First Vruzend build.. voltage just short of 42v

djbrandonr

10 mW
Joined
Jan 15, 2018
Messages
31
Hi,

new here but looked around... at any rate I just built my first battery with a vruzend kit

36v 10s6p with new 30q cells Ill break down my process but when I finished full assembly the pack was reading 34v... not biggie..so i charged the pack up and it stopped at 40v not a close to 41.5-42 like my other pre-fabbed batteries

anyways heres everything i did to this point and any thoughts are greatly appreciated..

new 30q cells arrived...i unpacked all and tested each to make sure all had charge and same voltage...all reading 3.7

i start building parallel groups of 6.. add the top caps and compress them all down onto the cells and then re-verify each cell has contact in the caps and showing voltage...YES

I start assembling the parallel groups in series (rectangle shape) ... now before adding the bus bars for parellel and series connections I DIDNT do one last voltage test of each cell as the whole pack was assembled...i assumed all was still good after parallelling them

I add all bars for series and parallel connections... add the BMS and balance wires and all that final wiring stuff... END

so what could it be? all cells cheacked out on the way in... and at the end we are where we are.

thanks
 
Can you check each group with a volt meter? that would surely tell you if there is a problem in the pack vs a prob with your charger.
 
DanGT86 said:
Can you check each group with a volt meter? that would surely tell you if there is a problem in the pack vs a prob with your charger.

yeah its def not the charger as I been using for a couple months now on other packs no issues.. im sure its might be a small error on my end being my first build... that said isolating each parallel group would require me to pull all the bars off from series connections, but i dont know if this is kinda the same but if I count the voltage from 1 - 10 it progresses like this if its any indication or help

1- 3.8 - 3.9 so string 1 seems to start a little low
2- 7.9 +4v
3- 12 +4.1
4- 16.1 +4.1
5- 20.2 +4.1
6- 24.3 +4.1
7- 28.4 +4.1
8- 32.5 +4.1
9- 36.5 36.6 +4.0 - 4.1 ish
10- 40.6 +4

can we isolate based on this? perhaps string 1,2 and 10?
 
Looks like string 1 and 10 are definitely outliers. Can you charge them separately? Do you have an rc balance charger or adjustable power supply you could use to catch them up with the others? Maybe the the highest cells triggered the bms to shut things down before the whole thing caught up. Some chargers wont reset on thier own if this happens. I would try to manually balance then all then see what happens.
 
Possible sequence of events that caused this.

Poor connection in string one and two caused the bms to see higher voltage on string one and two and start burning off the excess charge. bms then trips the pack before the voltage on these strings goes too high causing the whole charge to terminate early and miss the important current taper that would have reduced the problem.

Possible solutions.

1) fix the poor connections in string one and two.

2) Reduce the charging rate. This problem may not have occurred at all with a lower charging rate.

Tests.

1)Measure the voltage of the parallel groups under charge to see if there is one that is higher.
Also feel the connection, a parallel group with poor connection should get hotter.

2) Use/build a discharger and see how the pack performs under discharge. Bad connections and adjacent cells in the same group should be warmer.
 
good info so far... no balance charger as I havent needed one..yet... i have 2 chargers i use but mainly for these big packs I use a 42v 5amp charger...im not sure exactly how it works nor what exactly triggers them to stop charging aside from the bms limiting a max input... but that would be another thread...

at any rate.. i feel easiest starting option would be breaking the ends, 1 and 10 from series connection and see what that reading is or even test each cell in that string while im there... but.. although not difficult more like kinda time consuming would be to just unscrew all the top and bottom nuts n connections and do what I should have done ONE LAST TIME, just in case, before I actually started laying the bars down. =/

will report back later with findings but still open to any other info anyone feels compelled to provide!
 
The silver lining here is that you used a Vruzend building kit and didn't spot weld your batteries. It may be irksome to remove the nuts and bus bars, but that's way easier than having to rip apart a welded pack.

Side note:
I recently got some LG MJ1 cells and a BMS from Vruzend. The cells were all within a 100th of a volt of each other and the BMS looks good, though I have no idea how good the quality is yet. I welded my pack however, as my current build requires an absolutely minimum size battery and the Vruzend end-caps just added to much to the pack size and weight. They seem very well designed and engineered though.
 
djbrandonr said:
good info so far... no balance charger as I havent needed one..yet... i have 2 chargers i use but mainly for these big packs I use a 42v 5amp charger...im not sure exactly how it works nor what exactly triggers them to stop charging aside from the bms limiting a max input... but that would be another thread...

at any rate.. i feel easiest starting option would be breaking the ends, 1 and 10 from series connection and see what that reading is or even test each cell in that string while im there

If your charger is the common ebike type then it will supply constant current up to a pre determined voltage likely just below 42v in your case. When it hits this point it reduces current and goes into the cv phase where the current is reduced to a few hundred milliamps. This is where the balancing occurs when a bms is used. The current at this phase is low enough such that the bms can bleed off the high voltage cells while the low ones catch up. The charger doesnt care if there is a bms or not. It always does the same thing. It will supply current until the cutoff voltage (42v for 10s) is achieved. So if your pack starts out close to balanced and you have a bms then everything works well because the bms can do its job in time before the highest cells run away such that the sum total of the voltage is 42v causing the charger to shut down. In the real world the bms cant always keep up especially with a new unbalanced pack so the charger will turn off before balacing is done. I find it pretty frustrating how poorly these systems work but they are cheap and better than nothing.

Bottom line is that you want to start with a really well balaced pack so the bms is doing slight maintaiance rather than initial grunt work balancing. Rc chargers are nice for that initial balance work.

If the bms saw one cell group get too high it may have shut everything off as well. Some chargers dont turn back on automatically. There is a small board that needs to be removed if this is the case. If this is what happened i would expect to see the voltages closer to 4.2 so you may still have a problem.

If you are going to build packs its a good idea to have some cell-logs or battery-medics. Any little display device where you can see many cells at once is a big help. I also found that charging through my cycle analyst where I could watch the current and voltage of the charger in real time was super educational.

If your connections are all good then you dont need to take the pack apart to measure voltages. The individual cell groups dont care about the other cells in the pack. You can balance them through the bms wires or cell connections with the pack assembled. Since you dont have a way to charge individual groups you are going to have to balance by uisng a load to bring the others down to the lowest voltage group. When I am in a hurry I use a lightbulb and some alligator clip leads to pull down the high voltage cells while im charging. Like a human bms. To do that you need a good method of watching the cell voltage readings all at once. Its an annoying process but does work faster than the bms.
 
ok, so i just pulled all the bars off and brought the pack back to starting position of fully assembled but zero connections... each cell in each string test out exactly at 4.05v each

EXCEPT string 5... all cells tested at 4.12v and heres the kicker with 1 of the 6 coming back at 3.42v

as a reminder when the individual cells arrived I tested each before starting the assembly process and each came out to a solid 3.7v out of the box.

charger was reading 41.9v

what do we got doc??
 
From your latest results it seems that one cell in string 5 has a very bad connection or is disconnected from the rest of the pack. Thus all the rest of the cells in the string got more charge and therefore are a bit higher. It could also be a bad cell.

I still suspect the bms killed the charge. Try re-assembling string 5 with a new cell then put pack on charge again. Measure the strings as you're charging to get clearer idea of what is happening.

The other odd thing is your initial reading of strings 1 and 2 having a lower voltage. Are you sure your multimeter is working OK?
 
NickF23 said:
From your latest results it seems that one cell in string 5 has a very bad connection or is disconnected from the rest of the pack. Thus all the rest of the cells in the string got more charge and therefore are a bit higher. It could also be a bad cell.

I still suspect the bms killed the charge. Try re-assembling string 5 with a new cell then put pack on charge again. Measure the strings as you're charging to get clearer idea of what is happening.

The other odd thing is your initial reading of strings 1 and 2 having a lower voltage. Are you sure your multimeter is working OK?
I dont remember I may have had the meter at 200 instead of 20 when i started which was giving at 3.8ish reading..then as i worked up each group from the negative end the resulting numbers you saw are there, same as this morning testing each cell solo..the meter was at 200 giving me 3.8, but dropped to 20 gave a more accurate #


i could try replacing the caps on the one cell as that could have been one issue... but this is like the runt cell now that needs to be brought up to the others now yes? So do i need a balance charger to bring it up to 4.05 or 4.12... or do i discharge the 5 from 4.12 to 4.05.. whats the best plan?

or do i just string it all back together as is... 90% at 4.05v... string 5 at 4.12 with the one off cell and hope it all balances right??

multimeter is basically new (although that doesnt necessarily mean anything)
 
well were up to 41 - 41.1v now... i broke string 5 apart and replaced the caps on the low one just in case... then marked each cell that had made it to 4.12... I re-dispersed them though a few other strings hoping that in a row of 6 if one was a lil high the bms would balance easier instead of one high string only cutting it out...

I even used my slow 1.5amp charger just to take it slow and see what happens... unless anyone else has any other ideas or hacks Im feeling like buying just a cheap balance charger and tune the few cells up/down that are a bit off.. thoughts?
 
999zip999 said:
If balanced. Put a light blub on it and drain it down a little and recharge. Check balance.

how balanced is balanced?

4.12v on strings 1, 2, 6
4.14v on 10
4.09v on the rest

thx
 
getting closer! the light bulb helped i assume...

string 5 still the lowest but heres the most current results after charge...

1- 4.2
2- 4.2
3- 4.15
4- 4.16
5- 4.07
6- 4.2
7- 4.16
8- 4.17
9- 4.15
10- 4.18

So all very close with in hundredths for the most part...
 
one last bump and update... i ran this pack this week while doing deliveries...

most times I run it till its empty..meaning not empty empty as in WOT and no response but more like the pack is low enough to where I need to start pedaling more than necessary at which point I plug in my backup battery... I dont have a precise value but lets say 10% left..

Now Ive used it about 4-5times and the max charge Ive gotten is 41.6v is this good enough? All my other pre-fabbed batteries stop at 42 or just a hair above.

thx!
 
Sounds a lot better. If its stopping at a different voltage than your other packs using the same charger then that would point to the BMS terminating the charge. Thats pretty close to 42 so they'll probably even out better over time since they get dome bms balancing on each charge. I'd say your close enough to rule out any major problems with the components.
 
As a a rule don't run to lvc as with or if trouble parallel set of cells have a lower capacity thats how much ah the battery has. I would put a set of sense wires on for ez checking with cell meter and have outside accessibility to each parallel string for balancing ect.
Number 5 is to low. 4.15v
You can use a phone charger if you check polarity and voltage just because it says 5 volts on the case does it mean it doesn't put out 7 volt. YOU MUST SIT AND watch it as it hits 4.15 plus it will charge past 4.2v fast. The danger zone. Good luck if taking a piss unplug it. Sit haha
 
Back
Top