Does the Classic have Battery voltage sense wire Inputs?

[finalman]

OK so I've got my first Midnite Solar product ever on the way a Classic 150 to me in Australia.

I am looking for confirmation as to whether the Classic has battery voltage sense wire inputs and if not how do we get the Classic to correctly compensate for the voltage drop over the battery cables?

[Vic]

Hi finalman,

There are no separate Sense Leads for battery voltage on the Classics.

Just try to make the CC-to-battery cables as large as possible,  and short.

For the standard Classics,  it can be convenient to mount the Classic CC very close to the Battery breakers  (and have short leads from the breaker to the battery),  to allow very short total cable length.

The MNGP can be mounted in a convenient place,  by removing it from the Standard Classic,  which may have been placed in a place that would make using the MNGP that is attached to the Classic inconvenient.

FWIW,  Have Fun with the new Classic.   Vic

[finalman]

Thanks.

So I read yesterday something like you measure the battery voltage at the battery terminals with no PV input present and somewhere in the classic menus you enter that voltage and the classic will compensate for that instead of the voltage at Classics battery input terminals.

Is that correct?

[Vic]

Hi finalman,

Yes,  IIRC,  in te Tweaks menu  you can calibrate the Classic verses the battery voltage that you measure at the same time,  with your accurate MulitMeter.

As you would guess,  the voltage drop in the cables connecting the Classic to the battery will vary  depending upon the current that the Classic is delivering to the battery and loads.

And,  YES,  if you shut off the input,  this is the best time to calibrate the Classic's battery voltage reading.

You might want to increase the Absorb voltage setting to help compensate for voltage drop in the cable with current flowing.  AND,  also,  if you have Flooded batteries,  increasing the EQ voltage  a bit  might be a good idea,  as well.

But,  short large diameter Classic-to-battery cables will help reduce the voltage drop.

FWIW,   Vic

[finalman]

Yes,  IIRC,  in te Tweaks menu  you can calibrate the Classic verses the battery voltage that you measure at the same time,  with your accurate MulitMeter.

Is it better to do with no loads (inverter and appliances) using the battery or doesn't it matter?

[Vic]

Hi finalman,

There are some variables,  and dependencies  involved.

If you are using Sealed batteries  --  AGM or Gel,  then you would want to keep Absorption voltage at or below the Temperature-Compensated Maximum specified by the battery manufacturer.

But,  voltage drop in the cables and breakers between the CC and the battery vary with the current flow in that circuit.  Loads on the battery (the inverter and its loads) will cause the current that the CC needs to supply to vary (if there is sufficient power from the assumed PV array).

So,  much of this is not particularly clear-cut.

Flooded batteries are very forgiving,  AND,  the State Of Charge of the battery bank is quite simple to measure,  with one's Hydrometer.   With Sealed batteries,  Absorption voltage is much more critical,  and it is much more difficult to measure any effects of voltage drop/calibration issues on just how well the Sealed battery is being charged.

Sorry for the ambiguous "answer".   FWIW,   Vic

[boB]

The important thing is, what is the current when the Classic goes into Absorb and comes out of
Absorb to Float ?

Remember that in the Bulk MPPT charge stage, the battery voltage is not as important
as when in Absorb.  When in Absorb, the battery current is less than MPPT mode and
the voltage drop is lower so it is not as important as one might think.

The battery voltage reading at the start of the Absorb cycle is the part where it
would be worse because the battery current is just starting to decrease from full
current (and worse voltage measurement) at this time.  It gets better as Absorb
time goes on.

boB

[Vic]

The important thing is, what is the current when the Classic goes into Absorb and comes out of
Absorb to Float ?

Remember that in the Bulk MPPT charge stage, the battery voltage is not as important
as when in Absorb.  When in Absorb, the battery current is less than MPPT mode and
the voltage drop is lower so it is not as important as one might think.

The battery voltage reading at the start of the Absorb cycle is the part where it
would be worse because the battery current is just starting to decrease from full
current (and worse voltage measurement) at this time.  It gets better as Absorb
time goes on.
boB

Not to try to argue,  boB,

But,   for systems with significant loads during the later portion of Absorb (many of us run opportunity loads),  even when using Shunt EA via the WBjr,  the increased current that the Classic needs to supply,  and the attendant added voltage drop in the CC-to-battery cable does actually make a real difference in how well charged the battery is for that charge cycle  ...   I have whined about this previously,  in the New Product Ideas area of this Forum.

If the opportunity load was consistent,  and any other loads were low,  and time variable (switched on and off over minutes of time),  then the Vabs and or Shunt EA value could be tweaked to compensate for this.  But,  here,   the main Opportunity load is the 10K BTU A/C that tries to keep the batteries and power electronics in the Power Room as cool as possible.

One could increase the thermostat setting in this A/C,  making certain that the A/C would always cycle near the end of Absorb,  but this would raise the average temps in the power room a number of degrees F.   AND,  when the A/C does cycle,  initially,  the battery supplies a considerable amount of the power for the A/C load,  causing the battery Charge Current to diminish ...   slowly increasing as the A/C runs.  Then,  when the A/C cycles off,  that amount of discharge of the battery must be made up,  so the battery charge current increases for a few minutes,  so,  moderating these effects (at least in the Summer),  becomes more difficult,  especially with varying WX,  and would require a significant increase in thermostat setting to allow the delta battery I to settle out.

Folks that run Sealed batteries are considerably more sensitive to not having voltage sense capability,  of course,  as the Max charge voltages are much more absolute,  plus the inability to directly measure actual SOC.   We use only FLAs here,  so any absolute risk to battery health resulting from no sense capability is diminished.

I have NO complaints with the marvelous Classic CC Family,  just seems to me,  at the very least that a MN WB Plus,  or whatever,  might be able to send Sense Lead battery voltage to the Classic (or KID)  via ModBus,  or whatever fairly frequently,  and this issue could be resolved.

Know that the Classic wiring compartment is relatively busy as it is,  so thinking about adding Kelvin Leads into it seems,  to me,   unlikely.

All FWIW,  Just my read on things.  73   Have Fun,  Vic

[australsolarier]

voltage sensing cables would be a real advantage. i suppose with a 48volt system it is not so problematic, but it is with a 12 volt system with lifepo4 battery. the system goes into absorption at 75% soc. the problem is not so much the cables, but the resistance of the isolators and fuses between the midnite classic and the battery. also the midnite classic hangs in absorption mode a decimal point lower than programmed. it is not a problem with 10a charge, but it is with 40. i had to rise the absorption time to 3 hours to  fully charge the battery.

[keppelk]

i suppose with a 48volt system it is not so problematic, but it is with a 12 volt system with lifepo4 battery. the system goes into absorption at 75% soc.

What voltages are you using for float, absorb and equalise (for whole pack or individual cell)? Interested to hear a bit more about your lithium pack.

[australsolarier]

for my 12volt lifepo4 system  absorption voltage is 14 volts. the battery pack is 300ah. the whizbang jr is configured to float at 13.4 volts when charging currents goes below 6 amperes. (they are pretty much standard voltages for lifepo4's) you want to avoid very low discharges and high charging. so you are using approximately 90% of the capacity.
lifepo4 do not need equalizing. the BMS, battery managment system, is doing that. so every two months i raise absorption to 14.6 volts. the bms starts to "equalize" at 14.4 volts. so far the largest voltage difference i have found between the cells is about 60mV. but i understand in 48volt systems the differences are more of a problem than a 12v.
after the midnite classic hits float, for a minute or so the battery discharges, till it settles in again at the float voltage, so -1ampere and 99% soc.
the advantage of a 48volt system is also and fine tune the voltages more precisely.
i am planning to install a 48 system in about 2 years time and going of the grid permamently. so my small system, providing about 40 to 50% of my electricity consumption, is a good training ground.

[RossW]

I've been running a 300AH/48V LiFePO4 bank for the last 18 months, and wish I'd done it earlier.

I'm using a set of discrete cell balancers (rather than top-of-charge levelers) which keep the cells within a few millivolts regardless if they're charging, discharging or just sitting. A seperate BMS (Battery Monitoring System) lets me see individual cell volts, generates alarms if any cell gets out of whack, etc.

I've been trying to get hold of the supplier to get more, 300AH of LFP performs similarly to my old 1000AH AGM, but if I can double it up, I probably won't have to touch them again in my lifetime! (ok, perhaps once!)

They're stiff, they hold charge brilliantly, their slightly higher voltage reduces the inverter currents a little. The smaller size and significantly lower weight are bonuses. They don't gas, they don't get hot. I'm sold