Can anyone help with basics of a charge controller's absorption stage?

kwb · August 2015

Hello,

I'm trying to model an off-grid solar system and any insight into how a charge controller works would be very helpful!

To begin, this is a three-stage charge controller that goes through "bulk," "absorption," and "float" modes of operation.

The bulk stage seems very straightforward- the controller finds the panel's MPPT, steps the voltage down to the battery's terminal voltage, and passes as much current as is possible until the battery's voltage rises to the level where it switches over to the absorption stage.

Absorption is a little more confusing, though. I understand that what the controller is trying to do is hold battery terminal voltage more-or-less steady while it slowly throttles the current flowing into the battery by letting the solar panel move away from its MPPT. The controller I'm modelling (An Outback FLEXMax) does that by setting a timer for the absorption stage. But, what exactly is it doing during that time period? Is it setting a current that is allowed to flow into the battery? Does that mean that the controller is trying to find an operating point on the panel's IV curve that it can convert to the battery terminal voltage and the current it wants? If so, how does it know where its set points should be when you connect a load to the charge controller on the battery side, where it will also draw current?

Thanks in advance for your help!

BB. · August 2015

Bulk is the controller pumping maximum current (limited by solar array/charge controller) into battery.

Absorb is when the battery voltage reaches the "absorb" set point. The charge controller holds the absorb voltage for 2-4 hours or so--And the battery "naturally" accepts less current as it approaches 100% full.

Float is just a lower set point voltage (to avoid over charging the battery bank)... Battery (and loads) accept current as needed.

The MPPT charge controller "sweeps" the array (takes from 0 to 100% of available current, while logging panel voltage) every 5-15 minutes or so. Goes back through list of Current*Voltage to find the "Maximum" power. MPPT controller will attempt to hold Vmp of panel by adjusting panel current up or down as needed (guessing a "close enough" explanation--there are many ways of doing this MPPT function)..

If the battery + loads need less power than the controller+solar array can provide, the charge controller simply takes less current from the solar array and lets the Vpanel rise (don't care).

If the output load is more wattage than the panel can supply--then the MPPT controller simply reduces its output current as needed to keep Vmp (voltage maximum power) at its calculated value. If the DC loads exceed the MPPT charge controller output current, then the battery bus voltage will fall and the battery will switch from charging to actually outputting current to support the loads.

-Bill

PNjunction · August 2015

BB. wrote: »

Absorb is when the battery voltage reaches the "absorb" set point. The charge controller holds the absorb voltage for 2-4 hours or so--And the battery "naturally" accepts less current as it approaches 100% full.

Exactly - many new to battery charging don't realize that -naturally- means that the *battery itself* is the one doing the current limiting taper in the absorb stage, as long as the voltage is held limited at that point.

What is happening is that with a limitation on the upper voltage in the controller, as the battery terminal voltage continues to rise to try and meet the chargers upper voltage limit, less current flows because the difference in voltage (pressure if you will) becomes smaller and smaller as the battery charges and the voltage at the battery terminals eventually rises to the same voltage as the controller.

No difference in voltage means no more current flow, but it takes awhile to get to that point seen as a tapering of current in the absorb stage. All done by the battery simply charging up.

vtmaps · August 2015

PNJ, you lost me, what am I missing?

PNjunction wrote: »

What is happening is that with a limitation on the upper voltage in the controller, as the battery terminal voltage continues to rise to try and meet the chargers upper voltage limit, less current flows because the difference in voltage (pressure if you will) becomes smaller and smaller as the battery charges

Huh? You were writing about absorb, but if the battery terminal voltage is rising, you are in bulk. If the current decreases during bulk, it is usually not the battery... it is the charger's power limitation. If charging power is limited, then as battery voltage goes up, battery current must go down. That is not the same thing as the tapering of current during absorb, where the voltage is constant.

PNjunction wrote: »

and the voltage at the battery terminals eventually rises to the same voltage as the controller.

If the voltage at the battery terminals and at the controller is not the same, then the cable is too long and thin.

--vtMaps

BB. · August 2015

I think we are getting a bit lost in the weeds here... The battery terminal voltage is "set" by the charger during absorb... The battery itself has something like (what we would call in motors) "Back EMF" (electro-motive force).

More or less the state of charge, internal resistance, chemical reactions, etc. serve to "resist" current flow from the charger. The higher the state of charge, the less current will be allowed to flow--And, sort of, the higher Back EMF by the battery.

-Bill

Photowhit · August 2015

I actually think the understanding that the battery limits the amount of current it accepts, is an important one for 'Off Grid' people, who deal with batteries every day.

It might be good to note that if the system has minimal loads, the Flex Max (I believe) is capable of letting the amount of amps passing into the battery turn off absorb. Called 'End Amp' this will work if you have minimal loads on the battery since the charging rate is measured at the charge controller so loads are not taken into account.

Can anyone help with basics of a charge controller's absorption stage?

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