BMS now can barely balance when discarging

[william2021]

Hi, I bought the following:

4 x 230AH cells from Docan
VNSZNR BMS LiFePO4 4S 12V 150A from Amazon
GoWISE Power 1500W Pure Sine Wave Power Inverter from Amazon
Renogy MPPT charger from Amazon

They essentially works, except sometimes when using 1100W rice cooker the system would stop working while beeping.

On the BMS' app I noticed at that time the voltage difference between highest and lowest cells became large, even to 0.6V. So then I bought:

4S active balancer from Amazon

That worked perfectly - while using 1100W rice cook the voltage difference between highest and lowest cells dropped to like 0.005V. I could even use 1500V air fryer and that voltage difference was still minimal so everything worked perfectly.

Until one day it beeped again and I found it was redhot around the negative pole on the battery set. I immediately stopped discharging. I believe it was because, I did not properly connectors there, so just stuffed the wire of the active balancer between metals there - close to plastic wire protector there, which heated up those plastic there.

Then I properly solder them.

However, now whenever I discharge, even at 150W, there can be like 0.7V voltage difference and sometimes disconnect; and for 600W discharge it's essentially impossible since voltage difference is even higher so it keeps disconnecting.

More worrisome is, it's always that cell with that burnt connector that has the lowest voltage.

Furthermore, now the active balancer makes no difference at all - regardless of whether or not to connect the active balancer, they always have that huge voltage difference.

I thought that active balancer was burnt, so I ordered a new one - but the problem remains.

I measured, they are now all properly connected. That is, for example, from the negative pole of the battery cell to the corresponding lead in that socket in the other end is 0ohm.

But as long as there is no discharging, the cells are quickly balanced by the BMS even without the active balancer. And in such situation any cell can be of lowest voltage.

Now where is the problem? Is it the BMS, or that cell, or some connection? How to diagnose?

And why even the active balancer now can not balance?

Thanks!

 

[sunshine_eggo]

Have you verified all cell readings with a separate multimeter?
Have you checked that all connections are properly torqued and of high quality?
Did you top balance your cells prior to assembly?
Can you charge your battery to 14.4V without triggering cut-off?

Note that you shouldn't confuse voltage with balance. Voltages are typically very narrow even when the cells are imbalanced when cells are between about 3.1 and 3.4V.

The only time voltage is a representation of balance is below 3.1 and above 3.4V, thus a battery at rest will often show very similar voltage even if the cells are significantly imbalanced.

 

[william2021]

Hi, thanks so much for your prompt help - and they are spot on!

I measured voltage with multimeter before, so thought that's unnecessary to do that again. But I followed your advice anyway - and I'm happy I did!

The reading of that last cell sometimes is of the same reading just like other cells, but sometimes fluctuates agreeing with the reading on the BMS' app.

I tried quite some times but could not figure out the inconsistency.

Eventually I just dismantled the cables and bolts on that pole. I saw quite some surfaces there were highly oxidized because I was inexperienced before, and including the last incident. So I just carefully sanded them (actually just scratched with knife) until all surfaces reveal shining metal. Then assembled them back.

Now, even at 1500W, the voltage difference is within 0.005V - and without the active balancer!

So looking back, the later addition of that active balancer was already the result of my previous issues at that pole before.

This also taught me, that it's *that* important to keep good contact at all connections.

Thank you very much again!