Charging efficiency LifePO4

[createthis]

I don't have a Midnite Classic but I would have thought that if you have programmed a float voltage of 13.6 volts that it will keep the battery voltage at 13.6 volts as long as the sun is up and the current going into the battery will taper off to nearly zero. With the current at zero your battery will be around 99% full.

Simon

The Midnite Classic did indeed enter a resting state and stayed there as the voltage fell below 13.6v toward it's final resting voltage. I didn't let it sit long enough to find the final resting voltage. I plugged in my inverter and charged my laptop at that point. The Midnite Classic kicked back in and supplied all of the inverter's power via solar. I'm mostly pleased with how this is working. However:

I'm disturbed that you keep telling me my battery is 99% full. That wasn't my intention with this. I'm worried that will mess up my bottom balance.

I did my homework (or tried to) before this. In this post, PNJunction indicates that a charging voltage of 3.45v per cell (13.8v) will yield a 90% SOC: https://www.solarpaneltalk.com/forum...138#post302138

By using a 13.6V charging voltage I was hoping to even be a little more conservative than that.

[karrak]

I was looking at the blue line. Isn't that resting voltage?

There are not enough data points on the blue line. We need some more data points between 90% and 100% to get a more accurate figure at the voltages you are looking at. We also haven't been told how they performed the test so we don't know exactly what their "resting voltage" really is.

[karrak]

I was expecting the Midnite Classic to enter resting state, but now I'm wondering if it will continue to float with 0 amps input or something.

I don't have a Midnite Classic but I would have thought that if you have programmed a float voltage of 13.6 volts that it will keep the battery voltage at 13.6 volts as long as the sun is up and the current going into the battery will taper off to nearly zero. With the current at zero your battery will be around 99% full.

Simon

[createthis]

The yellow line is a charging voltage, a resting voltage of 3.35 V/cell is more like 98%-99%

I was looking at the blue line. Isn't that resting voltage?

[createthis]

I'm finally seeing the charge amperage fall this morning. My four 100w panels are only putting about 20w into the battery in float mode. So it's looking like my charge cycle is almost complete. I'm not sure what will happen at the end of the cycle however. I was expecting the Midnite Classic to enter resting state, but now I'm wondering if it will continue to float with 0 amps input or something.

[karrak]

Yeah, so I can expect a final resting voltage of 13.4v (4s) or 13.35v per cell, which is 92% soc according to that chart. Ok. Yeah. I'm fine with that.

The yellow line is a charging voltage, a resting voltage of 3.35 V/cell is more like 98%-99%

[createthis]

Resting voltage is around 0.05 volts less per cell than the charging voltage up to a charging voltage of 3.45 V/cell at low charge currents

Yeah, so I can expect a final resting voltage of 13.4v (4s) or 3.35v per cell, which is 92% soc according to that chart. Ok. Yeah. I'm fine with that.

[karrak]

If I charge at 13.6v, will the battery come up to that voltage when the charge is done? Or do you have to charge a few Volts higher to achieve a 13.6v resting voltage?

Resting voltage is around 0.05 volts less per cell than the charging voltage up to a charging voltage of 3.45 V/cell at low charge currents

[createthis]

If I charge at 13.6v, will the battery come up to that voltage when the charge is done? Or do you have to charge a few Volts higher to achieve a 13.6v resting voltage?

[karrak]

You all misunderstand me. I'm not trying to get a full charge. I went into this expecting to get an 80% charge because I'm bottom balancing. I'm confused because the battery appears to be absorbing way more than the 80% I expected.

If you look at the graph below you will see that 13.6 volts (3.4V/cell) is equivalent to around 99% full

I have graphed SOC v Cell Voltage from two sources. The data for the blue line comes from here advrider.com/index.php?threads/motorcycle-batteries-agm-gel-wet-lithium-iron-phosphate-lifepo4.757934/page-9#post-18206897"]http://advrider.com/index.php? and the data for the yellow line comes from some tests I have performed recently.

My tests involved setting Bulk=Absorb=Float to the same voltage at the start of the day and taking an SOC reading when the charge current had dropped to around zero.

SOCvVoltage.jpg

[karrak]

You all misunderstand me. I'm not trying to get a full charge. I went into this expecting to get an 80% charge because I'm bottom balancing. I'm confused because the battery appears to be absorbing way more than the 80% I expected.

If you look at the graph below you will see that 13.6 volts (3.4 V/Cell) is equivalent to around 99% full.

I have graphed SOC v Cell Voltage from two sources. The data for the blue line comes from here http://advrider.com/index.php?thread...#post-18206897 and the data for the yellow line comes from some tests I have performed recently.

My tests involved setting Bulk=Absorb=Float to the same voltage at the start of the day and taking an SOC reading when the charge current had dropped to around zero. SOCvVoltage.jpg

[createthis]

Also, yes, I'm using a fluke 87, but no, I haven't calibrated the Midnite Classic against it.

[createthis]

Should I expect to see the Absorb (CV) current taper off to zero at 13.6v? And when that happens, will the battery be not quite 100% charged because my voltage set point is too low? That's what I expect to see happen. I'm just weirded out by how many kW hours are going into this thing. I only expected 900wh or so to go in and I'm way over that.

[createthis]

You all misunderstand me. I'm not trying to get a full charge. I went into this expecting to get an 80% charge because I'm bottom balancing. I'm confused because the battery appears to be absorbing way more than the 80% I expected.

[karrak]

A couple of minor corrections to your logic.

I've got a GBS 100ah 12V pack (1200wh)

The average resting voltage of a LFP cell is around 3.27 volts so the power stored in a 12 volt LFP battery is around 1308Wh (3.27*4*100)

So, 1500wh - 210wh = 1290wh. I've always read LifePO4 is 99% efficient, so that's 1290 * .99 = 1277wh.

Should be 1290/0.99

Simon