Charging efficiency LifePO4

[createthis]

Hello,

I've got a GBS 100ah 12V pack (1200wh) that I'm charging up via solar. I've had it on the shelf for a few months as it came from the factory, and just this week I finally had the time to tackle bottom balancing and charging it up for the first time.

After bottom balancing, I started charging it up using my Midnite Classic 150 charge controller with the Bulk (CC), Absorb (CV), and Float (CV) voltages set to 13.6V and EQ and temp compensation disabled. For two days I charged it like that using just a single 100W solar panel. I got maybe 6 hours of between 60-70W input. Looking back at the history graphs, the Midnite Classic says it drew .1kwh from the PV on the 9th, .5kwh from the PV on the 10th (I added a second 100W panel half way through the day for about 120W actual input), and like .9kwh on the 11th (because half way through the day I added two more panels for a total of 400W input, ~280W actual).

Now, by my math, that's 1.5kwh. The classic draws at least 5W at idle times 14 dark hours times 3 days = 210wh.

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

My pack should be full, right? Charging voltage today was maxing out at like 13.65V in the constant voltage phase and my resting voltage after about 10 minutes idle is about 13.41V. I checked each individual cell and they're all at the exact same voltage.

I'm not worried my pack is damaged yet. It seems fine. But why doesn't my math add up? Is the charging efficiency of LifePO4 much lower than 99% when charged below .1C? Or is something else going on?

Thanks!

[PNjunction]

You are close but not quite full - if one wants to play the voltage game, which as we know has issues. But...

The canonical end-amps for absorption with LFP, is .05C. With your 100ah battery, and 100W panel, that is best case about .05C. So you are basically end-amp absorbing your entire charge!. Ok, that being the case, you want to be conservative with your upper voltage, which you are.

However - for a "full" charge for GBS, if one wants to play the voltage game, should come to *rest* at about 3.38v after 12 or more hours with no charge. You are slightly beneath this.

What I'd do is instead of running at 13.6v total (3.4v per cell), is raise it slightly to 3.45v per cell (13.8v total). A fully charged LFP needs to be charged to at least 3.4v, and allowed to absorb until .05C. You are already there with current, BUT to actually get to 3.4v, one needs to run with a higher charge voltage of 3.45v. You won't actually ever fully charge if you set to 3.4v instead of 3.45v - you gotta' be a little higher to actually force current to reach the target.

AND, since you are bottom balanced, all you have to do is stop when the FIRST cell reaches 3.45v, since you started out with a low end-amperage figure.

Even with your secondary panel attached, you can still stay conservative, but if you want to play the voltage-vs-full game, 13.8v (3.45v per cell, or in your case first cell to reach that) will do the trick at your low charge rate. You can verify by *resting* the battery for 12 hours at least, and you'll see your GBS cells sitting around 3.38 - 3.39v.

Now run a load, and calculate your runtime to see.

Of course, the whole lead-acid mentality of "full" takes on a different meaning with LFP, as in "is there enough capacity to do what I want with the way that I charge them?", rather than obsess over keeping them truly fully charged as with Pb.

And once again, with LFP all the voltage banter is MEANINGLESS unless you trust your voltmeter and that of your charge controller to be accurate. In all cases, when I talk about voltages, I'm using FLUKE's, (from low end 11x series to 87V's) and measured at the cell terminals as the standard for all the rest of the gear. Have you vetted your Midnite Classic with a voltmeter standard you trust?

[PNjunction]

Quick followup about your .05C charge current and wanting to get to a "full" charge:

Ordinarily, one charges faster than .05C, and there is a direct relationship between charge current vs top-end voltage which is then allowed to taper naturally to .05C and STOP. You are already starting out at .05C with your setup, so a lower voltage and stopping when that first cell voltage is reached, is the way to go.

"Absorbing to zero amps", if you reach 3.45v or more under charge, is a BAD thing. Not that it is electrically impossible, but from a parasitic-reaction standpoint vs the miniscule amount of extra charge you'd obtain by absorbing-to-zero amps, the parasitic reactions are exacerbated needlessly by extending the time spent at full charge.

If you do an absorb-to-zero once or twice, no big deal. BUT, over the lifetime of the battery, doing this often just extends the parasitic-reaction time clock, which is bad.

[karrak]

The overall (charge/discharge) coulomb (current*time efficiency) of prismatic LFP batteries is around 99%. From my logged data over a period of around eighteen months I have 99.4%.

The overall power efficiency which is a combination of the coulomb efficiency and the voltage efficiency for my battery is around 95%. This matches the figure I have seen in allot of literature on the subject. For just the charging part of the cycle given the low charge current I would say that your charging power efficiency should be between 97-98%.

Simon

[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

[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.

[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]

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

[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

[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

[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]

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]

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]

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]

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.