losing 2v across BMS normal?

[Raged]

Hi everyone, I've been losing about 2v to the controller for a while now (even when I charge to 57.4v, the voltage from the BMS is ~55v when the charger stops charging. I checked the charger voltage and it is trying to push 57.4v). The BMS and charger is the 14S one from EM3EV (they're designed to work together)

I thought it was a balance issue, so I decided to pull everything apart and do a balance of all my lipo cells (4 x 7s of Hobbyking Zippy Compact Lipo to make 14s2p) with my turnigy accucel 8 balancers. They all balance true to 4.1v per cell... (I set my end voltage to 4.1v). End voltage of each 14s pack is 57.4v. All good.

I put it back together and plug in BMS (no balance wires connected) and it reads 55.4v from the BMS power terminals (power to controller). Nothing is connected except BMS. Double check voltage before the BMS and its 57.4v... I'm still losing 2v =(

Questions :
1 ) Is my BMS borked?
2 ) any way to quickly troubleshoot/repair?
3 ) If my BMS is borked. Is it safe to keep using it even though it's chewing 2v?
4 ) Where can I get a cheap 14s bms from for cheap in Australia?

Thanks in advance.

 

[amberwolf]

Hi everyone, I've been losing about 2v to the controller for a while now (even when I charge to 57.4v, the voltage from the BMS is ~55v when the charger stops charging.

I'm not sure what you mean by "losing about 2v to the controller", given the later part of your post.


Do you mean that when you hook up the controller to the output of the pack, the pack output drops by 2V, but that it doesn't do so with any other small load hooked up? (like a wattmeter, etc)

Or do you nean that it drops 2V when you apply throttle (put a definite load on the pack)?

If it drops 2V with any small load hooked up, it's not your controller, it's "surface charge" on the pack, probably. If it is LiFePO4 then this is pretty normal.

If it drops 2V with a heavier load on it, that may also be normal depending on C-rate of the cells, age, etc.


Given the part below:

I put it back together and plug in BMS (no balance wires connected) and it reads 55.4v from the BMS power terminals (power to controller). Nothing is connected except BMS. Double check voltage before the BMS and its 57.4v... I'm still losing 2v =(

I'm assuming that yoru controller itself has nothing to do with your problem, and may not even be what you meant with your original question. You may mean "losing about 2v to the BMS", instead (which your title implies).

If this is the case, what happens when you apply an actual load to the system? Does the voltage difference still exist? Does it increase? Decrease? Stay the same?

Do the FETs on teh BMS get hot even with a minimal load?

If the last is true, and the voltage drop stays the same or increases, when you put the system under a significant load, it's possible the FETs are not fully turning on. DUnno what the specific cause of that would be without further testing.


So, need more info to diagnose the problem.

 

[Raged]

Ahh.... sorry for the confusion. When I say lose 2V to controller, I mean the controller gets 2v less than what the battery is giving.

Here's a text picture :

Battery pack===> (57.4v) ===> BMS ===> ( 55v) ====> Controller.

The voltage readings are the same when the controller isn't connected (zero load) which is why I believe its the BMS. The batteries are Zippy Compact Lipo (full charge is typically 4.2v per cell, but I personally set it to 4.1v per cell).

I don't think the voltage will increase with a load applied, but I can give it a go. It's just a bit tricky when the pack is apart and trying to lift the back wheel up.

If I need to crack open the bms, is there anything I need to have handy first? (heatsink paste? etc?).

 

[amberwolf]

I don't think the voltage will increase with a load applied, but I can give it a go. It's just a bit tricky when the pack is apart and trying to lift the back wheel up.

YOu don't have to use the motor/controller as a load; you could hook up high-wattage house ilghtbulbs to it instead (alligator clips from battery output to prongs on a lamp cord). Or a heater, or almost anything "resistive" meant to draw a lot of power at a voltage similar to that of the pack. Car headlights could be used,t oo, but you'd need to put a few in series or you'll probably burn them out at so much higher than normal voltage.

If you have a wattmeter you can put it in series with the load just like you would with the controller, to see what the lamp(s) load is, to be sure how much you're loading it down.

 

[dnmun]

you should use the 20V DC scale on the meter and put the black probe on the B- spot on the BMS and then put the red probe on the P- spot and then report that voltage. it would be help if you posted up a vga picture of the BMS too.

 

[Raged]

Sorry for the late reply, but there's a heatwave in Melbourne at the moment. It's only just cooled down enough for me to enter the shed =(

I measured from the battery positive and the -ve of the BMS at 55v. This diagram shows it a bit better :

This is how it's setup in the bike frame :

 

[Raged]

Ahh crap... found the issue... one of the soldier joins for a pin on my balance cable fell out. So dammn simple yet had me wracking my brain for 2 days!!!

So if your BMS voltage output is less than what your battery is supposed to give, Check the first few connections on the balance connector.

Thanks for the help and sorry for the un-needed thread guys.

Happy New Year =)