Calibrating Voltage - Tweaks

[Muskoka]

I've never understood how this is supposed to work.

The MNGP battery voltage reading never seems to be happy. It's always 2/10's high, or low, depending on the state of the battery. Resting, it's 2/10's low, while charging it's 2/10's high. If I adjust it while in rest, but still a high'ish voltage, it will match my meter, but gradually get out of whack once the battery starts depleting, or charging ramps up.

When I meter my battery terminals, and the terminals in the Classic, they pretty much always match. But the MNGP is always out. When are you supposed to adjust the voltage in the tweaks menu? And how is that affecting the charging perameters, if at all. Is the voltage displayed on the MNGP just a close guess as to what the acutal battery voltage is, or is that value critical to changing charge modes, I sure hope not cause it's always different from metered readings, and never seems to be accurate once the battery is drawn down, or during charge.

Should also say, on these Lithium batteries, I don't want the readings to be any 10'ths out. It's disconcerting when I look at the voltage of my battery, and it doesn't match the MNGP displayed voltage, especially the transition times around Absorb and Float. It happened today, the controller went to Absorb, and the battery was still 2/10'ths away from truely transitioning. When I adjusted the tweaks menu to match the battery terminals, the MNGP was then 2/10's above that new tweaked value. It doesn't seem to matter when I apply a tweak tp battery voltage, it's always out, at the high end, or the low end.

[ClassicCrazy]

The voltage drop of your battery cables are most apparent when the voltage is highest and at  the most current . For your lithium batteries , this is when they are at peak of Absorb. So you would use the tweaks setting Voltage Battery offset at that time to bring the voltage of the batteries to the same that the Classic is seeing.
You are correct , that means the voltage will then be off when the voltage drops and no big current. There isn't really much I know of that you can do about that.
But since the only setting that will matter much at that point will be float , but you can raise or lower your Float setpoint as needed to do the control that you want.
The Classic doesn't have voltage sense which are independent wires to sense the battery voltage at the battery terminals which would eliminate what you are talking about. The Classic was designed for lead acid batteries which really weren't affected  much by the small voltage differences.
Larry

[Muskoka]

Thank you Larry. Problem is, it only seems to be the MNGP thats continually out, different. When I meter the battery terminals, or the terminals in the Classic, they match, it's the MNGP battery voltage displayed that doesn't agree. Is the MNGP displayed voltage used in the programming? To me it doesn't make sense that the battery and Classic terminals agree, but the MNGP doesn't match them.

[ClassicCrazy]

Thank you Larry. Problem is, it only seems to be the MNGP thats continually out, different. When I meter the battery terminals, or the terminals in the Classic, they match, it's the MNGP battery voltage displayed that doesn't agree. Is the MNGP displayed voltage used in the programming? To me it doesn't make sense that the battery and Classic terminals agree, but the MNGP doesn't match them.

I am not sure how to answer that question . The MNGP and the terminals in the Classic should match unless the tweaks is set for an offset.
What kind of meter are you using ? I had a meter that was off and I didn't realize that until I got a better meter !
Maybe Bob can weigh in on this.
Larry

[Muskoka]

Its a Greenlee DM-820. My batteries are very close, 2ft from the controller, there's no loss in the 2ft, battery terminals always match the Controllers terminal voltage. The MNGP never matches that voltage, and when adjusted it's higher on one end, lower on the other end. I can make it match it the middle, not under load or charging, but thats not really a critical spot for accurate voltage.

Like I say, I did adjust it today when the bulk was ramping up, and it matched, for a while. Then the controller transitioned to Absorb, but it was 2/10ths too early, according to the battery terminal voltage. But a 1/2hr or so before that I had adjusted the tweaks to match. It should be set, and forget.

And yes, I do have the sense wires on my Tristar, and its always spot on.

[ClassicCrazy]

Its a Greenlee DM-820. My batteries are very close, 2ft from the controller, there's no loss in the 2ft, battery terminals always matche internal terminal voltage.

What awg are your battery cables and what is the maximum amps going through  them ?
Larry

[Muskoka]

2ga from the busbar to the battery, 4 awg from controller to busbar, end modified to fit in the controller, about 2.5 ft total. Lets call it 3' to account for a bend. I've seen low 50's in the winter. Roughly 1540 watts in panel, not all together but basically the same angle to the sun, pointing south.

[ClassicCrazy]

The Classic will sometimes go to Absorb early with lithium batteries.
I think this has been discussed before.
It is just something that has to be compensated for in the absorb setpoint.
There should be some leeway in that setting for your batteries.
Larry

[Muskoka]

Thanks Larry. I can work around it, kinda always have. I watched a couple videos on adjusting the tweaks, but nothing was said about how well it worked. I've always have voltage discrepancies, but thought I'd ask incase I was missing a step.

Ya, I'm still very new with these batts, I'll get used to how they, it,  reacts.

[ClassicCrazy]

It has taken awhile for me to understand all the ins and outs of lithium batteries and how they work . Then toss in the bms that are all a bit different with different settings that make it not quite the same when people talk about lithium batteries of different brands. And of course there is always a lot of technical debate about all these things with different opinions and approaches. I am getting to trust Off Grid Garage suggestions since he usually backs things up with some detailed testing that he shows.
Larry

[Muskoka]

I do enjoy watching his videos, never a shortage of data.

[boB]

The Classic's voltage reading may need a bit of calibration.

The tweaks menu voltage offset adjust should help and possibly make the Classic (MNGP) read the same as your Fluke meter on the battery terminal connector of the Classic but only within a range of voltages.

Since you have a 24V system, the offset should help for at least a few volts.  The range of battery voltage offset should be adjusted right around your Absorb voltage set point which is the important voltage.

You may have to adjust the trimpot inside the Classic to get it better.  Your Classic may not have been calibrated as well as it could have been up in Arlington.  I have been working on this aspect down here in Phoenix, AZ and it can be adjusted to be pretty darn accurate between say 12V and 60V battery range. What I need to do is to make a voltage calibration paper for ones that are not as good as they can be but are already out in the field.

The transition voltage for Absorb and Float is another issue.  I am almost ready to release a Classic version of code that will help this a lot.  There is also a Lithium battery selection that will tighten that transition voltage up by a factor or 2 or 3 over what you have now.  It's been too long in coming for that but I will let folks know how to get that beta in hopefully just a couple of days.

The charge controller has to have a slight bit of hysteresis at and just slightly below the Absorb and Float set points.  BUT as I have found, it can and will be much better for lithium batteries.

I am going on a cruise and will be away for about 18 days starting Tuesday next week so I hope I get this out before then.  Monday most likely.

But the beta won't help if your battery readings are off too much.  Sorry you have had to futz with this.

boB

[ClassicCrazy]

Have a nice time on your cruise Bob !
Larry

[Muskoka]

Thank you Bob for the information. I've definitely looked at the pot inside, haven't touched it, yet. Supposed to have at least 4-5 more good looking sun days, so I can at least test.

And yes, have a great time on your cruise.