Charge Factor and Efficiency

[timmartin]

When I first got my FNDC and Mate2 I mostly ignored these numbers as I figured they would take a while to be accurate (They need a fair amount of data I supposed). But now I've had the FNDC for two years and have been using the Mate3 for a few months. Both these numbers are in the 40% to 50% range. This "seems" bad to me, but perhaps I just don't understand what these numbers are supposed to be.

Cycle Charge Factor compares the amp-hours removed from the battery and those returned to the battery while charging. It displays the comparison as a percentage. This number can be compared against the programmed charge factor (see page 112) to judge battery charging efficiency.

My charge factor is set to 94% or something close to that. So how do you compare these numbers to judge efficiency? It says you can, but it doesn't explain how to compare them, or what is considered good versus bad. I thought this is the real-world recorded number of the Ah in and out, so it should be similar to my programmed charge factor. But it's not, it's very very different.

Cycle kWH Charge Efficiency compares the kilowatt-hours removed from the battery and those returned to the battery during all activity (such as float charging). It displays the comparison as a percentage. This number can be used to judge overall battery efficiency.

Again, how do you use this to judge efficiency? Is a high number good? My number is below 50% so that sounds very inefficient to me. My system seems to be running perfectly and we always have plenty of power. But these two numbers have always been a mystery to me.

[Kent Osterberg]

Tim,

What is the typical daily depth of discharge on your batteries? Charge efficiency isn't really a constant, below 14 volts the amphour efficiency is probably close to 100%. Above 15 volts, it is probably less than 25%. If your batteries aren't cycled very deeply, the charge efficiency will be low. Don't worry about it. Just make sure that the batteries are being brought to a full charge frequently.

The kWh efficiency will always be lower than the amphour efficiency because the battery's charging voltage is higher than the discharging voltage.

[timmartin]

Tim,

What is the typical daily depth of discharge on your batteries?

20% on a day with no sun. Maybe 10% overnight between sunny days.

Charge efficiency isn't really a constant, below 14 volts the amphour efficiency is probably close to 100%. Above 15 volts, it is probably less than 25%. If your batteries aren't cycled very deeply, the charge efficiency will be low. Don't worry about it. Just make sure that the batteries are being brought to a full charge frequently.

Okay, Indeed they are. Typically they get fully charged every day.

The kWh efficiency will always be lower than the amphour efficiency because the battery's charging voltage is higher than the discharging voltage.

I don't really understand what the math is that it's doing, but okay. I'd love to simply know how it's computing these numbers that way I could better understand what they really are.

[Kent Osterberg]

Here's a link to an article from Home Power that may help. On the graph of voltage while charging, observe that the C/20 curve at 50% state of charge is at 13 volts. On the discharge curves, C/20 at 50% state of charge is about 12.3 volts. The 0.7 volts that is one cause lost energy or power. If C/20 is 20 amps, which is about right for a L16 battery, it takes 260 watts to charge the batteries at C/20 but during discharge at C/20 the batteries only deliver 246 watts. Do the same exercise at 90% and you'll see the voltage losses are even greater. So far we've only looked at losses related to the fact the the voltage changes, it is lower during discharge than it is during charging. There are also amphour losses that reduce the energy efficiency. Both the amphour losses and voltage losses get large when the battery is above 90% SOC.

Don't trust the curves in this article is be accurate for your batteries or for any battery; every battery is different. In fact the discharge curves look a little too high to be believe. But the basic shape of these curves is realistic and useful for understanding battery behavior.

[timmartin]

This number can be compared against the programmed charge factor (see page 112) to judge battery charging efficiency.

This was never really answered. Or I'm confused... because I understand everything that's going on, and I understand how DoD makes a difference, but how can you use with your set charge factor to judge battery charging efficiency? What can you DO with this number that is at all helpful?

[Kent Osterberg]

Tim,

I don't think there is a good answer. Batteries are simply more complicated than the algorithm in the FNDC can account for.

[tallgirl]

I believe the "charge factor" is based on amp-hours, and tends to be much more accurate than anything involving watt-hours.

If I had a short answer / analysis it is this -- people focus entirely too much on watt-hours.

[JRHill]

I've not been able to find a clear explanation of what a change to the CHARGE FACTOR actually does - only guesses. Has anyone found a definition more clear than the one in the Mate3 manual?

Thanks,
Jim

[SteveHiggins]

Jim,

Are you talking Battery Charge Factor? The setting was defaulted at 94%?

[JRHill]

Are you talking Battery Charge Factor? The setting was defaulted at 94%?

Hi Steve,

Thanks for chiming in here. In the time since this discussion was active I was again addressing the absorb time setting that worked best for my batteries. For a number of years I was under charging due to too short of an absorb cycle (the consequence of which cost me a new set of batteries at just over the four year mark). After replacement, I realized that things worked much better if I had a much longer absorb time. the SGs were more consistent and much closer to a full charge but still not there esp. with relatively new batteries. But I was having trouble absorbing for any longer as the charge cycle kept ending before I could get a full charge per the SGs. I was pointed to check the charge cycle control and disabled it. I had also changed the Battery Charge Control Factor setting from 94% to 90% and monitored the system - it didn't seem to do anything.

So, I was absorbing for 5 hours and the SGs look great EVERY DAY and much more consistent across the bank. I've never had this happen before and I was not really using that much more water. This is the point at which I asked the question of what the battery Charge Cycle Factor setting actually did if changed?

In the time since, what I (TBD) learned is the battery Cycle Charge Factor is useless if I was not charging the batteries correctly in the first place. As of this writing I still have my CCF set at 90% and the Cycle kWH Charge Efficiency has bottomed out at 85%. I could probably dial back the absorb time little by little while watching the SGs at cycle end and find the exact point in time. I am guessing the reported efficiency would improve a few points.

I write this explanation out so it may help someone else as there are many discussions about battery charging on this forum. But, frankly, I still can't figure out what the Charge Control Factor actually does ;-)

Best,
Jim

[SteveHiggins]

Jim,

The BCF% is basically the battery charging efficiency if you take out 100 amps and the BCF% is set to 90% then when 110 amps went back into the battery, it's full. Generally flooded or lead based batteries are 80-85% efficient when they are new and in good health...after about 3 years you lose about 1% per year.

Where this effects the FNDC is it uses three parameters to determine a full charge and where to set the SOC%...

BCF%
End Amps (Parmeters Met for Timer)
Absrob Timers..

If anyone of those three are met and the FNDC sees a draw on the batteries for 60 seconds the FNDC sets the SOC at 100%, 60 seconds in the past...

The issue with most and the FNDC is most people have these settings wrong, and generally these settings change depending on how hard the clients use the batteries. You can put 10 different customers in your home, and you will have 7-8 different programmed set points... the is no one magic number you program to that works for everyone....

I see you have Trojan Batteries... what are your Specific Gravity after a full charge and when the FNDC reads 100% or close?

[JRHill]

...what are your Specific Gravity after a full charge and when the FNDC reads 100% or close?

Ahh - now that's a curious question. The answer depends on how far into the absorb cycle I take the SGs when the FNDC shows 100% or close. If I take the SG reading as soon as the FNDC shows 98 to 100% I'll be at around 3 to 3.5 hours into the absorb and the reading will be low - about 1.250 to 1.255. If I take the SG at around the 4.5 to absorb end/float I'll be at 1.270 to 1.275. I have reset the FNDC multiple times when the batteries are full per the SGs. I just can not get the top end of the SOC to correlate with the SGs.

But this is OK for me. If I've seen anything often repeated it would be Tim, aka Blackswan555, saying not to trust the reported SOC but to get out and take SGs. I tried to fight against his advise 'cause I'd rather pick lint off the walls than take SGs). But, sigh, he's right on the money (or should I say battery budget).

Thanks for your input,
Jim

BTW, the dealer I use must move a lot of Trojans so the pricing is good and that's what I've purchased as a result for the house and the well systems. Since he knows I'll be back I get a little tickle here and there. No offense plz!

[SteveHiggins]

Stop resetting the FNDC by unplugging and re plugging... Let it learn where the top end points are...

Your Absorb timers should be around 4.5 hours... but I would also adjust your BCF to 80%

End amps should be about 1.5 to 2% of the bank capacity, with the Charged voltage .2 to .4 below the absorb set point.

You do this, after about 10-20 cycles the SOC should get closer... It'll never be perfect because the FNDC doesn't take in effect peukerts.

Good Luck!

[blackswan555]

But this is OK for me. If I've seen anything often repeated it would be Tim, aka Blackswan555, saying not to trust the reported SOC but to get out and take SGs. I tried to fight against his advise 'cause I'd rather pick lint off the walls than take SGs). But, sigh, he's right on the money (or should I say battery budget).

Nice one,,,, It took a while (I also hate doing SGs) But you time sounds just about perfect, If you are consistently hitting good SGs every day, May be worth trying backing your voltage down a notch or two & see if you can get the same result, A little kinder on the batteries & a little less watering maybe, but just a small detail,

Tim

Did I mention DO NOT TRUST YOUR FNDC !! Go check you SGs

[JRHill]

My system has been reliably charging to full (per SGs) for about a month. Steve, I haven't reset the FNDC in the last three weeks. My FNDC is a slow learner ;-)

For my purposes, I have saved this Mate3 profile as "Summer.xml" 'cause the days are coming when these parameters just won't work with the transition to generator season. There are four months when I can not get the batteries charged with solar. 1.5 of these months I only get half an hour of sunlight per day as I'm in the shadows. When this happens, I just can't have a daily four to five hour absorb for obvious reasons and the "Days since met charge params" output will be handy.. Based on previous winter seasons I'll do an extended absorb for at least two days per week on the "Winter" profile. In short, I'll be charging the batteries longer and lighter for the summer and, in the winter, shorter and heavier.

So, this question: before changing profiles, should I reset the FNDC? I'm thinking yes....
And another: what's the difference between resetting the FNDC to factory defaults from the menu vs. un/replugging it before changing profiles?

Thx folks,
Jim

[SteveHiggins]

Well, as for replacing water in batteries... It's much less costly to replace water than replacing batteries. With Flooded Lead Acid generally if you are cycling them once every few days they really want to see a full charge at least ever 7-10 days. The harder you work them the more often they want to see that full Spg...

If you just unplug from the hub that'll reset the soc%

If you reset to factory defaults you'll reset the soc% and the internal programmed settings.

The FNDC isn't like the mate there there are buried settings all over the place, there are really only a few to play with so resetting isn't usually required.