I am trying to plan a stand alone system, and for that I will need a charge controller. I am looking at a few models from the company MorningStar. I have understood what MPPT is, and decided to not use MPPT because of the cost.

Currently I am looking at their series, ProStar, which they themselves call a "mid-range solar controller". Datasheet.

Below is a screenshot from the datasheet, which is relevant for my question:

enter image description here

I do understand that each type of battery: gel, sealed and flooded, have different specifcations. I plan to use a deep cycle gel battery. And I do understand the temperature compensation. I also understand the load disconnect and load connect voltage thresholds.

What I don't understand are the different stages of the charging. I believe it is called a charging algorithm, even though the data sheet does not use that word. Am I right?

Here is what I think I understand. But please correct any misunderstanding I have.

1) Full charge: When the insolation is not enough for the solar panels to reach the optimal charging voltage, it charges with whatever voltage is yields. Of course this only happens if the battery is below some voltage threshold where it is safe to charge. But which threshold?

2) PWM regulation: The insolation is so intense that panels yield a voltage greater than what is good for charging, in which case the voltage is reduced, using PWM. I think it must be charging with the "Regulation voltage", since regulation means changing (increasing) the charge/voltage of the battery.

3) Float: When the voltage of the batteries is at the optimal level, it should not be "regulated" any longer, but instead just maintained at that level. So I guess it charges with the voltage that is also the maximum healthy voltage for the battery. I think this number is what they call "Float".

4) Equalize: I don't know what this does, please teach me!


3 Answers 3


When cells are mismatched, one cell can over charge while another is undercharged. This time is called equalize. Since you are using gel cell, you don't have to be concerned about this. So after mode 2 duration, optimal battery life is regulated at float if PV can supply it.

I am not sure of the algorithm for going back to Mode 2 voltage depending on duration of voltage below Mode 3 but it is likely counting coulombs of charge by I*t and putting up to twice as much back as depleted or simply goes to float Mode 3 when supply current = load current ( 0 charge into battery) .

PV's are in theory current sources but lossy so the max power MPPT, curve peak finder hunts for this, but it is usually around 75 to 80% of the no load max voltage . If you are close to this the curve is fairly flat so no big deal, but this may depend greatly on PV chemistry.


The equalize function is a controlled overcharge which will ensure that any charge imbalance between the cells is removed. With a flooded (wet cell) battery, it will cause significant bubbling in the cells, causing a loss of water. For this reason, equalization must not be done with gell or AGM batteries as you cannot replace the lost liquid.

It should not be done with "maintenance free" flooded cells either, as you are (officially) not able to check and replenish the water. (I sometimes call these batteries "maintenance resistant", as a determined user can break into them to check fluid levels, and can add water as necessary>)

  • \$\begingroup\$ Maintainance free flooded = sealed? \$\endgroup\$ Apr 22, 2014 at 23:57
  • \$\begingroup\$ Why is the equalization part on top of the float part on the chart? Shouldnt one be after the other? \$\endgroup\$ Apr 23, 2014 at 0:00
  • \$\begingroup\$ @MadsSkjern: Yes, "Maintenance free" = sealed flooded battery. Equalization is probably shown on top of float as it would only be done on a fully-charged battery. \$\endgroup\$ Apr 23, 2014 at 0:07

You are approaching it with the solar panels as your reference, but the terminology all refer to the battery's condition, not the state of the solar panels. I.E. These terms (equalize, float, full charge, etc) refer to the state at which the battery is in and thus what the charge controller will do to the battery regardless of whether the battery is being charged by solar panels, wind, or even plugged into your AC plug in.

Each term refers to the level of charge or state of charge of the battery (usually determined by the voltage of the battery and thus dependent on the type of battery).

For example, "float charge" means that the lead acid battery is essentially full and you are just applying a small amount of charge to it to maintain it at full charge to account for the small amount of self discharge that will occur. It doesn't know or care about the insolation of your solar panels.

To learn what all of the terms mean, I recommend that you read this:


It defines the terms that you are asking about. It is important to understand the proper charging of your battery or it will not last long.

Good luck.


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