PINNED - 230Ah (245Ah certified) A grade cells Amy, Docan

[emilevirus]

Still balancing...

[Burgerman]

Its only balancing at half an amp, between 8 cells... Expect a wait.
That might increase some if the voltage spread was greater at 3.55 or 3.6V. Remember the state of charge doesent change, only the visible spread difference.

[shirley_hkg]

That might increase some if the voltage spread was greater at 3.55 or 3.6V.

I think he has a 0.6A balance current BMS, which doesn't vary with voltage differences.

[emilevirus]

Yeah, It's only really useful on first balance. After that, 0.6A is enough. So I got this one, was cheaper. It's almost done now. Only 0.04v difference. A few hours and it's good.

[emilevirus]

Is it normal cell voltage drop to around 3.1v under a 100A load? They're around 70% SOC.

[Burgerman]

Several things.

That depends on many things.
1. CELL impedance and state of carge. Its lowest IR at 50% charge and a little higher a full and at discharged ends.
2. The resistance across all the cell connections. So some 16 joints, and the cables.
3. Where you measure it.

Theres only one place. Across EACH individual cell. Not the connectors. The connectors add more resistance than the cells do.
The actual voltage drop across all of these wires and cells and connectors doesent sound unreasonable to me for what its worth. As long as each is the same.

If you do the same on lead it drops over time (a very short time!). Over say 0.1 of a second it falls in a curve to around 18 volts. From 25 plus. To give some comparison.
It then drops more slowly but after a second, the control system is reducing torque as the battery falls below the point where it can stay above the 16 to 17V cut off point. So that it doesent. It does that proportionally rather than chop off power like a BMS.

[Burgerman]

This is actually one of the issues with BMS as you set your low over discharge protection voltage. And it chops off power when you hit that point. But it might do this with a 50% charged battery as you were going up a train ramp... Especially on the drop in replacement ones.

Because voltage drop under load is greater than that of normal discharge empty voltage. This is why the mobility system that reduces motor power proportionally as voltage sags below x voltage instead is a better system.

Watch this.

Volts drop from 25 plus to 18. something in an instant. You dont deed a long ramp or hill.

http://www.wheelchairdriver.com/gopro/voltdrop.mp4

Theres 3 sets of figures. The bottom, the lowest one, is the PEAK lowest voltage. So if it dips then we see that there. And in a literal 2 feet of movement on a full battery I can make it drop by SEVEN volts! Thats resistance.

If I drive up the steep ramp in the pub for example, it drops to the 16V limit and stays there while the controller reduces current to "save" the battery, prevent further drop, and to maintain function. Thats why crappy batteries reduce torque.

[emilevirus]

That's something you can program with Linx. Reduce power under x volts. That's what a RC ESC will do too. Reduce power by 50% under 3.1v per cell (programmable). Does it actually harm the cells if it goes under 2.5v? I was going uphills and voltage was around 3.07v but that was on almost full charge. So it could technically go under 2.5v.

[Burgerman]

That's something you can program with Linx. Reduce power under x volts.

You dont want that.
You want it to MAINTAIN a lower limit under load by varying the power (amps) proportionally as you load the battery at your choosen low voltage level. Which PG stuff - even old stuff already did/does.
Its set to 16V for lead so set to a higher figure (22V) for lithium. That allows for a low battery to drop to 22V under load.

That's what a RC ESC will do too. Reduce power by 50% under 3.1v per cell (programmable).

That all depends on what your RC ESC is and does. I have a few high end ones that are a lot more programmable that that! Hundreds of options that you can program and adjust and then watch and listen to telemetry in the Tx in flight and program to warn you with voice etc, read out low cells at x current and allow you to build a map. The reason that cheap ones reduce power to say half at some arbitrary voltage is to tell you the battery is low in flight when you have no telemetry. Time to land.

Does it actually harm the cells if it goes under 2.5v? I was going uphills and voltage was around 3.07v but that was on almost full charge. So it could technically go under 2.5v.

The answer to that depends on WHERE its measuring the cell voltage. Because 3.07V is after several battery terminal connectors. So the real cell voltage is higher than the one you are reading. Because of cable connector resistance. It might be very low resistance. But so is a cell. 0.2 of a single ohm! And it depends on how long you do it for.

As I say theres no sudden arbitrary limits. Theres no particular voltage point that suddenly hurts a cell. You begin speeding up demise anytime you leave that 30 to 70% level. Its all a matter of how much you leave the safe area, for how long. Setting some arbitrary point is senseless. Thats why BMS protections are a waste of time. The correct thing is to use the correct battry thats up to the job. That 244Ah pack at 1C continuous easily copes with our occasional peak above 100 battery amps. If it drops the cell voltage lower only while under load then no big deal. Its not "real"... What matters here is discharge level not measured voltage.

If you only do it intermittently and the batt voltage off load is above 2.5V then you will not do much harm to it. Its all about how low and for how long. If its way above that 3V low point most of the day, then it should be OK and you should not see much of an issue. This all mattered a lot more whay it was costing £2k to buy a lithium battery in early 2020s. When lead was £240. Now they are half the price of lead so its not much of a worry!

[emilevirus]

That's the voltage I see from the balance lead

[Burgerman]

So its showing the cell + the voltage drop across 2 terminals when loaded driving up a hill.

So the real voltage will be someplace between what your balance lead says and the unloaded voltage. When charging for e.g the PL8 gets an accurate reading by charging with a DC pulsewidth, and measuring cell voltage during the "off" periods. This all happens at high (audio) frequencies and while that seems too fast to do for us, its easy for electronics that can do this stuff even at radio frequencies.

Or it would see a higher voltage while charging across each cell based on the cell, the connectors impedance.

[Burgerman]

Look at it this way.
The damage is caused by low (or high) STATE OF CHARGE which is not the same thing as Voltage. At least while under load because a cell has an internal resistance. To know the real state of charge you would need to remove the load and then measure it.

When charging then the same applies. Charge and terminate charge FAST at 3.65V. Or charge and terminate after say 2 or 3h at 3.50V. Or split the difference and charge 3.550V and terminate at 30mins CV. Which does the most harm? They are all the same. The lower voltage allows longer to fully charge and balance the cells and it NEEDS longer as balance is less visible and so ends up worse. The "reach" 3.65V and then terminate way has a different problem. What happens if cells are way out of balance? Then it sits with a bunch of cells at a voltage way above resting voltage for days while it balances and you cant stop the charge without stopping the balancing... This is why BMS are bad news. They may be 2A balance! And thats much slower than a 1A balance hobby charger for several DIFFERENT reasons (ask).

Damage is also be caused by high discharge currents. Again theres no sudden point! Its like this. Less is better. More is worse. Its proportional. That said lower impedance cells are better for high currents. Less drop, less hat under load. And smaller cells with lots of surface area like the old cylindrical headway cells, are also better because HEAT also damages cells. And the greater the surface area, or the better the cooling under high continual loads the better. Its why teslas have water cooled batteries. But in our case we only draw high currents for seconds at a time. So irrelivant if the batt is big enough for the job.

[shirley_hkg]

As I say theres no sudden arbitrary limits. Theres no particular voltage point that suddenly hurts a cell.
If you only do it intermittently and the batt voltage off load is above 2.5V then you will not do much harm to it. Its all about how low and for how long. If its way above that 3V low point most of the day, then it should be OK and you should not see much of an issue.

Temperature is an issue. How is it in NE of Canada ?

UVP seems totally unnecessary for a 235Ah battery. I'd set it low enough that it won't trigger under load.

UVP 2.5v
UVPR 3.0v

[emilevirus]

Not that cold now. Around 5c.

[wchl25]

So if I want to just charge the cells to 80% for longevity, how would I set the charger up to do that?

[emilevirus]

You can't do that with LifePO4 as it needs to hit 100% in order to balance. You can only balance above ~3.45v per cell as resistance is higher so the imbalance shows. What you can do is limit your charger to around 3.50-3.55v rather than 3.65v.

[LROBBINS]

If you are going to store it and not use it, you can then discharge it by e.g. 30% to 70% and store it disconnected for 6 months.