Midnite Classic 150 — floating below Float voltage.

[William.M]

I’ll admit I haven’t scoured the forum before posting this, so if the answers to my question are already in another thread, please let me know.

I have a new system that’s using 48V AGM SLA batteries (eight 6Vs in series), and, as the subject line indicates, my Classic 150 charge controller is not floating at the set voltage value (54.4V). It usually cruises right around 53.9V after it’s started floating and then just gradually drops down til the next day â€" the batteries are usually at about 50/51V the next morning when it starts to Bulk again.

Maybe I’m worrying over nothing, but isn’t it important to ensure the batteries are floating at full voltage in order to make sure the batteries are being charged fully each day? Since the system is new, I want to make sure I get the most life out of these batteries by not abusing them from the get-go. Can anybody comment on the seriousness of floating under the set Float voltage value and perhaps give some guidance as to how I could achieve floating at 54.4V?

I was thinking that setting an end amps parameter for the Absorb stage might help with this (potential) problem. However, I’m not sure how to calculate the correct end amps value for my system. Right now I have my absorb stage set to 3 min long, as I’ve heard that AGM batteries don’t like being overcharged / held at high voltages for extended periods of time (by the way are those the same thing?).

Your wisdom and insight is greatly appreciated. The intricacies of solar power are an enigma of which I’ve barely scratched the surface.

[Vic]

Hi William,   welcome to the Forum.

First,  if the Float voltage,  as read on the display on the Classic is below the setpoint,   and the charge stage shows Float,  not Float-MPPT,  then it may well be,  that the batteries are warmer than the Reference temperature.   If the charge stage shows Float-MPPT,  then there is too little input power (usually from your solar panels),  to maintain the set Float voltage.

Are you using the Battery Temperature Sensor (BTS),   attached to the side of one of the batteries?
Have you set the Temperature Compensation value in the Classic,  as recommended by the battery manufacturer?

Battery charging heats LA batteries.  It can take some time for this heat to migrate from inside the battery to the surface of the battery cases,  and delay any effects of temperature compensation.

More later,   Vic

[William.M]

Hi Vic, thanks for the speedy reply.

Yes, in the situation I’m referring to the display reads ‘Float’ (not ‘Float MPPT’). If a big load kicks on while it’s floating then it will switch into Float-MPPT. Usually the voltage doesn’t drop much in those cases because my panels can produce about 5kw, and the biggest load that I have hooked up is a 2kw electric water heater which is on a timer to run once while the controller is in Bulk stage and a second time at midday. Anyway I’m straying off topic now. My concern is that while in the Float stage the controller display has been reading 54.0V or less as opposed to 54.4V.

Yes, my BTS is installed, and I have the T-Comp value set to -4mv as per the manufacturer specs (they have T-Comp values listed under Float Charging Voltage and Cycle Use Voltage. The T-Comp value under the Cycle Use Voltage is -4mv, and -3mv is listed under the Float Charging Voltage. I figured float charging refers to if you’re leaving the batteries in float for extended periods of time, and cycle use refers to when you’re cycling the batteries regularly which is what I am doing).

So let me try to reiterate what it is you’re saying: as the batteries are charging they heat up, and that increase in heat may not be detectable right away by the BTS. This could mean that in fact the batteries are floating at the correct voltage (54.4V) for a while even though the display only shows about 54.0V before it begins to fall steadily throughout the day..? â€" that potential delay in temperature compensation being the reason why the display only reads 54.0V while the battery has actually reached 54.4V?

To me it would seem a more likely theoretical scenario would be that the machine would read 54.4V while in actuality the battery reaches 54.8V. The attainment of those extra 0.4V being the result of delayed T-Comp effects. You’re saying the controller is smart enough to do its job well? Or have I completely missed the mark in my understanding of what you said..

[ClassicCrazy]

Did you follow any manufacturer recommendations for breaking in the batteries ?
Some you need to cycle down a few times to get full capacity .

Larry

[boB]

Typically, Absorb time wants to be a couple of hours even for AGM batteries.  Typically.  But whatever the manufacturer says to do there.

There is no intentional delay for temperature compensation.  What does the TEMPS menu say the batterie's temperature is when you see the lower float voltage ?

Also, in Absorb and Float (not MPPT) the voltage can be the temperature compensated voltage or possibly a tenth or 2 of a volt below that target set point.

Then below that tenth or two below, the Classic will return to FLoat-MPPT or Absorb because it cannot be near enough to the set point voltage.

You can VIEW the temperature compensated target voltage in the CHARGE menu....  Now I may not get this right but check the T-Comp menu and press the soft key under VIEW (top right I think) and it should show you the voltage that the Classic thinks it should be going to...  Plus zero and minus a couple of tenths of a volt.

[William.M]

@Larry: Yes, I left the Absorb stage run time at two hours for a couple of months to break in the batteries per the instructions of the dealer.

@Bob: Right now it’s floating at 54.1V, and the TEMPS menu displays the number 27.9 below the word BATTERY. Meaning the battery is 27.9°C I’m guessing. Can anybody tell me what the FETS and PCB temperature values are referring to? I think I remember reading that one of them is the internal temperature of the controller or something like that...

See that makes sense to me, but my controller doesn’t do that for some reason. While there’s still plenty of sunlight around 2 or 3pm, the float voltage will have dropped to somewhere around 53.5V but it never kicks into Float-MPPT mode to boost it back up. I did notice the default Rebulk voltage setting is all the way down at 8V, but Rebulk would start a whole new cycle as opposed to Float MPPT which would just be letting a little more juice in to boost up the voltage back toward the target float voltage. Can anybody think of a setting which isn’t allowing it to switch into Float-MPPT when it’s floating down at 53.5ish V?

Okay in the VIEW menu of the T-Comp menu, it’s saying that the TEMP COMP SETPOINT is 54.2V as it’s floating at 54.0V. Is that TEMP COMP SETPOINT a value that I can or should change? Or is it  automatically calculated based on my input for the Float voltage?

[ClassicCrazy]

Set temp compensation value as per manufactuer  . It is -3 mv for me.
So in the MNGP  Charge/ T-Comp setting I put in -3  . The classic will do all the rest figuring out how many cells to compensate for based on battery voltage.
Then in the temperature compensation setpoint view I think you are looking at it is showing you the Classics Temp compensated setpoint ( above or below what you entered as the setpoint depending on temperature )

If you have an Android phone I recommend getting Grahams  free Classic Monitoring app . It shows all the info you are going to want to see about what is going on in the Classic and your batteries.

Larry

[boB]

If your temp-comp is set to -4.0 mV per degree C per cell and your battery temp is almost 28 C,  that would be 4mV * 24 * 3 = 0.30 volts less than your Float and Absorb set point voltages.  Those set points are based on 25 degree C batteries so it sounds like the calculation in the Classic is about right.

Actually it comes out to a negative 0.288 volt offset.  Then, after it is in Float or Absorb, it can drop 0.2 or 0.3 below that voltage before it goes back to MPPT again.

The reason for bit of negative voltage allowance is to keep it from going from MPPT to Float or Absorb and back again all the time.  And it keeps the Absorb counter timer running.  0.2 volts or there abouts is close enough especially at a 48V battery setting.  It is not exact.

Remember that the old standards from Trace Engineering did had barely a plus and minus 0.4 volt resolution on their 48V battery voltage readings.  But we can do a bit better than that these days.


One thing that I like to watch when using AGM batteries is to make sure one battery is not too far off voltage and temperature wise from the others.  AGM batteries like to "plump when you cook 'em"  as I say.

[Vic]

William said,  "...   While there’s still plenty of sunlight around 2 or 3pm, the float voltage will have dropped to somewhere around 53.5V but it never kicks into Float-MPPT mode to boost it back up. I did notice the default Rebulk voltage setting is all the way down at 8V, but Rebulk would start a whole new cycle as opposed to Float MPPT    ...   "

When a charge state ends with -MPPT,   that means that there is not a sufficient amount of power input to the CLassic,  to reach or maintain a setpoint for that charge state.

SO,   when you see Float-MPPT,   that means that the Classic is trying to keep the Float voltage at its setpoint,   but it cannot reach it due to too little input power.

It does seem that the reason for the reduced voltage that you see on the MNGP display (or Local App),  IS   due to the battery temperature having risen,  during battery charge,  and/or,  the ambient temperature in the environs around your batteries has risen.   This change in charge voltage  vs  battery temperature IS because the Classic is doing its job of compensating charge & Float voltages,  for the temperature that the BTS is measuring.

But,  to that earlier question:
Where have you mounted the BTS for the Classic?

Thanks,   Vic