Solar Panel Voc-MPPT?

pianotuna wrote:
Hi Ken,

So how does an MPPT controller "know" the voltage increase necessary?

I have added a switch so I can "force" equalization voltage when ever I so wish.

On an entirely different note I checked all electrolyte levels yesterday. Really there is almost no loss--the levels are about 3/4 of an inch above the plates. One set of batteries is 8 years old and the others are four years old. I attribute this--rightly or wrongly--to very low amperage charging.

ken white wrote:
Instead he focuses on a reduction of amps as a degradation of system performance which simply is not true especially when voltage increases to keep power constant...

How does it know?? the algorithms programmed into the controller, which may be fixed, or user setable or both.

On my morningstart sunsaver MPPT controller you can either use the factory defaults for charging profile; times, voltages etc. for bulk,float, equalize OR you can use the morningstart SW to make a custom profile.
Want to equalize daily?? you can
Want to change equalize voltage? you can
want to change the max time or max voltages for any profile? you can.

And actually most (maybe all) MPPT controllers output try to operate as a current source for bulk charging. An ideal current source will supply a given current without regards to any voltage limit. The voltage will rise to whatever the battery presents in order for the battery to accept that current. Most good RV battery charges behave in a similar fashion, at least during initial bulk charging.

How close to an ideal current source it is will be determined by they charge controller, which if well designed, should behave very close to ideal within the specified range of output current.

Once it gets to float or equalize, then it moves to a more constant voltage mode. An ideal voltage source will hold a constant voltage and deliver whatever current it takes to hold that voltage.

so in this mode battery voltage is set and current is determined by what current the battery will accept for a given voltage.

Again, the same thing most good battery chargers behave.


On the input side, a MPPT controller tries to behave in a impedance matching mode. In this mode you will get the maximum power transfer between the source (solar panel) and load (MPPT controller input) when they achieve this goal input voltage with Match the panels Vmp for the specific solar conditions, and input current will match Imp for the specific solar conditions.


And yes, battery sense wires, or "4 terminal" measurements would be great. Many controllers look at what the output voltage rises to in order to determine when to move out of bulk mode. So with additional voltage across the controller to battery interconnect the controller is going to move out of bulk mode earlier than it should.

And lack theroff is a good reason to keep voltage drop to a minimum between the controller and battery bank. BTW this would be nice on chargers as well!

A PWM controller should act somewhat like an ideal current source for bulk charging mode as well. However, how close to ideal is determined by the solar panels output characteristics rather than the controller. How close to ideal that is I haven't a clue, but it must be reasonably close from observation of PWM output current vs. battery state of charge during bulk mode charging. Carefull analysis of a VI curve for a panel would yied some insight.

When designing electronics years ago we often used what was called a "long tail" current source. Hook a resistor to a high voltage (say 25V) at one end, and the other end was connected to the other devices in the circuit where voltage was much lower but somewhat variable (say around 5V, varying 0.5V) so the end result was a relatively constant current over the expected change in either supply voltage (25V) or load voltage (5V).

A solar panel is somewhat similar. Voc is near 20V for a 12V panel, and under high light conditions, the panel output is pretty much "current limited" so it is someone of a constant current source under high output conditions, so panel voltage can swing a fair amount to let the current to the battery remain reasonably constant.

And, if you could set the same bulk mode output charging current for a MPPT controller, PWM controller and RV converter/charger they ALL would give very close to the same output voltage readings into the same battey bank configuration. Any difference would from slight variations from a constant current source.

Likewise if you could set all three to the same output voltage for say float mode (13.2v) they would ALL give very close to the same current, with any variations based on deviation from a constant voltage source.