How bad are these voltages?

[Ampster]

I will indeed once do a proper top balance and see the difference.

If you have the time use the charger with the lower Amperage and maybe over time the BMS will balance them. However it is also possible that the voltage settings of the chargers were different and the lower setting terminated the charge. If you can, use that setting on the other charger.

 

[John Frum]

If you have the time use the charger with the lower Amperage and maybe over time the BMS will balance them. However it is also possible that the voltage settings of the chargers were different and the lower setting terminated the charge. If you can, use that setting on the other charger.

Also make sure to only balance while charging.

 

[Q-Dog]

But now I realized; I had charged initially with my inverter; this one resulted in the cells being balanced when it stopped charging. The inverter is an Ective SSI 20, charging at 20A;
And now with a dc-dc convertor- renogy 50- and if was charging at 35A . At the end the BMS stopped the charging; at that moment charging was still at 20-25A.
With the inverter , the inverter stopped the charging. And it stoppen after some time charging at low current - 1A and many lower.

Is it normal that the BMS stops the charging and not the charger?
For the DC-DC charger, it stopped when it was at about 13.8 volt before one of the cells hit the 3.6V max the BMS is configured.

The BMS is sort of like a circuit breaker in that it is there to protect the individual cells. A properly set up charging system should stop charging the battery well before the BMS cuts off charging.

 

[Gazoo]

If keeping the cells voltages between the knees then delta is not a big concern. When I initially parallel top balanced my cells, my delta at the top was .100mv's and .500mv's at the bottom. As @smoothJoey said, delta at the bottom will always be bigger because most of us parallel top balance. If bottom balancing it would be the reverse. If buying capacity matched cells then this would not be nearly as big of a headache.

I am with everyone else. An initial parallel top balance should be done. If there are problems after the pack is assembled, a runner cell can be brought down using a resistor. And a weak cell can be brought up by putting a bit of charge into it. No need to disassemble the pack.

I am not familiar with active balancers. My thought is if they balance at the bottom then they would also need to balance at the top. In my case I don't feel the need for them because I am keeping my cells between the knees where the delta is negligible.

 

[John Frum]

As @smoothJoey said, delta at the bottom will always be bigger because most of us parallel top balance.

Just want to clarify this point.
Most people parallel top balance but the parallel part is just a means to an end.
All top balancing means is getting each cell to 3.65 at consistent rate of charge.
Another simpler way to say this is all cells full.

 

[walter-]

Going to start the top balance ; I discharged 2Ah first; cells are now at 3.33V average and max range is 0.022V.

Disassembling is starting...

Walter

 

[walter-]

Damn; need more busbars lol!

 

[walter-]

Initial voltage of parallel battery 4p: 3.335V
My lab Power supply; a Manson hcs-3400 is normally a good one... good in the sence it is very accurate but still can do up to 600W.
But...; voltage is only controllable to the 0.1V (but it is accurate).
So I start charging at 3.6...

Have been charging now for about 2 hours; started at about 8A, now to 4A; after shutdown of the power supply voltage is 3.485V and dropping slowly...
I continue tomorrow (not going to run it while sleeping).

 

[John Frum]

Initial voltage of parallel battery 4p: 3.335V
My lab Power supply; a Manson hcs-3400 is normally a good one... good in the sence it is very accurate but still can do up to 600W.
But...; voltage is only controllable to the 0.1V (but it is accurate).
So I start charging at 3.6...

Have been charging now for about 2 hours; started at about 8A, now to 4A; after shutdown of the power supply voltage is 3.485V and dropping slowly...
I continue tomorrow (not going to run it while sleeping).

Now you are on the right track
Watch is like a hawk tomorrow.
Once the amperage starts to drop you are probably minutes away from completion.

 

[walter-]

Started again this morning:
Initial voltage 3.433V
Continue charging 3.6V with 5.9A.
After about 1 hours the current was down to 0.3A (not sure anymore about the voltage, something like 3.50).
I switched the charger to 3.7V; it started charging at about 3A; I closely checked the voltage of the batteries. In about 15 minutes it went up to 3.63V.
My voltage meter is 0.6% accurate in the range 0-6V. So the accuracy at 3.6V is about 0.022V. So if I measure 3.63 is could be as well 3.652V or 3.608V.
My reasoning is I shouldn't go higher anymore because I could be over 3.65V
After about 10 minutes without power supply the voltage dropped back to 3.60V.
Is that the resting voltage after a top balancing?

Roughly calculated; for the whole top charging it loaded about 16Ah; that is 4Ah per cell; before the start I discharged about 2Ah; so roughly it has now 2Ah charged more then I could charge with the DC-Dc convertor.

Note: re-read the top balancing manual; the charging with supply at 3.7V, is maybe too risky; don't do this at home. I should maybe have just waited for the current to drop completely at 3.6V. The amount of energy going in the battery extra with this 3.7V step is limited.

An now an hour after charing the 4p is down to 3.591V.

I let the pack rest and maybe I do this evening a load and capacity test.

 

[Gazoo]

An now an hour after charing the 4p is down to 3.591V.

That's good. My cells sat for two days and all of them were over 3.5 volts before I connected them is series. When a load is connected to them the voltage will drop rapidly.

Some have suggested letting the cells sit in parallel for a day or two after top balancing.

 

[walter-]

Just build up the battery again;

Above the start situation;
One stupid thing; the BMS thinks it is only about 70% charged but it doesn't allow to set the SoC via the app; strange since I think I could do it last time (at least that is what I thought I could).

I am doing the test now;
Last time I used a heat gun. I now have the display of the inverter turned towards me. I see the inverter in constant alert -didn't notice last time. The reason I think because the current is not stable. I think the heat guns power is constant being adjusted and unstable. I see indeed the current swich constant between 120A and 200A (that seems an extreme fluctuation). The amphere clamp is confirming the fluctuations. I can imagine that the true sine inverter has issues with this power fluctuation.
So I swap to a simple electrical heating.
This is indeed more stable; a little higher power (can choose between 950W and 2kW). The inverter stops complaining.
Below screenshot after 101Ah pulled from battery (ignore SoC; it is wrong...)

I am measuring Ah via a renogy 500A shunt; I have put the battery to 290A capacity to be sure it counts the complete test if it would go over the 270Ah.

Side note; last test the cells kept cold during the complete test ; except the last 20Ah (or was it below 3A) or so the temperature raised to 40°C. Is this normal that the cells produce more heat near the end of the capacity? I can understand since the voltage is higher, current is higher and there is more heat produced. But it seamed to me the heat was significantly higher (even if heat produced ~I²).

Walter

 

[walter-]

Note; start difference between the cells is 0.002V: that is smaller I think than the measuring error of the meters.
Now at 140Ah removed; difference is 0.02V (actually fluctuates between 0.02V and 0.010V). Balancing doesn't kick in -does it during discharge?

 

[walter-]

Ps; invertor is delivering 1.905W; input is 2.150 (measure with current in and voltage at its terminals); that is en efficiency of 87%.
Or differently put; the inverter has to get rid of about 230W of heat...
The inverter is blowing air out of 43°C.

The BMS measure 31°C on top of the cells. But that is more due the heat from the inverter; it is blowing against a panel and the air is reflected towards the cells.
The cells themselves don't seem to produce heat.
But a good reminder: I my RV I have to make sure the air from the inverter is not towards a component that shouldn't receive extra heat. The design of the inverter -renogy CSI 20- is not really helping here. The heat is blown towards the 12V terminals which normally would point towards the battery.

 

[walter-]

The inverter for me seems to have a strange design issue;
When used as a charger it has constantly the -loud- fan on; but then the invertor only delivers 250W (+ losses). Constant as the fan start after 5-10mimutes of charging and never stops.
When in use as inverter, the inverter can produce 600W without the fan running.
And actually when going from 2kW to zero, the inverter immediately stops the fan. Although it clearly is still warm (about 40°C). Actually the case seems to heat up due to the remaining heat inside.
It is so strange the invertor needs the fan when charging and producing really no heat whereas when in use he even stops the fan when clearly still being warm...
The fan on I guess will be annoying in the RV because i think I would typically charge at night. And then I don't want the sound...
Maybe it is because the charger is so inefficient it very quickly need the fan.

 

[walter-]

The test completed;
The inverter shutdown when the battery was at about 10.7V; at that moment the shunt has counted 270Ah;
AC side produced 2.89kW.
Possible the cells could have delivered the missing 2Ah when I would bring the battery the 10V. But for me the test is fine. I expect not to need to go so deep ever ;-).
Let me see what the charging brings.
View near the end of the test in the app;

View 15 minutes after the test; battery recovered to 12.2V. Note app give 0 current; shunt says 1A...

 

[walter-]

Interesting view when charging with the DC-DC charger; differences is very low...; 0.004V (and often lower)

 

[walter-]

I am about at 40% in charging again;
Balance between cells is scary good;

 

[RGIvy]

Your cells are very similar to mine.
..perhaps not quite as good quality. We were trying out a new supplier, while the deal was good their stated 300ah capacity turned out to be 285ah, so we're sticking with tried and trusted suppliers. My cells have gone into my own motorhome, we'd not be comfortable selling them.

 

[walter-]

Your cells are very similar to mine.
..perhaps not quite as good quality.

Based on what would the quality be less?