Rebulk Guidance

[SM-Viper]

Hi all!

I'm looking for a bit of guidance as to what some of you have for the Rebulk setting in the Kid.

As I understand it, this is the point at which the load pulls the battery down to a certain voltage, and the the CC should go from FloatMPPT (when in sun/charging), toBulkMPPT. Right?  What's a good voltage for that to happen while I'm running loads?

Also, if someone could clarify for me what the difference is between FloatMPPT and BulkMPPT, that would be most helpful.

Thanks!

-S

[Vic]

Hi SM-Viper,

ReBulk  is an additional Bulk charge stage on a given day.  If one has a system with limited battery Capacity,  and somewhat high daytime loads or inclement weather,  then,  perhaps trying ReBulk might be an OK idea,  because it has the promise of allowing the battery bank to be more fully-charged when night arrives.

In general,  batteries are rated for a certain number of charge cycles,  so the more charge cycles in a given day,  the more rapidly your battery bank might be "consumed" -- die in fewer months or years than if limited to one or fewer charges per day.

But,  if your battery bank is small,  compared to the demands on it,  and perhaps during inclement weather,  ReBulk might help a bit.

Have never had a need to use ReBulk,  and finding the best voltage setting for it may take a bit of experimentation.

And,  regarding the MPPT suffix on the charge stage noted on MidNite (and OB)  CCs,  this generally  an indication that there is not sufficient available input current to maintain (or reach)  a regulated voltage stage.   Bulk,  by definition,  is always a stage that is limited by too little input current to (yet) reach the next stage,  where voltage regulation is attained -- Absorb.  And,  EQ - MPPT  means that the CC is in the beginning of the EQ stage,  but has too little current to yet reach the EQ target voltage (this where the CC will display EQ).   Float - MPPT is where the float stage has been attained,  but after a period of time,  there is too little input current to maintain the Float voltage,  and  the battery voltage will begin falling below the Float setpoint.  At times  Float - MPPT is triggered by a load on the inverter,  and when this load is reduced,  the battery will have any current that it was supplying,  to the loads replenished,  and perhaps return to Float ...   and so on.

My take on things,  anyway.   FWIW,    Vic

[vtmaps]

Also, if someone could clarify for me what the difference is between FloatMPPT and BulkMPPT, that would be most helpful.

I think the -MPPT suffix means that the controller is tracking the maximum power point of the array.  It does that when it wants all the power your array can produce at any moment. 

That leads me to wonder: what does it display with an oversized array or with current limiting?  I presume, (but don't know) that during bulk charging if the Kid is putting out 30 amps it will not display the -MPPT suffix even though the voltage is still below Vabsorb.

I'm looking for a bit of guidance as to what some of you have for the Rebulk setting in the Kid.

As I understand it, this is the point at which the load pulls the battery down to a certain voltage, and the the CC should go from FloatMPPT (when in sun/charging), toBulkMPPT. Right?  What's a good voltage for that to happen while I'm running loads?

Where to set rebulk depends on your battery, your battery management plan, and on the load.   

You should have a management goal, here are two examples:
one charge cycle every day, or initiate charge cycle whenever SOC gets down to 70%.

The Kid does a rebulk based on the voltage falling, for at least 2 minutes, to a voltage which you can specify.  Therefore, you cannot directly specify that rebulk occurs at a particular SOC.  It should be possible to do this in the future if you have a whizbangjr.

You can, however, learn to correlate your battery's voltage to its SOC.  This will depend on the size of the load and how much voltage sag the load causes.   If your loads are very variable, it will be difficult to do.

example: 
let's suppose your battery's resting voltage is 12.36 volts when its SOC is 70% and you want to begin a charge cycle when the battery gets to 70% SOC.

Let's also suppose that your battery capacity is 100 amphours and it is at 90% SOC (resting voltage = 12.58).  You should be able to draw 20 amphours from the battery before it will get to 70% SOC.

Now comes the problem... discharging 20 amps for 1 hour is not the same as discharging 1 amp for 20 hours.  The battery voltage will be different during the discharge, the larger load causing more voltage sag.  The 1 amp draw will cause almost no voltage sag... the battery's actual voltage will be very close to its resting voltage.

As I mentioned, if your loads are fairly constant, you can learn to correlate your battery's voltage to its SOC while under load, and then pick a rebulk voltage that meets your management goals.

--vtMaps