ebay.co.uk/itm/DC-DC-St...-48V-60V-to-12V-5A-60W-Output-UK/223437163480

At first it worked great, but as the weather got colder and damper I found first my lights flickering and then my motor stuttering, this got rapidly worse to the point that when I turned the battery power switch on, my lights would flash once and then everything would be dead til I turned the battery power switch off and on again.

After thoroughly checking connectors I discovered the controller worked again if I disconnected the DC-DC converter above from the battery. I then tried connecting just the DC-DC converter to the battery on my workbench and got the shock of my life as 25mm long sparks arced out through the wire insulation and shorted across the battery terminals. I've seen enough arcing to respect sparks that long and stay well clear, they were very thin sparks though, and more of a crackling sound than the thick, fat pops of a high current spark. Suspect well over a thousand volts at a few milliamps by looking at them.

In the photo below the arcing was coming out of the thin wire I have added for the DC-DC converter, it was just below the cable ties and flickering around diagonally between black and red. After the sparks but before taking this photo I removed some of the tape round the battery negative connection to check the solder joint, but all was ok:

So my question is, where the hell did voltage like that come from and why is it only on my battery wires with the converter connected? And is it likely to have damaged my brand new 48V battery or my controller?

 

[amberwolf]

If there *is* that high a voltage (you'd need to measure to find out, with probes of an AC voltmeter on the DC-DC end of the connection), then you'd probably have to replace the DC-DC.

I don't know what would cause it; it'd have to have quite a design flaw to be able to do that; best guess is failure of all the input side filtering, so that the switching stuff going on inside the DC-DC is making high voltage AC riding on the DC input. (it is unlikely to be high voltage DC; I can't think of a way that could happen).


What's probalby causing the system shutdown is the BMS of the battery is trying to protect it from what it sees as overvoltage.

Damage...won't know unless something else fails, but I wouldn't put that DC-DC back on there, nor would I use that brand again.

If you happen to have any 14-15VDC output wallwarts / chargers for any devices you aren't using, that can output the amount of current your lights/etc need, there's a fair chance it will operate by connecting it's AC input prongs to your battery's output.

I used to use a 15VDC wallwart like this for running my LED lighting on the SB Cruiser trike, and before that it ran the lights on CrazyBike2. (presently it's just being a DC-DC powered by the 14s traction pack to turn on a relay that switches my lights on; the lighting power actualy comes from a separate 40Ah 4s battery).

 

[billvon]

So my question is, where the hell did voltage like that come from and why is it only on my battery wires with the converter connected? And is it likely to have damaged my brand new 48V battery or my controller?

A synchronous buck converter works almost as well backwards as it does forwards. Which means that if you have a 12V battery on the low voltage side, you can easily see current driven back to the HV battery.

However, if there's no energy source on the low voltage side, that can't happen.

If there's no external energy source, then nothing can push the HV battery into overcharge.

You can get arcing if the flyback diode is broken and the input caps are toast. Usually that destroys the input FET first, so it's odd that you are seeing it at all.