Today i learned that bullets and aluminum are not a good combination. Due to heat problems i am installing an aluminum case and was working on the lid. I moved a plate around and accidentally hit both bullets from the battery line. Then the optical and acoustic fireworks began . At first I flinched, but it didn’t stop because the bullets were sticking to the plate. The aluminum plate also partially touched bullets from the vescs where it sparked like crazy. I then pulled the plate away by hand, maybe not the best idea, but I didn’t get an electric shock and the short circuit was over. The end of the wires were quite hot and smoke was in the air.
One thing I really don’t understand is why the BMS didn’t shut off? I use BMS for discharge on every build exactly because of such situations. Once I had a massive short on the charging port with fireworks, sparks and material were hurled through the area. I could only stop it by disconnecting the charge extension inside the enclosure. The BMS didn’t shut off as well and the fuse which I put on the charge port was not even impressed so no damage at all I understand that a fuse needs some seconds to blow but a BMS not.
But all BMS I use like to shut off (over charge protection) when braking downhill with full battery. Also this bestech bms shuts off (over current protection) when doing funny stuff like burnouts above 110-120 battery amps. I have to disconnect the BMS from battery and connect again otherwise it doesn’t work. This BMS has never saved me from a short circuit which is ridiculous and very dangerous.
However I’m good and pray the battery and vescs survived without damage.
I hope you learn from my mistake and don’t do stupid things like me
I thought (but don’t know) that when a short circuit happens the amps increase dramatically and that’s why a fuse blows or BMS shuts off. For example this BMS has 120A over current protection (and that’s quite low).
I do but the fuse didn’t blow when I had the short circuit on charge port. The rating of the fuse was not much above charging amps. I read somewhere that those car fuses blow after 5-10 seconds at 125% of their rating.
Short circuit or in your case connecting positive and negative terminal via metal conductor it is not really a short circuit but more adding resistive load, as the distance between contacts was quite big and I guess the current was definitely lower than 120A otherwise damage would have been much bigger. Plus I guess from the damage it was touching and jumping around the metal which caused the “fireworks” otherwise it would have been just heating up as resistive load.
But that just my brain thought what could have happened.
Tis a shame, but I guess this is why most people use XT connectors so things like this can’t very easily happen, much less, if the female is on the battery side.
I suppose in your application, XT connectors might be large. I agree with others’ sentiment that you probably didn’t reach your discharge limit
You don’t have to be worried about that. Even up to 20s its not dangerous to touch the main battery voltage. It might tingle a bit but nothing serious.
German Safety regulations declare 60V AC and 120V DC as dangerous. Everything below that will definitely not kill you. I always touch the main voltage from my battery’s out of curiosity and you barely feel 20s DC.
AC is way more dangerous because it can cramp your muscles and stuff.
Since the board is not connected to the ground there are way less possibility’s to create a circuit though your body anyway.
That’s a valid point and XT is much safer but I dislike it for many reasons. I prefer bullets because they are way more flexible to work with and take almost no space. Also you can feed them through holes (e.g. enclosure) and remove them easily. However I forgot the golden rule that bullet connectors on cables that are close together should not be in the same place, but rather offset a little. Then a short is harder to achieve.
Well there was nothing connected.
In the first pic you see that the unconnected bullets of the battery laying on the rear binding. I was playing around with another aluminium plate (planned as lid) and managed to touch the bullets on the binding, that’s why the fireworks.
That’s not really how it works. The current capability of the power supply has little to do with its danger. Even 0.1A 50Hz AC through your heart is enough to cause ventricular fibrillation. But the body has a resistance depending on the location that limits the current.
Low voltage is save because the resistance of the body does not allow any signifikant current to flow. So your 12V 2000A spot welder is completely save to touch.
Only higher voltage sources start to get dangerous because they can generate a higher current through you.
Oh no. Terrible idea
You can touch the battery terminals with just one finger of the same hand. That’s safe because there is no path though you body.
Edit: Everything below 60V DC is considered save even without isolation.
. At first I flinched, but it didn’t stop because the bullets were sticking to the plate. The aluminum plate also partially touched bullets from the vescs where it sparked like crazy. I then pulled the plate away by hand, maybe not the best idea, but I didn’t get an electric shock and the short circuit was over. The end of the wires were quite hot and smoke was in the air.
I understand that a fuse needs some seconds to blow but a BMS not.
