Symptoms of a Damages BMS

[dogfood]

Hi All,
I'm riding on an EM3EV setup with a samsung 46.8v frame battery. Someone recently stole the seat and the batteries mount (ugly welded thing w/ no resale, wtf) so I had to improvise a way to connect the battery to the hub. While I was doing this I crossed the output wires and got a little spark. Fuse was fine, though. When my bike wouldn't run, I checked the voltage and got 25 V! Charger didn't activate when plugged in, so I opened her up. Please note that at this point I'm in way over my head. At the charger port and opposite ends of the cell pack read a full 54V, but output is half that. It seems like the BMS (I only know because of the pics on EM3EV) is faulty. Does this make sense to you guys?

Thanks,
D

 

[TheBeastie]

Hi All,
I'm riding on an EM3EV setup with a samsung 46.8v frame battery. Someone recently stole the seat and the batteries mount (ugly welded thing w/ no resale, wtf)

Some one stole just the battery mount? Geez what pieces of crap! I just wanna see what they took.. got a pic? (hey I at least gave u a free bump )

 

[Hugechainring]

Hello, DF,
I can give it a shot.
I'm having trouble understanding your first statement about connecting the battery to the hub. Normally your battery connections would go to your controller, then the power output of the controller is connected to the hub. Please be careful about what you are describing your actions because it makes a difference. I'm also having trouble understanding your voltage measurements as to where you are taking the readings. It sounds like you are going direct to the battery terminals and getting 54V, but only 25V at the BMS output. Is this correct? In any case, your battery /BMS should be disconnected from the controller when attempting these measurements. If the BMS has shut down due to a current spike, it will only "reset" if the load "the controller" is disconnected. So you would need to pull the connector between the BMS and controller. It sounds like you have done that.

Was your entire setup supplied by EM3EV? They have a writeup on their site about precharge circuits, and how their BMS is susceptible to damage by current spikes like what you have described. It is unclear to me whether the precharge circuit they construct is part of the battery assembly, or part of the controller they supply with their kits. If you have a complete EM3EV setup, it should have a precharge circuit somewhere. If not, it is entirely possible that your current spike has cooked your BMS.

It is also deceiving sometimes when taking voltage measurements due to high resistance in the circuit. A quality DVM can measure a voltage, but the circuit is still basically "open" due to the resistance, and it's impossible to run anything off it. In other words it's possible the 25V is effectively 0V. I don't know if this is helpful to you, but perhaps it gives you something to think about.

The fact that your charger will not initialize is a flag to me. If you are brave, you can bypass your BMS (temporarily!) and see if your system works. Watch the polarity! Don't bypass the fuse! If it runs, then you are likely correct, the BMS is toast.

 

[mlt34]

I had something similar with a 48V ping battery years ago. I was getting funky voltages, sometimes 20 something volts. Sometimes it would fluctuate with a period of about 1 second. very weird. Turns out it was just corrosion on one of the pins connecting the sense wires together. I'd check your bms for a bad connection to the little sense wires.

There's also a slight chance that it somehow way unbalanced itself, but there couldn't really have been enough energy flowing in that split second arc to do much unbalancing, in my opinion.

 

[dogfood]

Thanks for the replies!

@Beastie-It was a kids mongoose MTB seatpost with a welded offset to make it comfy for an adult. Very gnarly dirty off-the-clock fabrication. And it had a piece of plastic with a plug on it. Maybe the seat was worth a few bucks... I disguised it with an old milwaukee tool bag wrapped around it. They didn't even take that! Must have been an inside job.

@Huge-When I say to the hub I mean through the controller. Measurements were taken at the battery pack output (25V) initially and then I opened it up. Measurement of 25V continues from pack output through fuse, and into the thing that looks like the replacement BMSs that EM3EV sells. This is a complete em3ev package. I'm getting 54V when I jam the probes into opposite ends of the cell pack, the positive being anywhere between the charger input and the "thing that looks like a BMS." Which would explain the charger not charging an already full batt, right? All this is done while the battery pack is disconnected. Because someone stole my bracket.
What is a precharge circuit?
Can a BMS be repaired?
Are there any domestic (US) sources for BMSs? I'd like to not wait for international shipping while I'm pedalling around like a sucker, buying non-bulk catfood.

@Mtl- I will check that.

Thanks again,
D

 

[Hugechainring]

Fix it yet? I read what you said about taking the voltages at different points. It sounds to me like your BMS is cooked. Did you try doing the bypass thing? Cut the BMS out of the power circuit and go direct through the fuse to the controller? That could tell you a lot. The fact that you get a 25V voltage drop from one side of the "TTLLABMS" isn't right.

A precharge circuit is a device that limits inrush current to the controller when you first connect the battery. The controller has a capacitor bank in it that can draw several hundred or possibly over 1000A in some setups for a very short time while charging the capacitors. Hence, the "big spark" upon connection. This isn't good for the capacitors or the BMS.

If you need a BMS, buy it from EM3EV. He has quality stuff, but if you buy just a BMS from him, he offers virtually no customer support on those things after he tests and ships them because customers have found every possible way to blow them up by improper wiring, etc. Nothing personal, he just can't lose money selling BMS units. He may help you a little because you are a customer already. Send him a photo of what you got.
If it is a plug in deal, with minimal soldering, it might be worth it.

 

[d8veh]

25v means tha the BMS is shut off for whatever reason. It might just need a reset. Pull out the multi-pin connector and plug it in again, which sometimes works.

 

[dnmun]

it is hard to figure out what he shorted but if he shorted the output then the BMS would turn off for overcurrent.

no pictures so no way to know which BMS he is talking about either.